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Demographic and Fitness Consequences of Delayed Dispersal in The Cooperatively Breeding Acorn Woodpecker

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DEMOGRAPHIC AND FITNESS CONSEQUE NCES OF DELAYED DISPERSAL IN THE COOPERATIVELY BREEDING ACORN WOODPECKER By JUSTYN T. STAHL A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2006

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Copyright 2006 by Justyn T. Stahl

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iii ACKNOWLEDGMENTS I would like to thank my parents for always letting me choose my own path in life, and always being supportive of the decisions I make. I am grateful to my advisor, Madan Oli, and the other members of my advisory committee, Walt Koenig and Ben Bolker, for ad vice and assistance at all stages of my masters research. Madan served as an out standing mentor and editor, and my writing style has greatly improved thanks to him. I am especially thankful for Walts willingness to participate in this collaboration, because otherwise I would not have had such an amazing system with which to work. I thank Bill Searcy, my ornithology profe ssor at the University of Miami, for sparking my interest in avian ecology, a nd Jeff Hoover for teaching me proper field methodology and providing advice at vari ous stages of graduate school. This work would not have been possible without the support of 100+ field assistants and everyone else a ssociated with Hastings Natura l History Reserve. My stay at Hastings was made enjoyable by the following: Lauryn Benedict, Amber Budden, Catherine Dale, Janis Dickinson, Ben Harm eling, Joey Haydock, Jared Heath, Danika Kleiber, Amy Kochsiek, Alan Krakauer, Ja y McEntee, Kathleen Rudolph, and Mark Stromberg. I thank my current and former lab mates for providing work related help and relieving work related stress: Chris Burn ey, Jeremy Dixon, Elina Garrison, Ann George, Jeff Hostetler, Gabby Hrycyshyn, Melissa M oyer, Saif Nomani, Arpat Ozgul, Heidi

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iv Richter, Brian Spiesman and Matt Trager. Arpat, despite having a PhD to work on, was never too busy to answer my endless ques tions. I could never refuse an offer to go birding with Chris, whether it was chasi ng migrant warblers at Paynes Prairie or Crax rubra in Mexico and Costa Rica. Finally, I would like to thank various fr iends for providing support during the past several years: Mariola Alvarez, Matt Chambers, Joy Cox, Denny DuMey, Shannon Henn, Kathy Huala, Lisa and Friedrich Iglesias, Stacey Jones, Robyn Mericle, Sheda Morshed, Kymia Nawabi, Devin Peipert, Peejay Perez De Alejo, Alex Pries, Mark Rodriguez, Nikki Schiwal, Taylor Shie lds, and Autumn Tarleton.

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v TABLE OF CONTENTS page ACKNOWLEDGMENTS.................................................................................................iii LIST OF TABLES............................................................................................................vii LIST OF FIGURES...........................................................................................................ix ABSTRACT....................................................................................................................... ..x CHAPTER 1 INTRODUCTION........................................................................................................1 2 DEMOGRAPHIC CONSEQUENCES OF DELAYED DISPERSAL IN THE COOPERATIVELY BREEDING ACORN WOODPECKER....................................4 Abstract....................................................................................................................... ..4 Introduction...................................................................................................................5 Methods........................................................................................................................7 Study Species.........................................................................................................7 Study Area.............................................................................................................9 Field Methods........................................................................................................9 Survival and Transition Probabili ties: Estimation and Modeling.......................10 Results........................................................................................................................ .13 Discussion...................................................................................................................17 3 FITNESS CONSEQUENCES OF DELAYED REPRODUCTION IN A COOPERATIVE BREEDER: DOES HELPING HELP?..........................................29 Abstract.......................................................................................................................29 Introduction.................................................................................................................30 Methods......................................................................................................................31 Study Species and Study Site..............................................................................31 Field Methods......................................................................................................32 Estimation of Fitness...........................................................................................33 Statistical Analysis..............................................................................................34 Effect of Social and Envir onmental Factors on Fitness......................................34 Results........................................................................................................................ .35 Discussion...................................................................................................................37

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vi 4 CONCLUSIONS........................................................................................................45 LIST OF REFERENCES...................................................................................................48 BIOGRAPHICAL SKETCH.............................................................................................55

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vii LIST OF TABLES Table page 2-1. Quasi-likelihood adjusted AIC differences ( QAICc) for five models of the survival, recapture, and tr ansition probability in relation to sex and status for acorn woodpeckers (1972-2004)..............................................................................24 2-2. Estimates of apparent survival rates ( S ) for male, female, breeder and helper acorn woodpeckers (1972-2004) based on Mo del 1 (Table 2-1). Mean values (95% CI) are given...................................................................................................25 2-3. Transition probabilities ( ) between juvenile, helper and breeder stages for acorn woodpeckers (1972-2004) in relation to sex based on Model 1 (Table 2-1). Mean values (95% CI) are given, with M = male, F = female.................................25 2-4. Influence of social and environmental factors on apparent survival rate of acorn woodpeckers (1972-2004). Regression coefficients ( ) are given with 95% CI (significant effects are shown in bold ). SHM = survival of helper males; SHF = survival of helper females; SBM = survival of breeder males; SBF = survival of breeder females........................................................................................................25 2-5. Influence of social and environmenta l factors on transition probabilities of acorn woodpeckers (1972-2004). Regression coefficients ( ) are given with 95% CI (significant effects are shown in bold ). Symbols used are as follows: JB M = transition probability from j uvenile to breeder (male); JB F = transition probability from juvenile to breeder (female); HB M = transition probability from juvenile to breeder (male); HB F = transition probability from juvenile to breeder (female). The -values and 95% CI for JH and HH were of equal magnitude but opposite sign of JB and HB, respectively, for each sex..................................26 2-6. Quasi-likelihood adjusted AIC differences ( QAICc) for five models of the realized population growth of acorn woodpeckers (1972-2004) using Pradels reverse-time capture-mark-recapture model. Symbols are as follows: = survival, p = recapture, and = realized population growth rate. Sex effect is noted ( s ), time effect is noted ( t ), additive effect of sex and time is noted ( s + t ), and a period (.) indicates the constant value of the parameter.................................26 3-1. Fitness components and measures of fitness for female acorn woodpeckers (1972-2004) that were banded as juveniles, were recorded at least once after

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viii their first possible breeding season, and reproduced at least once. Means (SE) and ranges are shown. A t-test was used to compare helpers (N = 80) and potential breeders (N = 61), helpers and breeders that first reproduced at age 1 ( =1; N = 44), and helpers and breeders that first reproduced at age 2 or older ( 2; N = 17); P < 0.05 indicates statis tical significance..............................................43 3-2. Correlation between f itness components and measures of fitness for female acorn woodpeckers (1972-2004) that were band ed as juveniles, were recorded at least once after their first possible breedi ng season, and reproduced at least once. The two measures of fitness were lifetime reproductive success (LRS) and individual fitness ( ). Age at first reproduction is represented as The Pearson correlation coefficient is given above, a nd the p-value below (p < 0.05 indicates a significant relationship between the two variables, N = 141)...............................44

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ix LIST OF FIGURES Figure page 1. Annual variation in survival of acorn woodpeckers (1 972-2004). Mean rates (black lines) are shown with 95% confidence interval (gray shade) for male and female helpers ( SHM and SHF, respectively) and male and females breeders ( SBM and SBF, respectively). Estimates are base d on Model 1 (Table 2-1) and are compared to realized population growth rate ( ).....................................................27 2. Annual variation in transition pr obabilities of acorn woodpecker (1972-2004). Mean rates (black line) are shown with 95% confidence interval (gray shade) for the transition from juvenile to breeder stages for males (JB M ) and females (JB F ), and from helper to breeder stages for males (HB M ) and females (HB F ). Estimates are based on Model 1 (Table 2-1) and are compared to realized population growth rate ( )........................................................................................28

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x Abstract of Thesis Presen ted to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science DEMOGRAPHIC AND FITNESS CONSEQUENC ES OF DELAYED DISPERSAL IN THE COOPERATIVELY BREEDING ACORN WOODPECKER By Justyn T. Stahl May 2006 Chair: Madan J. Oli Major Department: Interdisciplinary Ecology Cooperative breeding in birds occurs when more than two individuals provide care for a single nest. In many species, the addi tional adults present are offspring from a previous year who have delayed dispersa l, and provision young who are non-descendant kin. Delaying dispersal and age of first breeding can influence the demography and fitness of individuals, but the fitness and demographic conse quences of helping behavior are poorly understood. Using long-term data (1972-2004), I examined the demographic and fitness consequences of help ing behavior in acorn woodpeckers Melanerpes formicivorus Using a multi-state capture-mark-re capture framework, I found that the apparent survival of breeders was higher th an helpers, and the survival of males was higher than females. Juveniles were much mo re likely to become helpers rather than breeders following fledging, and helpers were mo re likely to remain helpers rather than becoming a breeder. Both survival and tran sition rates varied annually and were positively influenced by the acorn crop. Fo r fitness estimation, I focused only on

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xi reproductive females who were banded as juveniles. Although helpers began reproduction significantly later and lived signifi cantly longer than breeders, there was no significant difference in lifetime reproductive success or individual fitness between the two groups. However, birds that successfu lly bred at age 1 without helping had a significantly higher fitness than those who helped and successfully bred at age 2 or older. My results suggest that delayed dispersal and reproduction in the acorn woodpecker lead to a loss of fitness when conditions are favor able for successful reproduction. However, if constraints in the environment prevent an indi vidual from breeding at age 1, helping is a viable option until reproduction is possible.

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1 CHAPTER 1 INTRODUCTION Cooperative breeding in birds occurs when more than two individuals provide care for a single nest (Brown 1987). Why so me individuals provide care for young who are not their own has been of substantial interest to beha vioral and population ecologists alike (reviewed in Stacey & Koenig 1990; Cockburn 1998; Koenig & Dickinson 2004). Recognized in 3.2% of the extant species of birds (Arnold & Owens 1998), cooperative breeding can manifest itself in a number of ways. Multiple males can share a single female mate (mate-sharing) and multiple females can lay eggs in the same nest (jointnesting). Offspring can disperse and attempt to breed independently when they become sexually mature, or they can delay dispersal for a year or more and remain at home on their natal territory. Dispersi ng individuals may return home at a later time, may breed on their own, or may immigrate into another group of kin or non-kin. One of the most interesting aspects common to many cooperativ e breeding systems is helping behavior, which can generally be described as dela yed reproduction past the point of sexual maturity and provisioning of young that are not direct descendants. It has been suggested that a shortage of a critical resource constrains the availability of breeding territories and th e ability of some i ndividuals to reproduce successfully. This idea, referred to as the ecological constraints hypothesis, has been proposed as one cause of cooperative br eeding (Koenig & Pitelka 1981; Emlen 1982). Often, the ecological constraint is a shorta ge of reproductive vacancies and can be associated with a lack of suitable nest cavities (Walters, Cope yon & Carter III 1992),

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2 habitat (Woolfenden & Fitzpa trick 1984), or food-related resources (Koenig & Mumme 1987). Until a reproductive vacancy becomes av ailable, helpers may remain on the natal territory and help raise younger siblings. The acorn woodpecker Melanerpes formicivorus is a cavity nesting species common throughout the oak woodlands of west ern North and Central America, ranging from Oregon south to Colombia. In Ca lifornia the acorn woodpecker practices cooperative breeding, consisting of mate-shari ng, joint-nesting and helping behavior, although the most common reproductive gr oup is a monogamous pair (Koenig & Mumme 1987). Displaying one the most comple x social systems of any vertebrate, the acorn woodpecker has been stud ied continuously since 1971 at Hastings Natural History Reservation in central coas tal California (MacRoberts & MacRoberts 1976; Koenig & Mumme 1987; Koenig, Haydock & Stanback 1998) and offers an opportunity to answer a number of questions related to cooperative breeding and helping behavior. The majority of demographic analyses regarding helpi ng behavior were done several years ago (summarized in Koenig & Mumme 1987). Since then, many years of additional field data have been collected, and new techniques for estimating fitness and population dynamic consequences of social behavi or have been developed. My objectives were to investigate de mographic and fitne ss consequences of helping behavior in the acorn woodpecker. In chapter 2 I investig ated the effect of cooperative breeding and helping behavior on survival, probability of breeding, and realized population growth rate. I used multi-state and reverse-time capture-markrecapture (CMR) modeling approaches to estimate the aforementioned parameters, to

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3 investigate the influence of helping on these parameters, and to test specific biological hypotheses relevant to my study system. In chapter 3 I investigated the effect of delayed dispersal (and thus delayed age of first reproduction) to help at home on indivi dual fitness of female acorn woodpeckers. I estimated individual fitness using two methods: lifetime re productive success (LRS), and individual fitness (( McGraw & Caswell 1996). I compared fitness of females who helped and those who dispersed as yearlings, al lowing me to test for an effect of helping and delayed dispersal on individu al fitness. I also compared helpers and dispersers that successfully bred at age 1 to investigate the fitness effects of vari ation in age of first reproduction which arises from differing disp ersal strategies. Finally, I examined the effect of a number of social and envir onmental factors on indi vidual fitness and its components. The application of recently developed capture-mark-recapture techniques (multistate models) and fitness estimation tools ( ) to long term data from the acorn woodpecker allowed me to address questions regarding cooperative breeding and helping behavior that were not po ssible when similar studies were conducted decades ago.

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4 CHAPTER 2 DEMOGRAPHIC CONSEQUENCES OF DELAYED DISPERSAL IN THE COOPERATIVELY BREEDING ACORN WOODPECKER Abstract Acorn woodpeckers ( Melanerpes formicivorus ) often delay dispersal and provision the offspring of close relatives. We investigated the demographic consequences of helping behavior using a multi-state capture-mark-recapture approach in Program MARK, and evaluated the influe nce of social and environmental factors on survival and breeding probabilities of acorn woodpeckers. Br eeders survived better than helpers, and males survived better than females. Juveniles were much more likely to become helpers rather than breeders following fledging, and he lpers were more likely to remain helpers rather than becoming a breeder. Both survival and transition rates varied annually. Group size and composition had a signif icant influence on survival of breeder males, breeder females and helper males. These social f actors often had a negative influence on the likelihood a female would attain a breeding position, while not significantly influencing the same rates for males. The acorn crop posit ively influenced surviv al and probability of breeding. Realized population growth rate vari ed annually and was positively affected by both survival and transition probabilities. Surv ival of male breeders, which was strongly influenced by the size of the acorn cr op, generally had the greatest influence on population growth rate.

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5 Introduction Social behavior influences surviv al, timing and probability of breeding, reproduction, dispersal, and population grow th rate in many species of birds and mammals (Brown 1987; Stacey & Koenig 1990; Solomon & French 1996; Koenig & Dickinson 2004). However, the population dynami c consequences of social behavior are still not well understood. This is particularly true for cooperati ve breeding birds, in which more than two individuals participate in providing care for a singl e nest (Brown 1987). One common characteristic of many coope rative breeding systems is the presence of helpers: individuals who delay dispersal, forego breedin g and provide care for another individuals offspring (Eml en 1982, 1995). In some species, individuals may return to help a family member after dispersing and failing to breed as in the western bluebird Sialia mexicana and long-tailed tit Aegithalos caudatus (Dickinson, Koenig & Pitelka 1996; MacColl & Hatchwell 2002). In others, birds nest independently but establish territories near their parent s and sometimes provision the young at both nests, as in Galapagos mockingbirds Nesomimus parvulus and western bluebirds (Curry & Grant 1990; Dickinson et al. 1996). Delayed dispersers can in some cases also stay without offering any help at the nest, as occurs in the Siberian Jay Perisorius infaustus (Ekman, Sklepkovych & Tegelstrm 1994). Delayed dispersal and helping behavior are often attributed to ecological constraints (Koenig et al. 1992; Hatchwell & Komdeur 2000) with constraints in some cases being so extreme as to cause helping behavior to be obliga te (Boland, Heinsohn & Cockburn 1997). Depending on the suite of envi ronmental and social factors experienced by an individual of dispersing age, the two po ssible strategies (disperse or stay) should confer differing fitness advantages to the individual. For example, Covas et al (2004)

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6 showed that when unlimited food was provide d to colonies of social weavers the proportion of birds that helped decreased and more birds initiated independent breeding. Similarly, Walters et al (1992) experimentally showed that red-cockaded woodpecker Picoides borealis helpers dispersed into suitable v acant territory when a key resource (a suitable nest cavity) was available, but generally remained at home a nd helped until nest cavities became available. Social factors, such as availability of mates, number of breeders on natal territory and group size and composition, may also affect an individuals dispersal decisi ons. When constraints on breed ing are high, juveniles should be more likely to stay at home and help ra ther than disperse and breed independently (Koenig et al. 1992). Individuals that provide help to relatives may increase their own inclusive fitness indirectly due to the promotion of shared genetic material (Hamilton 1964; Griffin & West 2002; Oli 2003). However, helping can ha ve a positive or nega tive effect on the direct fitness of an individual. Survival a nd the probability that an individual will breed are two major components of fitness and population dynamics, and can be affected by delaying dispersal and remaining on the natal te rritory past the point of sexual maturity. Survival of non-breeders can be enhanced by fa miliarity with the natal territory, parental nepotism, or by group augmentation (E kman, Bylin & Tegelstrm 2000; Kokko, Johnstone & Clutton-Brock 2001), while surviv al of breeders can be increased by the load-lightening associated with the pres ence of helpers or group augmentation (Kokko et al. 2001; Heinsohn 2004). By delaying immedi ate breeding, it is often possible to increase the likelihood of successful breedi ng through territory or mate inheritance, territory budding or by dispersing in coali tions of same-sex siblings (Woolfenden &

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7 Fitzpatrick 1984; Hannon et al. 1985; Komdeur & Edelaar 2 001; Dickinson & Hatchwell 2004). Changes in survival and the likelihood of breeding affect an individuals fitness and the growth rate of a population, and thus population dynamics. An understanding of how delayed dispersal and helping behavior influences probability of survival and breeding, and of the influence of environmen tal and social factors on these rates, is necessary for discerning the fitness and population dynamic consequences of helping behavior in cooperative breeding systems. Here we apply multi-state capture-mark-recapture (CMR) models (Williams, Nichols & Conroy 2002) to 33 years of data to investigate the su rvival and population dynamic consequences of delayed dispersal and helping in the c ooperatively breeding acorn woodpecker Melanerpes formicivorus Specifically, we ask the following: Do nonbreeding helpers and breeders ha ve different survival probabilities? Are there sex-specific differences in survival (within helpers and breeders, and overall) or the probability of becoming a breeder or helper after fledging ? Which social and environmental factors influence survival and probability of beco ming a breeder? Finally, we used Pradels (1996) reverse-time CMR model to estimate and model the realized population growth rate and investigate the effect of variation in survival and probability of breeding on the growth rate. Methods Study Species The acorn woodpecker is a cooperative br eeding bird common in oak woodlands of the Pacific coast. Breeding groups range from single monogamous pairs up to 15member mixed groups of breeders and non-br eeding helpers of both sexes (Koenig & Mumme 1987). Helpers are almost always offspring from prior years. Both breeders and

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8 helpers contribute to territory defense, f ood storage, and care of young (Mumme, Koenig & Pitelka 1990). Exhibiting one of the most complex social systems of any vertebrate, the acorn woodpecker is opportunistica lly polygynandrous, often prac ticing both mate-sharing and joint-nesting within the same group (K oenig & Mumme 1987). Mate-sharing occurs when 2-4 males, usually brothers or a fath er and son(s), compete for reproductive access to 1-3 females, usually sisters or a mother and daughter(s). Joint-nesting occurs when multiple females synchronously lay eggs in a common nest. The latter occurs in 22% of groups, while mate-sharing by males occurs in slightly over half (54%) of groups. However, the most common breeding syst em, making up 23.5% of all groups, is a monogamous pair (Koenig & Mumme 1987; Koen ig & Stacey 1990). Incest is rare; a genetic analysis by Haydock et al. (2001) found that only 14 of 400 (3.5%) offspring apparently resulted from incestuous breedi ng. In general, the high costs of inbreeding depression (estimated to be at least 1.2-1.8 lethal equivalents pe r individual) select against helpers breeding with related group members (Koenig et al. 1998; Koenig, Stanback & Haydock 1999). Helpers disappear at a higher rate than breeders, at least in part because of dispersal out side the study area (Koenig et al. 2000). The apparent limiting factor (or critical resource, cf. Walters (1990) for successful breeding acorn woodpeckers in California is an acorn storage tree or granary (Koenig & Mumme 1987). Members of the group drill a nd maintain holes in a tree throughout the year, collecting acorn mast and storing it in the granary. Larger granary sizes allow the food stores to last through the winter, enabling earlier nesting, larger clutches, and higher nesting success in the sp ring (Koenig & Mumme 1987; Koenig & Stacey 1990).

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9 Study Area The study site is located on Hastings Na tural History Reservation in the Santa Lucia mountain range in Monterey County, California. Elevation ranges from 450 to 920m. Summers are hot and dry, while winters are cool and wet with infrequent snow. Common trees in the study area include five species of oak: coast live oak ( Quercus agrifolia ), canyon live oak ( Q. chrysolepis ), blue oak ( Q. douglasii ), black oak ( Q. kelloggii ), and valley oak ( Q. lobata ), along with California sycamores ( Platanus racemosa ), willows ( Salix spp.), California buckeye ( Aesculus californica ), and madrone ( Arbutus menziesii ). The plant communities in acorn woodpecker habitat include foothill woodland, savanna-grassland, and riparian woodland. Acorn woodpeckers are most frequently found in generally open areas with a dense ground cover of grasses and few shrubs (Koenig & Mumme 1987). Field Methods Acorn woodpeckers at Hastings Reservati on have been individually marked and their survival and reproduction have b een monitored since 1971 (MacRoberts & MacRoberts 1976; Koenig & Mumme 1987; Koenig & Stacey 1990; Koenig et al. 1999). Nestlings were banded between 20-25 days after hatching; fledging occurs at approximately 30-32 days (Weathers, Koenig & Stanback 1990). Breeding status of adults (helper or breeder) was determined indirectly based on the history and known relatedness of birds. Genera lly, birds immigrating into a group were assumed to be potential breeders, whereas offspring rema ining from prior years were generally classified as non-breeding helpers. Howeve r, when all breeders of the opposite sex disappeared and were replaced by unrelated individuals, offspring were assumed to

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10 inherit breeding status within their na tal territory (Koenig & Mumme 1987; Haydock et al. 2001). Further details on field methodology are given in Koenig & Mumme (1987). Each fall starting in 1980, the acorn crop of the five Quercus species was estimated. For each tree, the number of acorn s counted in 15 s by each of two observers was tabulated. Each count value was log-tran sformed and the mean value was calculated and used as an index of the acorn crop (hereafter, acorn crop or AC) (Koenig et al. 1994a; Koenig et al. 1994b). Group size and composition of each group was determined in the spring of each year. The number of breeding males, breeding females, helper males and helper females was determined for each group (details in Koenig & Mumme 1987). Survival and Transition Probabili ties: Estimation and Modeling A multi-state CMR model was constructed separating birds into juvenile (J), helper (H), and breeder (B) stages. A bird was considered a juvenile from banding (just prior to fledging) until the following spring breeding s eason (1 March). Juvenile woodpeckers had several potentia l options available to them. They could remain on the natal territory past the point of sexual maturity and help, th ey could disperse to fill a breeding vacancy (either within or outside of the study area), they could inherit breeding status on their natal territory, or they could die. After beco ming a helper, an individual could die, continue to help, or become a breeder (either within the group, following the death of the opposite-sex breeder or by dispersing to anothe r group to fill a reproductive vacancy). Breeding birds could continue br eeding (either in the same group or after moving to another group), die, or (rar ely) revert to becoming a helper.

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11 Following Williams et al. (2002), we define rs i as the probability of being alive and in state s at time i + 1, given that the bird was alive in state r at time i The recapture probability, ,r i p is the probability that a bird alive in state r at time i is captured or observed after banding. Both rs i and r i p assume that survival and transition between i and i + 1 and capture at i depend only on the state at time i The parameter rs i is the product of both surviv al and probability of transition between states (Eq. 17.30 of Williams et al. 2002): ,rsrrs iiiS where r iS is the probability that an animal in state r at time i survives and remains in the study population until time i + 1, and rs i is the probability that an animal is in state s at time i + 1, given that it was in state r at time i and survived until i + 1 and remained in the study area (Williams et al. 2002). It is important to note th at, due to the finite study area, birds that disappeared could either have died or dispersed outside the study area. Therefore, these estimates are apparent surv ival, and should not be interpreted as true survival, especially in the case of helpers, a large fraction of which are known to disperse. Similarly, transition probabilities for help ers becoming breeders are to an unknown extent compromised by the probability of dispersal, as only the fate of individuals remaining in the study area could be determined. In cont rast, although breeders do sometimes disperse (Haydock and Koenig, unpublished data), appare nt survival for br eeders is likely to closely reflect actual survival. Additionally, fo r a given state, the tr ansition probabilities sum to 1 (i.e.1rs i s).

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12 In our analysis, we have th ree states, juvenile, helper and breeder. For example, J B is the probability that a juve nile bird in a given year will be a breeder one year later, and H H is the probability that a helper in one year will remain a helper the next year. By definition, a juvenile bird at i cannot be a juvenile at i + 1, as it must become either a helper or breeder. Therefore, 0.JJ Additionally, birds cannot return to the juvenile stage. Thus, 0.HJBJ We excluded from analyses individuals that were banded as fledglings but did not survive the winter. Because we included only th ose birds that survived to the first spring following fledging, all juveniles included in th e analyses became either a helper or a breeder. Juvenile survival was thus set to 1. We estimated the goodness of fit (GOF) for our multi-state model using Program U-CARE (Choquet et al. 2003). With two sexes (male, female), three strata (juvenile, helper, breeder) and 33 capture occasions, our data file was too large for Program UCARE to handle. Because of the large number of states in our model, we were unable to use the standard Arnason-Schwarz (AS) multi-state model and instead used TEST 3G and M of the Jolly-Move (JMV) model, as impl emented in U-CARE. This is a reasonable approach to GOF of the Arnason-Schwarz (AS) multi-state model, because the JMV model is unlikely to show signi ficantly greater fit to the data than the AS model (Cooch & White 2005). Further details regarding JMV and AS models are av ailable in Pradel, Wintrebert & Gimenez (2003). We implemented the multi-state model using Program MARK (White & Burnham 1999). For model comparison we used Akaike s Information Criterion (AIC), which considers both the deviance of the model a nd the number of estimated parameters to

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13 provide a parsimonious description of how we ll the models explained variation in the data when compared to one anot her (Burnham & Anderson 2002; Williams et al. 2002). A difference in AICc values ( AICc) of less than two indicated that the models were similarly supported by the data. A moderate difference in model support was shown when 2 < AICc < 7, and strong support when AICc > 10. We used the most parsimonious model (lowest AICc) to examine survival and transition estimates. Using the most parsimonious model, we i nvestigated the effect of environmental and social factors on survival and breeding pr obability by modeling the logits of survival and transition rates as linear functions of the factors. Values of all environmental and social factors were scaled such that value ranges from 0 to 1. The relationship between the survival or transition rate and a cova riate was considered significant if the 95% confidence interval of the slope parameter of the linear model ( ) did not include 0 (Williams et al. 2002). Pradels (1996) reverse-time CMR m odel, implemented in Program MARK, was used to estimate and model the realized population growth rate ( ). For model selection and parameter estimation, we used the same methodology as for the multi-state model. For the Pradels model, our interest was in the overall growth rate of the population, and the breeding status of birds (juvenile, breed er, helper) was therefore ignored. We used Program RELEASE, implemented in Progr am MARK (White & Burnham 1999), to estimate GOF for the general Pradels model. Results Our dataset spanned 33 years (1972-2004) and included data for 1570 individual acorn woodpeckers (845 males and 725 fema les). We included 1218 birds banded as juveniles (690 males, 528 females), and 352 birds banded as adults (155 males, 197

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14 female). Only juveniles that were present in the population during th eir first spring were included in our analysis. The fully time-dependent multi-state model of survival, recapture, and transition probabilities, {SJ (s*t) SH (s*t) SB (s*t) pJ (s*t) pH (s*t) pB (s*t) JB (s*t) HB (s*t) BB (s*t)}, fit the data poorly ( 2 105 = 219.9, P < 0.0001). The variance inflation factor, was 2.09, indicating over-d ispersion (Lebreton et al. 1992). Therefore, we used calculated to aid in parameter estimation and model comparison. The most parsimonious model (Model 1, Table 2-1) differed from the second and third most parsimonious models (Model 2 and 3, respectivel y, Table 2-1) by QAICc of 0.25 and 1.13, respectively, indicating that these 3 models were practically identical. Model 1 included an additive effect of sex a nd time in all parameters except recapture probability of breeders and helpers, probability of transitioning from breeder to helper, and by definition, all fixed parameters, which were constant across time and sex (Table 21). Recapture (p) rates for helpers and breeders were equal and constant across time and sex. The transition rate between breeder a nd helper was constant across time and sex. As the most parsimonious model, we used Model 1 in further analyses. Recapture probability (the probability of being seen again after banding, assuming an individual is still alive) for helpers and breeders was high (0.958, 95% CI: 0.947, 0.975). Juvenile recapture was fixed to zero (T able 2-1). Survival rates showed temporal variation and differed between sexes and br eeding status. The apparent survival was higher for males than females, and higher for br eeders than helpers (Table 2-2). Apparent breeder survival was approximately 10% higher than that of helper, and male survival was 8% higher than female survival. The difference between male and female survival

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15 was most pronounced among helpers, where males survived at a rate nearly 12% higher than female helpers. Survival probability of helpers of both se xes exhibited greater temporal variation than that of breeders (Figure 1). Juvenile acorn woodpeckers were roughly 4-7 times more likely to become helpers than breeders following fledging, but this differed be tween sexes. Juvenile males had a higher probability than juvenile females of becoming a breeder in the study area in the spring after fledging (Table 2-3). Additi onally, helpers were a pproximately twice as likely to remain helpers the following year as becoming breeders, with females having a slightly higher probability of remaining he lpers. Once an individual became a breeder, the probability of it returning to the helping stage was very low and similar for the two sexes (Table 2-3). Using the most parsimonious model (Model 1, Table 2-1), we investigated the effects of environmental and social factor s on survival and transition probabilities. Survival of male breeders (SBM) was positively influenced by the acorn crop, size of storage facilities and group size. The effect of group size was due to both the number of breeders and the number of helpers, since both positively influenced survival (Table 2-4). Survival of female breeders (SBF) was positively influenced by acorn crop, storage holes and group size. The group size effect on the survival of breeder females was due to number of helpers in the group (especially he lper males). Survival of male helpers (SHM) was positively influenced by storage holes and group size, which was due to the number of helpers (specifically helper males), but was unaffected by the annual acorn crop. Survival of female helpers (SHF) was positively affected by only the acorn crop, showing no influence of any social factor (Table 2-4).

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16 The probability of a male breeding in his second year (the year following his juvenile year, JB M ), was not significantly influenced by any social or environmental factor (Table 2-5). The same probability for females was negatively influenced by the number of breeders in the group (both the num ber of breeding males and females), while it was positively influenced by the acorn cr op. Females, however, were less likely to become breeders (JB F ) if there were more breeders in their natal group and more likely to become a breeder if the acorn crop wa s good. Male helpers were more likely to become breeders (HB M ) when the acorn crop was good a nd less likely when there were more helper females in the group. Female he lpers were less likely to become breeders (HB F ) in larger groups (which was due to th e number of helpers, specifically helper females), and also more likely when the acorn crop was better (Table 2-5). The fully time-dependent Pradels model, { (s t) p (s t) (s t)}, fit the data poorly ( 2 139 = 475.4, P < 0.0001), and a variance inflation factor ( ) of 3.42 was used to aid parameter estimation and model comparison. The most parsimonious model included constant capture probability (p), additive effect of sex and time on survival ( ), and time effect on realized popu lation growth rate ( ). A competing model that differed in QAICc by 1.23 ( QAICc) showed an additive effect of sex and time on (Model 2, Table 2-6). Because the QAICc was less than 2, there was no support for a difference between the two models, indicating th at the sex effect on was negligible. Although there was no significant difference between the two models, we chose the one yielding the smallest QAICc value (Model 1, Table 2-6), { (s + t) p (.) (t)}. The realized annual population growth rate ranged over time from 0.598 ( 95% CI: 0.530, 0.674) to 1.547 (95% CI: 1.296, 1.846, Figure 1).

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17 We investigated the influence of the underlying surviv al and transition probabilities on and found that all probabilities had a significant positive influence on Survival of male breeders (SBM) generally had the greater influence on ( = 2.023; 95% CI: 1.433, 2.612), followed by SBF ( = 1.718; 95% CI: 1.225, 2.212), SHM ( = 1.654; 95% CI: 1.810, 2.128) and SHF ( = 1.450; 95% CI: 1.035, 1.865). Among transition probabilities, HB M had the highest influence on ( = 0.443; 95% CI: 0.301, 0.585), followed by HB F ( = 0.442; 95% CI: 0.301, 0.582), JB F ( = 0.369; 95% CI: 0.221, 0.517), and JB M ( = 0.334; 95% CI: 0.200, 0.469). The regression coefficients and confidence intervals for the transition probabilities JH and HH were of equal magnitude, but opposite sign of JB and HB respectively for the corresponding sexes. Discussion In cooperatively breeding birds, there are costs as well as benefits associated with a birds decision to stay and help vs. disper se and attempt to breed independently (Brown 1987; Dickinson & Hatchwell 2004; Ekman et al. 2004). For example, survival of philopatric individuals is often higher than th at of dispersers due to factors such as territory familiarity (Ekman et al. 2004). A birds decision to di sperse or stay and help can also influence its probability of breedi ng. Survival and breeding probability are key components of fitness, and the effect of dispersal decisions on these demographic parameters can have important population dynamic consequences. In our analyses, helpers had lower appare nt survival than breeders, and thus permanently disappeared with much greater fr equency. However, this is clearly because many disappearing helpers are dispersing to fi ll reproductive vacancies outside of the study area (Koenig et al. 2000). Although breeder dispersal does occur (Haydock and

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18 Koenig unpublished manuscript), apparent survival of breeders is rela tively close to the real survival rate due to the fact that on ce an individual begins breeding at a site, it generally stays there until death or some catastrophic event occurs (Koenig & Mumme 1987). Of the 1570 individuals included in our analysis, 352 were banded as adults, the majority of which were immigrants from outside of the study area. Assuming that emigration equals immigration, 35.7% of helpers disappearing are filling breeding vacancies outside of the study area (Koenig et al. 2000). Therefore, both the survival rate and probability of attaining a breeding position for helpers were likely biased low. The dispersal confound also exists for female acorn woodpeckers, who, like many avian species, tend to disperse furthe r than males (Greenwood 1980; Koenig et al. 2000). Koenig and Mumme (1987), in a previous analysis of the Hastings population (1973-1986), estimated annual su rvival of breedin g males to be 0.824 and breeding females to be 0.712. Our estimates (0.756 for breeding males and 0.714 for breeding females) were similar to the extent that the earlier estimates fall within the 95% confidence intervals (Table 2-2). Koenig & Mumme (1987) hypothesized that the higher mortality of female breeders is attributable to increased energetic expenditure by females associated with egg laying and incubation, nest care, and territory defense. Additionally, the higher number of males banded as juveniles that remained in the study area reflects male-biased natal philopatry, while the higher number of fe males banded as adults (many of which immigrated into the study area) reflects female-biased dispersal. This male-biased philopatry is likely a major sour ce of the apparently higher surv ival of helper male acorn woodpeckers (Koenig et al. 2000).

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19 In many systems, it is not possible to differentiate between individuals that permanently emigrate from the study ar ea and those that die. The confounding of dispersal and survival is almost a univers al problem (Koenig, Van Vuren & Hooge 1996). Walters (1990) work on redcockaded woodpeckers has prob ably been the closest to avoiding the problem in a cooperative breed er. Survival rates of red-cockaded woodpeckers were similar to acorn woodpeck ers: 0.76 for breeding males and 0.68 for breeding females, but the survival of helper males was slightly higher than that of breeders (0.78 compared to 0.76, Walters 1990) Juvenile birds that survived their firs t breeding season were much more likely to become a non-breeding helper than to attain a breeding position in the study area. Males were almost twice as likely as females to become a breeder in the study area following their first year. We offer two possible explana tions for the higher breeding probability for males. First, cobreeding among males is mo re common, and dispersing coalitions of brothers were more successful at competing for breeding positions following a vacancy (Hannon et al. 1985). Thus, a larger number of help er males will generally fill a given number of vacancies than helper females. Secondly, male-biased philopatry typically leads to more males remaining in the study area and attaining breeding positions than females (Koenig et al. 2000). A woodpecker that remained a helper for the year following fledging was roughly twice as likely to become a breeder as a juvenile (compare J B and H B for both sexes, Table 2-3). But because the helper stage in cludes birds of varying age, the increased H B probability is likely an effect of age. Once an acorn woodpecker obtains a breeding position, however, it rarely reverts to he lper status (Haydock and Koenig unpublished

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20 manuscript), in contrast to species in whic h redirected helping by failed breeders is common, such as long-tailed tits and western bluebirds (Dickinson et al. 1996; MacColl & Hatchwell 2002). Analysis of populations wi th redirected helping would likely yield significantBH values. Previous work has shown the acorn cr op to be strongly correlated with reproductive success, territory stability, a nd breeder survival in acorn woodpeckers (Koenig & Mumme 1987; Koenig & Stacey 1990) Therefore it was not surprising that the size of the acorn crop had a significant influence on survival and transition probabilities of acorn woodpeckers. For br eeders, acorn production increased annual survival of both sexes. For helpers, ho wever, only the survival of females was significantly influenced by acorn production. In years with large acorn crops, juveniles (females) and helpers (males and females) had a higher probability of attaining a breeding position. Thus, more birds breed durin g years with bumper acorn crops, in much the same way that Ural owls Strix uralensis take advantage of th e vole cycle, and begin breeding at different ages depending on food availability (Brommer, Pietiainen & Kolunen 1998). Using granary size (t he number of storage holes avai lable for acorn storage) as a proxy for territory quality, Koenig & Mu mme (1987) found no relationship between territory quality and breeder survival. Ho wever, we found a positive influence of the number of storage holes on the survival of breeders of both sexes, and male helper survival. Larger granaries allow groups to ta ke advantage of larger acorn crops, leading to increased food availability in the wi nter (Koenig & Mumme 1987). Interestingly, however, territory quality had no direct effect on the likelihood of a juvenile or helper

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21 becoming a breeder. The dispersal decisions of western bluebirds are influenced by site quality (Kraaijeveld & Di ckinson 2001; Dickinson & McGowan 2005), but we found no direct relationship between te rritory quality and dispersal. However, because territory quality increases survival, it could indirec tly increase the probability of breeding for acorn woodpeckers because longer lived birds have more opportunities to breed. Longlived species place greater value on future reproduction (Stearns 1992), and this could be one possible explanation for the high surv ival characteristic of cooperative breeding species (Ekman et al. 2004). Koenig & Mumme (1987) found that surviv al of male breeders increased with group size, number of breeder s, and number of helper s, but found no relationship between the survival of female breeders a nd any group size or composition measurement. We found that group size and composition had a strong positive effect on survival of both sexes, with both the number of breeders and helpers positively infl uencing survival of breeder males, and number of helpers positivel y influencing survival of breeder females. Such increased survival in larger gro ups offers support for the group augmentation hypothesis, which states that if larger groups confer increased survival, it benefits all group members (especially breeder s) to recruit and maintain larger groups (Brown 1994; Kokko et al. 2001). The presence of helpers in a group can increase the survival of breeders due to the reduction in breeder workload (Khan & Walters 2002; Heinsohn 2004), in addition to simply increasing group size and thus diluting the effects of predation. The realized population growth ( ) fluctuated annually in a manner similar to the survival probability of adult birds. Alt hough the breeding probabilities had a significant

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22 influence on their influence on was generally less than thos e of survival probabilities, consistent with the observation that growth rates of fairly long-lived birds were more sensitive to survival parameters than repr oductive parameters (Sta hl & Oli in press). Survival of breeder males, which had the st rongest impact on the population growth rate, was higher in years of larger acorn crops. This reinforces the importan ce of the role of the acorn crop for group stability a nd individual survival (Hannon et al. 1987). For delayed dispersal to be maintained, be nefits of delayed dispersal must balance or exceed associated costs (Ekman et al. 2004). This study suggests that acorn woodpeckers that delay dispersal and help have a lower apparent surv ival than breeders. Even if we consider that death and disappear ance are not separable outcomes, and assume that survival is equal (Walte rs 1990), helpers are unlikely to be fully compensated, at least directly, for helping (Dickinson & Hatchwell 2004). The delayed start of reproduction is likely to reduce direct f itness (McGraw & Caswell 1996; Oli, Hepp & Kennamer 2002; Oli & Armitage 2003), but so me other component of fitness may be compensating individuals who follow this strate gy. Helpers are certai nly gaining indirect fitness by assisting related breeder s such that inclusive fitness of helpers is comparable to those that disperse and breed (Hamilton 1964) MacColl & Hatchwell (2004), using Olis (2003) methodology for estimating inclusive fitne ss, showed that for long-tailed tits with zero lifetime reproductive success (LRS), a non-zero fitness was possible through helping. However, inclusive fitness conse quences of cooperative breeding still remain poorly understood. Earlier work by Koenig & Mumme (1987) found that males delaying dispersal and helping may have a significan tly longer reproductive lifespan and a higher LRS than those who dispersed in their first year to begin breeding. Females that delay

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23 dispersal and help, however, had a shorter reproductive lifespan and lower LRS than first-year dispersers, with only the latter being signif icant (Koenig & Mumme 1987). A thorough examination of the inclusive fitne ss consequences of delayed dispersal and helping behavior is clearly desirable.

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24Table 2-1. Quasi-likelihood adjusted AIC differences ( QAICc) for five models of the survival, recapture, and transition probability in relation to sex and status fo r acorn woodpeckers (1972-2004). Number Model QAICc QAICc Weight Number of Parameters 1 SJ (.) SH (s+t) SB (s+t) pJ (.) pH = pB (.) JB (s+t) HB (s+t) BH (.) 0 0.407 129 2 SJ (.) SH (s+t) SB (s+t) pJ (.) pH (.) pB (.) JB (s+t) HB (s+t) BH (.) 0.25 0.360 130 3 SJ (.) SH (s+t) SB (t) pJ (.) pH = pB (.) JB (s+t) HB (s+t) BH (.) 1.13 0.231 128 4 SJ (.) SH (t) SB (s+t) pJ (.) pH (.) = pB (.) JB (s+t) HB (s+t) BH (.) 11.19 0.002 128 5 SJ (.) SH (s+t) SB (s+t) pJ (.) pH (.) pB (.) JB (s+t) HB (s*t) BH (.) 36.44 0.000 155 The symbols used in this table are defined as follows: S = survival; p = recapture rate; = transition rate; J = juvenile; H = helper; B = breeder; JB = juvenile to breeder trans ition (the effect was the same for juvenile to helper); HB = helper to breeder transition (the effect was the same for remaining a helper); BH = breeder to helper transition (the effect was the same for remaining a breeder); (t) = time effect, no sex effect; (s+t) = additive effect of sex and time; (s*t) = interactive effect of sex and time; (.) = constant parameter, no effect of sex or time. Parameters that had fixed values, and were thus constant (.) by definition, were as follows: SJ (1), pJ (0), JJ (0), HJ (0), and BJ (0). The complementary parameter for each transiti on probability experienced the same effect (i.e. HB and HH both experienced sex and time effects).

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25 Table 2-2. Estimates of a pparent survival rates (S) for male, female, breeder and helper acorn woodpeckers (1972-2004) based on Mo del 1 (Table 2-1). Mean values (95% CI) are given. Male Female Overall Helper 0.695 (0.452, 0.849) 0.577 (0.327, 0.771) 0.636 (0.389, 0.810) Breeder 0.756 (0.587, 0.862) 0.714 (0.531, 0.835) 0.734 (0.559, 0.849) Overall 0.725 (0.519, 0.855) 0.644 (0.427, 0.802) Table 2-3. Transition probabilities ( ) between juvenile, helper and breeder stages for acorn woodpeckers (1972-2004) in relati on to sex based on Model 1 (Table 21). Mean values (95% CI) are given, with M = male, F = female. To: From: Breeder Helper Juvenile M F 0.212 (0.067, 0.515) 0.130 (0.037, 0.392) 0.787 (0.485, 0.932) 0.870 (0.608, 0.963) Helper M F 0.365 (0.158, 0.617) 0.344 (0.143, 0.601) 0.635 (0.383, 0.842) 0.656 (0.399, 0.857) Breeder M F 0.995 (0.987, 0.998) 0.995 (0.987, 0.998) 0.005 (0.002, 0.013) 0.005 (0.002, 0.013) Table 2-4. Influence of social and environmental factors on apparent survival rate of acorn woodpeckers (1972-2004). Regression coefficients ( ) are given with 95% CI (significant effects are shown in bold). SHM = survival of helper males; SHF = survival of helper females; SBM = survival of breeder males; SBF = survival of breeder females. Covariate SHM SHF SBM SBF Group size 0.094 (0.010, 0.177) -0.032 (-0.125, 0.061) 0.205 (0.104, 0.306) 0.146 (0.026, 0.267) Breeders 0.069 (-0.030, 0.169) -0.055 (-0.161, 0.052) 0.080 (0.002, 0.158) 0.040 (-0.052, 0.132) Helpers 0.088 (0.014, 0.162) 0.010 (-0.070, 0.091) 0.210 (0.099, 0.322) 0.144 (0.022, 0.267) Breeder males 0.057 (-0.040, 0.154) -0.041 (-0.144, 0.062) 0.058 (-0.012, 0.127) 0.074 (-0.021, 0.168) Breeder females 0.085 (-0.014, 0.184) -0.056 (-0.159, 0.046) 0.081 (-0.004, 0.166) -0.016 (-0.099, 0.066) Helper males 0.076 (0.009, 0.144) 0.016 (-0.068, 0.100) 0.173 0.065, 0.282) 0.124 (0.004, 0.244) Helper females 0.064 (-0.011, 0.139) -0.005 (-0.076, 0.067) 0.144 (0.046, 0.243) 0.088 (-0.022, 0.198) Acorn crop 0.376 (-0.079, 0.831) 0.528 (0.047, 1.010) 0.420 (0.013, 0.826) 1.04 (0.580, 1.498) Storage facilities 0.191 (0.090, 0.291) 0.062 (-0.041, 0.165) 0.135 (0.039, 0.231) 0.127 (0.008, 0.247)

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26 Table 2-5. Influence of soci al and environmental factors on transition probabilities of acorn woodpeckers (1972-2004). Regression coefficients ( ) are given with 95% CI (significant effects are shown in bold). Symbols used are as follows: JB M = transition probability from juvenile to breeder (male); JB F = transition probability from juvenile to breeder (female); HB M = transition probability from juvenile to breeder (male); HB F = transition probability from juvenile to breeder (female). The -values and 95% CI for JH and HH were of equal magnitude but opposite sign of JB and HB, respectively, for each sex. Covariate JB M JB F HB M HB F Group size -0.008 (-0.260, 0.144) -0.052 (-0.263, 0.159) -0.079 (-0.176, 0.019) -0.166 (-0.318, -0.015) Breeders -0.026 (-0.166, 0.114) -0.354 (-0.635, -0.074) -0.063 (-0.178, 0.051) -0.101 (-0.263, 0.060) Helpers -0.037 (-0.182, 0.108) 0.097 (-0.061, 0.256) -0.068 (-0.155, 0.020) -0.174 (-0.312, -0.035) Breeder males 0.019 (-0.102, 0.140) -0.293 (-0.553, -0.033) -0.027 (-0.139, 0.084) -0.069 (-0.225, 0.087) Breeder females -0.100 (-0.258, 0.059) -0.317 (-0.590, -0.044) -0.101 (-0.217, 0.014) -0.123 (-0.279, 0.033) Helper males -0.033 (-0.163, 0.097) 0.077 (-0.077, 0.231) -0.019 (-0.097, 0.060) -0.174 (-0.315, -0.032) Helper females -0.029 (-0.179, 0.121) 0.074 (-0.078, 0.226) -0.111 (-0.202, -0.020) -0.118 (-0.237, 0.002) Acorn crop 0.637 (-0.205, 1.478) 1.514 (0.439, 2.588) 1.009 (0.359, 1.659) 1.547 (0.699, 2.395) Storage facilities 0.031 (-0.091, 0.152) -0.205 (-0.454, 0.043) 0.008 (-0.096, 0.112) -0.163 (-0.333, 0.008) Table 2-6. Quasi-likelihood adjusted AIC differences ( QAICc) for five models of the realized population growth of acorn woodpeckers (1972-2004) using Pradels reverse-time capture-mark-recapture model. Symbols are as follows: = survival, p = recapture, and = realized population growth rate. Sex effect is noted (s), time effect is noted (t), additive effect of sex and time is noted (s + t), and a period (.) indicates the c onstant value of the parameter. Number Model QAICc QAICc Weights Parameters 1 (s + t) p (.) (t) 00.6366 2 (s + t) p (.) (s + t)1.220.3467 3 (t) p (.) (t) 6.670.0265 4 (t) p (.) (s + t) 8.720.0166 5 (s + t) p (.) (s) 104.35036

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27 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 197219761980198419881992199620002004 0.5 1.0 1.5 2.0 HFSHMSBMSBFSYEAR Figure 1: Annual variation in survival of acorn woodp eckers (1972-2004). Mean rates (black lines) are shown w ith 95% confidence interval (gray shade) for male and female helpers (SHM and SHF, respectively) and male and females breeders (SBM and SBF, respectively). Estimates are ba sed on Model 1 (Table 2-1) and are compared to realized population growth rate ( ).

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28 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 197219761980198419881992199620002004 0.0 0.5 1.0 1.5 2.0 JB MJB FHB MHB FYEAR Figure 2: Annual variation in transition probabilities of acorn woodpecker (1972-2004). Mean rates (black line) are shown with 95% confidence interval (gray shade) for the transition from juvenile to breeder stages for males (JB M ) and females (JB F ), and from helper to breeder stages for males (HB M ) and females (HB F ). Estimates are based on Model 1 (Table 2-1) and are compared to realized population growth rate ( ).

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29 CHAPTER 3 FITNESS CONSEQUENCES OF DELAYED REPRODUCTION IN A COOPERATIVE BREEDER: DOES HELPING HELP? Abstract Cooperative breeding in birds occurs when more than two individuals provide care for a single nest. In many species, the additi onal adults present are offspring from a previous year who have delayed dispersa l, and provision young who are non-descendant kin. Delaying dispersal and age of first br eeding can potentially influence fitness, but fitness consequences of helping behavior are poorly understood. Using long-term data (1972 2004), we examined the fitness cons equences of helping behavior in acorn woodpeckers Melanerpes formicivorus. Although helpers began reproduction significantly later and lived si gnificantly longer than breeders, there was no significant difference in lifetime reproductive success or individual fitness between the two groups. However, birds that successfully bred at ag e 1 without helping had a significantly higher fitness than those who helped a nd successfully bred at age 2 or older. Our results suggest that delayed dispersal and re production in the acorn woodpecker leads to a loss of fitness when conditions are favorable for successful re production. However, if constraints in the environment prevent an individual from breeding at age 1, helping is a viable option until reproduction is possible.

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30 Introduction Cooperative breeding in birds occurs when more than two individuals provide care for a single nest (Brown 1987). Frequently helpers are offspri ng from a previous nest who have delayed dispersal, and provide care for their siblings from a later clutch (Cockburn 1998). Previous studies of the possibl e costs and benefits of helping indicate that fitness benefits of help ing are often lower than loss of fitness from not breeding independently (Reyer 1984; Woolfenden & Fitzpatrick 1984; Komd eur & Edelaar 2001; MacColl & Hatchwell 2002; Dickinson & Hatchwell 2004). One likely reason for the reduced fitness is a delayed onset of repr oduction, which can profoundly influence fitness (Cole 1954; Lewontin 1965; McGraw & Ca swell 1996). Early re production generally allows an individual to begin propagating its genes sooner than delayed reproduction (e.g., Oli et al. 2002; Oli & Armitage 2003), but doing so can have detrimental effects if survival, growth or reproductive potential is compromised (e.g., Pyle et al. 1997). If delayed age of first reproduction redu ced fitness, why would an individual delay reproduction and help? One explanation is that constraints in the environment limit individuals ability to successfully breed immediately upon reaching sexual maturity; helping is a strategy that enables some birds to make the best of a bad situation (Koenig & Pitelka 1981; Emlen 1982; Koenig et al. 1992). In many coopera tively breeding birds, a critical resource (such as su itable habitat, a food related resource or a breeding cavity) is often necessary for successful breeding, and the absence of this resource can lead to a shortage of breeding positions (Woolfe nden & Fitzpatrick 1984; Koenig & Mumme 1987; Walters et al. 1992). The acorn woodpecker Melanerpes formicivorus is a cavity nesting species and a common inhabitant of the oak woodlands of California (Koenig & Mumme 1987; Koenig

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31 et al. 1995). Practicing opportuni stic polygynandry in addition to helping behavior, the cooperatively breeding acorn woodpecker exhi bits one of the most complex social systems of any vertebrate, with group size ra nging from a single pair to a mixed group of up to 15 breeders and helpers. Territorial i nheritance is rare, yet many individuals delay dispersal for one year or more and help on their natal territory (K oenig & Mumme 1987). Using long-term (1972-2004) life-histor y data, we investigated the fitness consequences of helping be havior in female acorn woodpeckers. Specifically, we addressed the following questions: (1) Do es helping (and delayed age of first reproduction) reduce fitness? Specifically, do helpers have lower fitness compared to birds that disperse and breed without helping? (2) What social and environmental factors influence fitness? (3) If helpers have lo wer fitness than breeders, why does helping persist? Methods Study Species and Study Site The acorn woodpecker is a cooperatively breeding, cavity nesting species that ranges throughout the oak woodlands of Ca lifornia and possesses one of the most complex social systems of any vertebra te (Koenig & Mumme 1987). The reproductive system of this species is opportunistic polygynandry, a mix of mate-sharing (multiple males compete for mating opportunities with one or more females) and joint-nesting (multiple females breed with one or more males and lay their eggs in a single nest). Group size and composition range from a monog amous pair to a 15-member mixed group of breeders and non-breeding helpers of both sexes (Koenig & Mumme 1987). Both breeders and helpers contribute to territory defense, food storage, and care of young (Mumme et al. 1990).

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32 In parts of their range, acorn woodpeckers c onstruct granaries, or storage trees, in which they store acorn mast, food that supplem ents their diet of sap and flying insects (MacRoberts & MacRoberts 1976; Koenig & Mumme 1987). These granaries are a critical resource, allowing te rritories to be maintained year-round and may serve as an alternative to winter fattening wh en food is more scarce (Koenig et al. 2005). Groups with acorn stores remaining in spring e xperience earlier nests and higher reproductive success than those without stored acorn ma st (Koenig & Mumme 1987; Koenig & Stacey 1990). This study took place at Hastings Natural History Reservation in central coastal California. Plant communities at Hastings include foothill woodland, savanna-grassland, and riparian woodland. Common tr ees in the study area include five species of oak: coast live oak (Quercus agrifolia), canyon live oak (Q. chrysolepis), blue oak (Q. douglasii), black oak (Q. kelloggii), and valley oak (Q. lobata), along with California sycamores (Platanus racemosa), willows (Salix spp.), California buckeye (Aesculus californica), and madrone (Arbutus menziesii). Acorn woodpeckers are ma inly found in open areas, nesting in oaks and sycamores, with a dens e ground cover of gra sses, and few shrubs (Koenig & Mumme 1987). Field Methods Intensive research on acorn woodpeckers at Hastings Reservation began in 1971 (MacRoberts & MacRoberts 1976) and continues to da te (Koenig & Mumme 1987; Koenig et al. 2000; Haydock & Koenig 2003); data collected during 1972-2004 were used in this study. The major ity of natural cavity nests (loc ated 2 20m up in trees) were located and checked, and groups were censuse d to determine membership. Birds were individually marked with leg bands as nestli ngs 20-25 days after hatching or as soon as

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33 possible if immigrating as adults. Group si ze and composition (number of breeders and helpers) were determined by observation and the known relationship of group members based on banding record (Koenig et al. 1998; Koenig et al. 1999). Further details on field methodology can be found in Koenig & Mumme (1987). Each fall starting in 1980, the acorn crop of the five Quercus species was estimated. For approximately 250 oak trees, the number of acorns counted in 15 s by each of two observers was recorded. Each count value was log-transformed and the mean value was calculated and used as an index of annual acorn crop (hereafter, acorn crop or AC) (Koenig et al. 1994a; Koenig et al. 1994b). Estimation of Fitness Although there is a signifi cant amount of reproductive skew among breeding male acorn woodpeckers (Haydock & Koenig 2003), th e egg destruction behavior of jointnesting females enforces hatching synchrony an d leads to equal pare ntage of fledglings by breeding females (Mumme, Koenig & Pitelka 1983; Haydock et al. 2001). Therefore, to estimate the number of young fledged per female, we divided the number of young fledged for a group by the number of breeding females in the group. Survivorship was determined by repeated visits to each gr oup, and by monitoring the territory until all group members were seen (Koenig & Mumme 19 87). For the purposes of this analysis, because we only focus on individuals who succe ssfully reproduced at least once in their lives, birds disappearing were treated as dead. We used two measures of individual fitness. First, we calculated the lifetime reproductive success (LRS) as the total number of offspring produced by a female during her lifetime (Clutton-Brock 1988; Newton 1989). A second measure of individual fitness ( ) was estimated using the methods of McGr aw & Caswell (1996). Th is fitness measure

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34 represents the rate at which an individuals genes are propagated if the vital rates remain the same (McGraw & Caswell 1996). A populatio n projection matrix for each individual was constructed, with age-specific annual fert ility rates (estimated as one half times the number of offspring produced by a female pe r year) on the first row of the matrix, and survival probability of 1 along the lower s ub-diagonal until age of last reproduction. The dominant eigenvalue of the matrix is the estimate of individual fitness ( ). All fitness calculations were done using MATLAB. Statistical Analysis We used a t-test to compare mean fitness of individuals that helped as yearlings to those that did not. Because some individuals may attain a breeding position at age 1 but fail to breed, we also used t-tests to compar e the fitness of helpers to individuals who dispersed and bred successfully at age 1, a nd to those who dispersed at age 1 but did not successfully reproduce until age 2 or older. Effect of Social and Environmental Factors on Fitness We investigated the influence of soci al and environmental factors on both LRS and Social factors included group size (the number of adults in the group), the number of male, female and total helpers, and the number of male, female and total breeders. Environmental factors included the acorn crop of the previous fall (potentially still present in the spring), the acorn crop of th e current fall, and the number of storage holes in the groups granary (an estimate of territory quality). We collected data for each social and environmental variable in three ways: th e year the individual was born, the year the individual began breeding and the average over the individuals lifespan (Koenig & Mumme 1987).

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35 We used step-wise variable selection pr ocedure in multiple linear regression to identify variables signifi cantly influencing LRS or (System) 2002).Then, using only those variables selected by the stepwise variab le selection procedure, we fitted a multiple linear regression with LRS (or ) as response variable and th e aforementioned social and environmental variables as predictors. Results We had lifetime survival and reproductive data for 498 females that were banded as juveniles and were resighted at least once during or afte r their first potential breeding season. We excluded individuals who were stil l alive (last seen in 2004) and those that never successfully reproduced within the study area. Therefor e, all analyses were based on 141 females (61 potential breed ers and 80 potential helpers). On average, female acorn woodpecker s lived 4.35 0.25yrs, began breeding between at age 2.39 0.11, reproduced 2.55 0.19 times, and had a mean postmaturation survival (numbers of years surv ived beyond the age of first reproduction) of 1.96 0.22yrs (Table 3-1). The mean LRS wa s 6.90 0.65 and mean individual fitness ( ) was 1.27 0.04. The longest lived indivi dual (female 787) in our study lived 16yr, and had the largest LRS (37. 75), although not the highest (1.54, compared to the maximum of 2.62). This individual did not he lp in its first year, and first successfully reproduced at age 1. Lifetime reproductive success and fitness were strongly correlated with each other, and both fitness measures were str ongly correlated with lifespan, post-maturation survival, lifetime number of reproductive events and mean number of young fledged (Table 3-2). Neither measure of fitness was significantly related to age at first reproduction ( ). Only two fitness components showed a significant and positive

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36 relationship with : lifespan and mean young fledged. Li fespan and reproductive lifespan were positively correlated with one another, as well as with number of reproductive events, and mean young fledged. Comparing the fitness com ponents and fitness of female s that delayed dispersal and helped for at least one year (helpers; n = 80) to those that dispersed and attempted to breed at age 1 (potential breed ers; n = 61), we found the fo llowing (Table 3-1). Helpers began breeding at around age 3, which was si gnificantly delayed compared to potential breeders. Helpers outlived potential breeder s, living about 1.5 yr longer. There was no significant difference between helpers and potenti al breeders in terms of post-maturation survival, number of reproductive events, mean young fledged per reproductive event, lifetime reproductive success or individual fitness ( ). Because some individuals disperse but fa il to successfully reproduce at age 1, we also compared helpers to those individua ls who successfully reproduced at age 1 (breeders, n = 44). As expected, helper s began reproduction signi ficantly later (3.09 compared to 1.0, p < 0.05) but lived longer th an the breeders (5.08 compared to 3.20, p < 0.05). Birds that began breedi ng at age 1 had a significantly larger individual fitness ( ) than helpers (1.43 compared to 1.21, p < 0. 05). Neither LRS nor any other component of fitness differed significantly between the two groups. Comparing helpers to those potential breeders who dispersed and acquired a breeding position but failed to successfully reproduce until age 2 or older, the only measure of fitness or fitness component that differed significantly between the two groups was lifespan, with helpers living 1.2 yr longer (5.08 compared to 3.88, p < 0.05). Neither measure of fitness, nor any other co mponent of fitness differed. Dispersers who

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37 bred at age 1 had higher fitness than those w ho dispersed at age 1 but failed to breed until age 2 (1.43 and 1.18, respectively, p < 0.05). Of 29 candidate social and environmen tal variables considered, the stepwise variable selection procedure selected five variables as having significant influence on : the acorn crop the year before birth (AC), the granary size on the territory when the individual was born (GBirth), granary si ze the year the individual began breeding (GBreed), the average granary size over the i ndividuals lifetime (GLife), and the number of breeding females in the group the year th e individual began breeding (BF). A multiple linear regression model with these variables in cluded explained 44.3% of the variation in ( = 1.80094 0.25147AC 0.00012GBirth 0.0027GBreed + 0.000518GLife 0.19782BF; R2 = 0.443, p <0.0009). None of the so cial or environmental variables considered significantly influenced LRS. Discussion Two of the most interesti ng characteristics of cooperative breeding systems are offspring retention and helping behavior. Instead of dispersing when they are capable of breeding independently, many individuals stay at home for 1 yr or more and help to raise siblings. Helping is generally considered a r oute to lower fitness, and helpers attempt to, but rarely do, recoup the loss of fitness due to foregoing re production and remaining at home (Dickinson & Hatchwell 2004). The acorn woodpecker follows one of two routes upon reaching sexual maturity. Individuals can delay breeding for one or more years and help raise non-descendant kin, the path of so-called helpers. Alternatively, individuals can disperse and attempt to breed as soon as they are sexually mature at age 1 (potential breeders). In this study, helpers had a delayed age of first reproduction and a longer lifespan but neither measure of

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38 fitness, nor any other compone nt of fitness differed signi ficantly between helpers and potential breeders. However, it is important to note that some individuals disperse and gain a dominant position in a group, but for some reason (e.g. no mate, poor acorn crop, predation) fail to successfully fledge any young at age 1. Thus, we compared fitness and its components of birds who successfully reprod uced at age 1 (breeders) to those who delayed dispersal and helped ; this comparison should better elucidate the fitness consequences of helping and delayed of first reproduction. Females that successfully reproduced at age 1 had higher fitness ( ) compared to those who stayed home and helped. In other words, dispersing and breed ing is advantageous if the individual can successfully reproduce at age 1, otherwise ther e is no difference in fitness for helpers or breeders. There was no difference in LRS between the two groups. However, LRS may not adequately quantify fitness because it on ly considers the amount of reproduction and ignores the timing of reproduction, which can also substantially influence fitness (McGraw & Caswell 1996; Oli et al. 2002). Consequently, inferences based on may be more appropriate (McGraw & Caswell 1996; Oli et al. 2002; Oli & Armitage 2003). One obvious consequence of helping is that reproduction is delayed. Early work proposed that a delayed age of first re production could reduce fitness (Cole 1954; Lewontin 1965). Recent comparative life hist ory studies of birds and mammals found that Coles prediction strictly holds in sp ecies characterized by early maturity and high reproductive rates (Oli & Dobson 2003; Stahl & Oli in press); this conclusion is consistent with findings of several empirical studies that have examined the effect of age at first reproduction on fitness. In the w ood duck, for example, individuals beginning reproduction at an earlier age had a greater i ndividual fitness than those who delayed age

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39 of first reproduction (Oli et al. 2002), indicating that fitness benefits of early reproduction outweigh associated costs (e.g. Reznick 1985). In the goshawk, a species characterized by late age of first reproduction and low fecundity, individuals who delayed maturity had higher fitness than thos e who began breeding ear lier (Krger 2005). Using LRS as a measure of fitness, Koenig & Mumme (1987) and Koenig & Stacey (1990) found no significant difference in fitness between acorn woodpeckers of either sex that helped and those who reproduced without helping. Consistent with findings of Koenig & Mumme (1987) and Koenig & Stacey (1990), we found no significant difference in LRS betw een helpers and breeders. When was used as a measure of fitness, however, we found that de layed reproduction due to helping is costly in acorn woodpeckers, with significantly lower fitness for females that helped for one or more years compared to those who bred as yearlings. If helping is a route to lower fitness, why has it been maintained in the system? We offer three possible explanations, which are not mutually exclusive: (1) ecological conditions prevent some individuals from beginning to breed at sexual maturity, (2) females are more successful at dispersing and attaining a breeding position with a sister, and (3) inclusive fitness benefits. Ecological constraints have been proposed as the cause of cooperative breeding, especially offspring retention (Koenig & Pitelka 1981; Emlen 1982). In cooperative breeding systems, a critical resource is often necessary for a family unit to exist. In the case of acorn woodpeckers, the critical resour ce is the granary (K oenig & Mumme 1987). Without a tree in which to store acorns, a family unit may not persist and therefore ecological constraints may influence delayed di spersal, helping behavior and an average

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40 age of first reproduction over a year past the point of sexual maturity. Acorn woodpeckers invest a consider able amount of time construc ting and maintaining their granary much in the same way red-cockaded woodpeckers Picoides borealis must invest considerable effort creating cavities in liv e trees (leading to delayed dispersal and reproduction in a number of males Walters et al. 1992). In red-cockaded woodpeckers, males typically begin reproduction between 2 3yrs of age (for comparison, as female helpers are rare Walters 1990). Another coopera tive breeder that has been intensively studied, the Florida scrub-jay Aphelocoma coerulescens generally breeds for the first time at age 2, with only 3 females breeding at the age of sexual maturity (1yr) in 18 years of study (Woolfenden & Fitzpatrick 1990). Indivi duals remaining at home must wait for a breeding vacancy to become available, and it can be advantageous to disperse with siblings and compete as a unit for vacancies (Hannon et al. 1985). Up to 30 helpers from surrounding territori es vigorously compete for reproductive vacancies; the winners of such power str uggles are often larger groups consisting of same-sex sibling units (Koenig 1981; Hannon et al. 1985). Therefore, sisters (or brothers) dispersing together were more likely to attain a breeding position when one became available, and remaining at home past the poi nt of sexual maturity may be advantageous if it increases a females chances of successf ully dispersing with and competing alongside a sister for a breeding vacancy. Joint dispersa l leads to approximately one half of all joint-nesting occasions (Koenig & Mumm e 1987; Mumme, Koenig & Pitelka 1988). Because helpers in this system were nearly always related to at least one of the breeders (most often both breeders are the help ers parents) and th erefore the young they provision, helpers are most certainly accrui ng indirect fitness benefits (Koenig &

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41 Mumme 1987). Helper presence has been shown to increase reproductive success and survival of group members (Koenig & Mu mme 1987; Stahl, Koenig & Oli unpublished manuscript). Therefore the inclusive fitness tota ls of helpers are most certainly likely to be higher than our direct fitne ss estimates, and may lead to helping being advantageous in some or all situations. Stacey & Ligon (1987; 1991) showed that acorn woodpecker helpers remaining on high quality territories had higher LRS a nd survivorship. Both the acorn crop and territory quality positively influence surviv al, probability of breeding and reproductive success in acorn woodpeckers (Koenig & Mumme 1987; Stahl et al. unpublished manuscript), and this study has shown direct effects on individual fitness were also apparent. Using granary size (num ber of storage holes) as a pr oxy for territory quality, we found that birds breeding on higher quality territories over thei r lifetime had higher fitness. The most important social effect on fitness was that joint-nesting females had lower than females nesting alone. This result is consistent with that of Mumme et al. (1988) who found that the lifetime reproductive success of joint-nesting females was less than or equal to that of singly nesting indivi duals, despite increased survival and territory dominance resulting from co-breeding. Three caveats must be mentioned. First, the results presented in this manuscript are only for females, and may not apply to males. Males are the more common helping sex and co-breed more often than female s joint-nest (Koeni g & Mumme 1987). Unlike females, parentage in males of ten significant skewed (Haydock et al. 2001; Haydock & Koenig 2003). Second, breeders typically occupy a territory continuously once they begin breeding, and are therefore more likely to remain in the st udy area than helpers (Haydock

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42 & Koenig unpublished manuscript; Stahl et al. unpublished manuscript). Because of this we only have fitness estimates for those help ers who remained in the study area, and the fitness of those leaving the study area to fill breeding vacancies could possibly be different than those remaini ng in the study site (Koenig et al. 2000). Third, we have only considered the direct fitness of individuals and our estimate s must therefore be seen as conservative. Indirect fitness benefits coul d possibly make up for the cost of delayed dispersal. Future work should employ th e methodology of Oli (2003) to accurately quantify the inclusive fitness th at Hamilton (1964) suggested. In conclusion, helping in the acorn woodp ecker is a route to lower fitness, but only if dispersers successfully reproduce at age 1. If the pros pects of successful reproduction look grim, staying at home can be worthwhile. Females breeding on territories with larger granaries had higher fitness. The only obvious social factor that affected fitness was that joint-nesting females suffered reduced fitness. At least in the acorn woodpecker, helping is maintained due to constraints placed on individuals in terms of limited reproductive vacancies. Help ers are possibly attempting to make due by accruing inclusive fitness benefits and/or wa iting to disperse with siblings, which increases the likelihood of atta ining a breeding position (Hannon et al. 1985).

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43Table 3-1. Fitness components and measures of fitness for female acorn woodpeckers (1972-2004) that were banded as juveniles, w ere recorded at least once after their first possible breeding season, and reproduced at least once. Means (SE) and ranges are shown. A t-test was used to compare helpers (N = 80) and potential breeders (N = 61), helpers and breeders that first reproduced at age 1 ( =1; N = 44), and helpers and breeders that first reproduced at age 2 or older ( 2; N = 17); P < 0.05 indicates statis tical significance. Helpers vs. Potential Breeders Helpers vs. Breeders Fitness Measure/Component All females Helper Potential Breeders P = 1 P 2 P Age at first reproduction ( ) 2.39 (0.11) 3.09 (0.14) 1.48 (0.11) <0.05 1.00 (0) <0.05 2.70 (0.16) 0.08 Lifespan 4.35 (0.25) 5.08 (0.32) 3.39 (0.36) <0.05 3.20 (0.47) <0.05 3.88 (0.43) <0.05 Post-maturation survival 1.96 (0.22) 1.99 (0.28) 1.92 (0.36) 0.88 2.20 (0.47) 0.67 1.18 (0.43) 0.21 Reproductive events 2.55 (0.19) 2.58 (0.24) 2.52 (0.30) 0.89 2.77 (0.39) 0.65 1.88 (0.36) 0.21 Lifetime reproductive success (LRS) 6.90 (0.65) 7.21 (0.84) 6.50 (1.01) 0.59 7.21 (1.31) 1.00 4.66 (1.24) 0.19 Individual fitness ( ) 1.27 (0.04) 1.21 (0.03) 1.36 (0.07) 0.06 1.43 (0.10) <0.05 1.18 (0.05) 0.72 Mean young fledged per event 2.42 (0.09) 2.55 (0.13) 2.26 (0.12) 0.10 2.23 (0.15) 0.13 2.31 (0.18) 0.41

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44Table 3-2. Correlation between fitness components and measures of fitness for female acorn w oodpeckers (1972-2004) that were banded as juveniles, were recorded at least once after their first possible breeding season, and reproduced at least once. The two measures of fitness were lifetime repr oductive success (LRS) a nd individual fitness ( ). Age at first reproduction is represented as The Pearson correlation coefficient is given above, and the p-value belo w (p < 0.05 indicates a significant relationship between the two variables, N = 141). LRS LifespanPost-maturation su rvival Reproductive events 0.618<.001 0.058-0.1460.4940.083 Lifespan 0.8370.390 0.441 <.001<.001 <.001 Post-maturation survival 0.9030.509 -0.0150.891 <.001<.001 0.856 <.001 Number of reproductive events 0.9210.531 -0.0370.857 0.974 <.001<.001 0.665 <.001 <.001 Mean young fledged per event 0.5660.674 0.325 0.423 0.306 0.305 <.001<.001 <.001 <.001 <0.001 <0.001

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45 CHAPTER 4 CONCLUSIONS Group living can affect the f itness and population dynamics of social animals. In cooperative breeding birds, some individuals delay reproduction and assist in raising young that are not their own dir ect descendants, whereas othe rs disperse and attempt to breed independently when they attain sexua l maturity (Cockburn 1998). Direct benefits of delayed dispersal and/or helping behavior can include increased survival (e.g., Ekman et al. 2000) and territory acquisition via budding (Woolfenden & Fitzpatrick 1984; Komdeur & Edelaar 2001). An individual can ga in indirect fitness by helping aids kin (Hamilton 1964). However, because delayed repr oduction is a characteristic of helping, this behavior can also have fitness costs. Despite decades of research, the fitness and population dynamic consequences of coope rative breeding are poorly understood. In this thesis I investigated both the fitness and demographic consequences of cooperative breeding and the stra tegy of delayed dispersal and reproduction for helpers in the cooperatively breeding acorn woodpecker Applying recently developed techniques for estimating survival and f itness to a long-term data set, my results were based on robust calculations. In chapter 2, I investigated th e demographic and population dynamic consequences of helping beha vior and cooperative breeding and found th at survival and breeding probabilities of acorn woodpeckers di ffered depending on sex, status and year. The apparent survival (the pr obability of surviving and rema ining in the study area) of helpers was lower than breeders. However, this difference was likely biased towards

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46 breeders as they are more likely to rema in in the study area; many helpers that disappeared were filling reproductive vacan cies outside the study area. Males (both helpers and breeders) survived at a greater rate than females. Juvenile males, when compared to females, were nearly twice as likely to become breeders immediately after their first year of life. Fo r both sexes, though, the most common strategy was to delay dispersal and help for at least one year. A number of social and environmenta l factors influenced the survival and breeding probabilities. Acorn woodpeckers were more likely to survive and also had a higher probability of becoming a breeder in years when the acorn crop was high. Larger granaries (i.e., better qualit y territories) also positivel y influenced survival, but surprisingly had no effect on breeding proba bility. Group size and composition both increased survival of group members. The realized population growth rate ( ) varied annually and was most influenced by su rvival, followed by breeding probabilities. Because the acorn crop positively influences survival and probability of attaining a breeding position, the changes in realized grow th rate reflect the variation in the annual acorn crop. In chapter 3, I examined the fitness consequences of helping behavior. Although individual fitness of helpers di d not differ from that of poten tial breeders (i ndividuals that dispersed as yearlings without helping), female acorn woodpeckers that successfully reproduced at age 1 had a significa ntly higher individual fitness ( ) than those that helped and delayed age of first breeding until age 2 or later. This difference was due to a delayed onset of reproduction for helpers. Furthe rmore, yearling dispersers who did not

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47 successfully reproduce until age 2 or later did not have a signif icantly greater fitness than helpers (who also did not begin breeding until age 2 or later). Four factors signi ficantly influenced : the acorn crop prior to birth, two measures of territory quality (lifetime average, and year of first reproductive event), and the number of co-breeding females in the gr oup when the individual began breeding. Females living on higher quality territories showed higher f itness, but individuals who shared reproduction with other females suffered a fitness loss. In conclusion, my study has shown that helping in acorn woodpeckers generally is a route to lower fitness if dispersers successfully reproduc e as yearlings. However, if successful reproduction is not possi ble staying at home is the ne xt best strategy. In years with large acorn crops, more birds are able to acquire breeding positions within a group and fitness is increased on territories with larg er granaries in which to store these acorns. When the acorn crop fails, the likelihood of breeding is significantly decreased and staying at home is suitable in the face of such constraints. These inferences are made based only on direct fitness, and the resu lts may differ if inclusive fitness were considered. Therefore, inclusiv e fitness calculations should be a main target for future research in this and other cooperative breeding systems.

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48 LIST OF REFERENCES Arnold, K.E., & Owens, I.P.F. (1998) Coope rative breeding in bi rds: a comparative analysis of the life history hypothesis. Proceedings of the Royal Society of London Series B, 265, 739-745. Boland, C.R.J., Heinsohn, R., & Cockburn, A. (1997) Experimental manipulation of brood reduction and parental care in cooperatively breeding white-winged choughs. Journal of Animal Ecology, 66, 683-691. Brommer, J.E., Pietiainen, H., & Kolunen, H. (1998) The effect of age at first breeding on Ural owl lifetime reproductive suc cess and fitness under cyclic food conditions. 67, 359-369. Brown, J.L. (1987) Helping and communal breeding in birds. Princeton University Press, Princeton, New Jersey. Brown, J.L. (1994) Historic al patterns in the study of avian social-behavior. Condor, 96, 232-243. Burnham, K.P., & Anderson, D.R. (2002) Model selection and multimodel inference. Springer-Verlag New York, Inc., New York. Choquet, R., Reboulet, A.M., Pradel, R., Gimen ez, O., & Lebreton, J.D. (2003) User's manual for U-CARE. Mimeographed document, CEFE/CNRS, Montepelier (ftp://ftp.cefe.cnrs-mop.fr/biom/Soft-CR/) last accessed September 2005. Clutton-Brock, T.H., ed. (1988) Reproductive success: Studies of individual variation in contrasting breeding systems. University of Chicago Press, Chicago. Cockburn, A. (1998) Evolution of helping be havior in cooperatively breeding birds. Annual Review of Ecology and Systematics, 29, 141-177. Cole, L. (1954) The population conse quences of life-history phenomena. Quarterly Review of Biology, 29, 103-137. Cooch, E., & White, G.C. (2005) Program MARK: A gentle introduction (http://www.cnr.colostate.e du/~gwhite/mark/mark.htm) last accessed December 2005.

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51 Koenig, W.D., Knops, J.M.H., Carmen, W.J., Stanback, M.T., & Mumme, R.L. (1994a) Estimating acorn crops using visual surveys. Canadian Journal of Forest Research, 24, 2105-2112. Koenig, W.D., & Mumme, R.L. (1987) Population ecology of the cooperatively breeding acorn woodpecker. Princeton University Press, Princeton. Koenig, W.D., Mumme, R.L., Carmen, W.J., & Stanback, M.T. (1994b) Acorn production by oaks in central coastal Ca lifornia: variation within and among years. Ecology, 75, 99-109. Koenig, W.D., & Pitelka, F.A. (1981). Ecol ogical factors and kin selection in the evolution of cooperative breeding in birds. In Natural selection and social behavior: Recent research and new theory (eds R.D. Alexander & D.W. Tinkle), pp. 261-280. Chiron, New York. Koenig, W.D., Pitelka, F.A., Carmen, W.J ., Mumme, R.L., & Stanback, M.T. (1992) The evolution of delayed dispersa l in cooperative breeders. Quarterly Review of Biology, 67, 111-150. Koenig, W.D., & Stacey, P.B. (1990). Acor n woodpecker: group-living and food storage under contrasting ecolog ical conditions. In Cooperative breeding in birds: Longterm studies in ecology and behavior (eds P.B. Stacey & W.D. Koenig), pp. 413453. Cambridge University Press, Cambridge. Koenig, W.D., Stacey, P.B., Stanback, M.T ., & Mumme, R.L. (1995). Acorn woodpecker (Melanerpes formicivorus). In The birds of North America, No. 194. (eds A. Poole & F. Gill), The Academy of Natural Sciences, Philadelphia, PA and American Ornithologists' Uni on, Washington, DC. Koenig, W.D., Stanback, M.T., & Haydock, J. (1999) Demographic consequences of incest avoidance in the cooperati vely breeding acorn woodpecker. Animal Behaviour, 57, 1287-1293. Koenig, W.D., Van Vuren, D., & Hooge, P.N. (1996) Detectability, philopatry, and the distribution of dispersal di stances in vertebrates. Trends in Ecology and Evolution, 11, 514-517. Koenig, W.D., Walters, E.L., Walters, J.R., Kellam, J.S., Michalek, K.G., & Schrader, M.S. (2005) Seasonal body weight variati on in five species of woodpeckers. Condor, 107, 810-822. Kokko, H., Johnstone, R.A., & Clutton-Brock, T.H. (2001) The evolution of cooperative breeding through group augmentation. Proceedings of the Royal Society of London Series B, 268, 187-196.

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52 Komdeur, J., & Edelaar, P. (2001) Male Se ychelles warblers use territory budding to maximize lifetime fitness in a saturated environment. Behavioral Ecology, 12, 706-715. Kraaijeveld, K., & Dickinson, J.L. (2001) Fam ily-based winter territoriality in western bluebirds: the structure a nd dynamics of winter groups. Animal Behaviour, 61, 109-117. Krger, O. (2005) Age at first breeding and fitness in goshawk Accipiter gentilis. Journal of Animal Ecology, 74, 266-273. Lebreton, J.-D., Burnham, K.P., Clobert, J., & Anderson, D.R. (1992) Modelling survival and testing biological hypot hesis using marked indivi duals: a unified approach with case studies. Ecological Monographs, 62, 67-118. Lewontin, R.C. (1965). Selection for colonizing ability. In The genetics of colonizing species (eds H.G. Baker & G.L. Stebbins), pp. 79-94. Academic Press, New York. MacColl, A.D.C., & Hatchwell, B.J. (2002) Temporal variation in fitness payoffs promotes cooperative breeding in lo ng-tailed tits Aegithalos caudatus. 160, 186194. MacColl, A.D.C., & Hatchwell, B.J. (2004) Determinants of lifetime fitness in a cooperative breeder, the long-tailed tit Aegithalos caudatus. Journal of Animal Ecology, 73, 1137-1148. MacRoberts, M.H., & MacRoberts, B.R. (1976) Social organization a nd behavior of the acorn woodpecker in cent ral coastal California. Ornithological Monographs, 21, 1-115. McGraw, J.B., & Caswell, H. (1996) Estimati on of individual fitness from life-history data. American Naturalist, 147, 47-64. Mumme, R.L., Koenig, W.D., & Pitelka, F.A. (1983) Reproductive competition in the communal acorn woodpecker: sisters destroy other's eggs. Nature, 306, 583-584. Mumme, R.L., Koenig, W.D., & Pitelka, F.A. (1988) Costs and benef its of joint nesting in the acorn woodpecker. American Naturalist, 131, 654-677. Mumme, R.L., Koenig, W.D., & Pitelka, F. A. (1990) Individual contributions to cooperative nest care in the acorn woodpecker. Condor, 92, 360-368. Newton, I., ed. (1989) Lifetime reproduction in birds. Academic Press, London.

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53 Oli, M.K. (2003) Hamilton goes empirical: es timation of inclusive fitness from lifehistory data. Proceedings of the Royal Society of London Series B-Biological Sciences, 270, 307-311. Oli, M.K., & Armitage, K.B. (2003) Sociality and individual fitne ss in yellow-bellied marmots: insights from a long-term study (1962-2001). Oecologia, 136, 543-550. Oli, M.K., & Dobson, F.S. (2003) The relativ e importance of life-hi story variables to population growth rate in mammals: Cole's prediction revisited. American Naturalist, 161, 422-440. Oli, M.K., Hepp, G.R., & Kennamer, R.A. (2002) Fitness consequences of delayed maturity in female wood ducks. Evolutionary Ecology Research, 4, 563-576. Pradel, R. (1996) Utilization of capture-mar k-recapture for the study of recruitment and population growth rate. Biometrics, 52, 703-709. Pradel, R., Wintrebert, C.M.A., & Gimenez, O. (2003) A proposal for a Goodness-of-Fit test to the Arnason-Schwarz multistate capture-recapture model. Biometrics, 59, 43-53. Pyle, P., Nur, N., Sydeman, W.J., & Emslie S.D. (1997) Cost of reproduction and the evolution of deferred bree ding in the western gull. Behavioral Ecology, 8, 140147. Reyer, H.-U. (1984) Investment and relatedness : a cost/benefit anal ysis of breeding and helping in the pied kingfisher (Ceryle rudis). Animal Behaviour, 32, 1163-1178. Reznick, D. (1985) Costs of reproduction: an evaluation of the empirical evidence. Oikos, 44, 257-267. Solomon, N.G., & French, J.A., eds. (1996) Cooperative breeding in mammals. Cambridge Unviersity Press, Cambridge. Stacey, P.B., & Koenig, W.D., eds. (1990) Cooperative breeding in birds: Long-term studies in ecology and behavior. Cambridge University Press, Cambridge. Stacey, P.B., & Ligon, J.D. (1987) Territory quality and dispersal options in the acorn woodpecker, and a challenge to habitat sa turation model of cooperative breeding. American Naturalist, 130, 654-676. Stacey, P.B., & Ligon, J.D. (1991) The be nefits-of-philopatry hypothesis for the evolution of cooperative breeding: varia tion in territory and group size effects. American Naturalist, 137, 831-846.

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54 Stahl, J.T., Koenig, W.D., & Oli, M.K (unpublished manuscript) Demographic consequences of delayed dispersal in the cooperatively breeding acorn woodpecker. University of Florida, Gainesville, FL. Stahl, J.T., & Oli, M.K. (in press) Relative importance of avian lifehistory variables to population growth rate. Ecological Modeling. Stearns, S.C. (1992) The Evolution of Life Histories. Oxford University Press, Oxford. SAS (Statistical Analysis System) (2002). SAS/STAT user's guide, version 9. SAS Institute, Cary, N.C. Walters, J.R. (1990). Red-cockaded woodpeckers: a "primitive" cooperative breeder. In Cooperative breeding in birds: Long-te rm studies of ecology and behavior (eds P.B. Stacey & W.D. Koenig), pp. 67-101. Cambridge University Press, Cambridge. Walters, J.R., Copeyon, C.K., & Carter III, J. H. (1992) Test of the ecological basis of cooperative breeding in re d-cockaded woodpeckers. Auk, 109, 90-97. Weathers, W.W., Koenig, W.D., & Stanb ack, M.T. (1990) Breeding energetics and thermal ecology of the acorn woodpecker in central coastal California. Condor, 92, 341-359. White, G.C., & Burnham, K.P. (1999) Program MARK: survival estimation from population of marked animals. Bird Study, 46, 120-139. Williams, B.K., Nichols, J.D., & Conroy, M.J. (2002) Analysis and management of animal populations. Academic Press, San Diego. Woolfenden, G.E., & Fitzpatrick, J.W. (1984) The Florida scrub jay: Demography of a cooperative-breeding bird. Princeton University Press, Princeton, New Jersey. Woolfenden, G.E., & Fitzpatrick, J.W. (1990) Florida scrub jays: a synopsis after 18 years of study. In Cooperative breeding in birds: Long-term studies of ecology and behavior (eds P.B. Stacey & W.D. Koenig), pp. 241-266. Cambridge University Press, Cambridge.

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55 BIOGRAPHICAL SKETCH Justyn Stahl was born on September 13, 1980, in Alton, Illinois. He is the oldest son of Ralph and Bette Stahl. After gr aduating from Alton High School in 1998, he moved to Miami, Florida, to attend the Un iversity of Miami where he received his bachelors degree in biology in 2002. He then spent two field seasons working on prothonotary warblers in Southern Illinois. He moved to Gainesville, Florida, and the University of Florida in the fall of 2003.


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DEMOGRAPHIC AND FITNESS CONSEQUENCES OF DELAYED DISPERSAL IN
THE COOPERATIVELY BREEDING ACORN WOODPECKER















By

JUSTYN T. STAHL


A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE

UNIVERSITY OF FLORIDA


2006

































Copyright 2006

by

Justyn T. Stahl















ACKNOWLEDGMENTS

I would like to thank my parents for always letting me choose my own path in life,

and always being supportive of the decisions I make.

I am grateful to my advisor, Madan Oli, and the other members of my advisory

committee, Walt Koenig and Ben Bolker, for advice and assistance at all stages of my

master's research. Madan served as an outstanding mentor and editor, and my writing

style has greatly improved thanks to him. I am especially thankful for Walt's willingness

to participate in this collaboration, because otherwise I would not have had such an

amazing system with which to work.

I thank Bill Searcy, my ornithology professor at the University of Miami, for

sparking my interest in avian ecology, and Jeff Hoover for teaching me proper field

methodology and providing advice at various stages of graduate school.

This work would not have been possible without the support of 100+ field

assistants and everyone else associated with Hastings Natural History Reserve. My stay

at Hastings was made enjoyable by the following: Lauryn Benedict, Amber Budden,

Catherine Dale, Janis Dickinson, Ben Harmeling, Joey Haydock, Jared Heath, Danika

Kleiber, Amy Kochsiek, Alan Krakauer, Jay McEntee, Kathleen Rudolph, and Mark

Stromberg.

I thank my current and former lab mates for providing work related help and

relieving work related stress: Chris Burney, Jeremy Dixon, Elina Garrison, Ann George,

Jeff Hostetler, Gabby Hrycyshyn, Melissa Moyer, SaifNomani, Arpat Ozgul, Heidi









Richter, Brian Spiesman and Matt Trager. Arpat, despite having a PhD to work on, was

never too busy to answer my endless questions. I could never refuse an offer to go

birding with Chris, whether it was chasing migrant warblers at Paynes Prairie or Crax

rubra in Mexico and Costa Rica.

Finally, I would like to thank various friends for providing support during the past

several years: Mariola Alvarez, Matt Chambers, Joy Cox, Denny DuMey, Shannon Henn,

Kathy Huala, Lisa and Friedrich Iglesias, Stacey Jones, Robyn Mericle, Sheda Morshed,

Kymia Nawabi, Devin Peipert, Peejay Perez De Alejo, Alex Pries, Mark Rodriguez,

Nikki Schiwal, Taylor Shields, and Autumn Tarleton.
















TABLE OF CONTENTS



A C K N O W L E D G M E N T S ................................................................................................. iii

LIST OF TABLES ........ ........... .. ....................... .. ........... ............. vii

LIST OF FIGURES ......... ....... .................... .......... ....... ............ ix

A B ST R A C T ................. ................................................................................... ........

CHAPTER

1 IN T R O D U C T IO N ............................................................................... .............. ..

2 DEMOGRAPHIC CONSEQUENCES OF DELAYED DISPERSAL IN THE
COOPERATIVELY BREEDING ACORN WOODPECKER .............. ................4

A b stract ...................................... .................................. ...................... 4
Introduction..................................... ........................... .... ..... ........ 5
M e th o d s ............................................................. ................ 7
Stu dy Species .................................................. ................. 7
S tu dy A rea ...................................... ............................... ................ 9
Field M methods ......................................... .... ...... .................................. ..
Survival and Transition Probabilities: Estimation and Modeling ......................10
R e su lts ...................................... .................................................... 13
D iscu ssio n ...................................... ................................................. 17

3 FITNESS CONSEQUENCES OF DELAYED REPRODUCTION IN A
COOPERATIVE BREEDER: DOES HELPING HELP? .......................................29

A b stra c t ...................................... ................................................... 2 9
Introduction ....................................................................................... .. .. .... 30
M eth o d s ................................................................................... 3 1
Study Species and Study Site .......................................................... ..... .......... 31
Field M methods .................................................................. .... ... ........ .... 32
E stim ation of F witness ............................................................. .. .... ......... .. 33
Statistical A analysis ............. ........ ....... ...................... ............. 34
Effect of Social and Environmental Factors on Fitness ....................................34
R e su lts ...........................................................................................3 5
D isc u ssio n .............................................................................................................. 3 7









4 C O N C L U SIO N S ........................ .... .... .................... .. .. ........ .... ... .......45

L IST O F R E F E R E N C E S ...................................... .................................... ....................48

B IO G R A PH IC A L SK E T C H ...................................................................... ..................55















LIST OF TABLES


Table page

2-1. Quasi-likelihood adjusted AIC differences (AQAICc) for five models of the
survival, recapture, and transition probability in relation to sex and status for
acorn w woodpeckers (1972-2004) .................................................................. ....... 24

2-2. Estimates of apparent survival rates (S) for male, female, breeder and helper
acorn woodpeckers (1972-2004) based on Model 1 (Table 2-1). Mean values
(95% C I) are given ..................... .. ........................ ...... .. .... .... ........... 25

2-3. Transition probabilities (') between juvenile, helper and breeder stages for
acorn woodpeckers (1972-2004) in relation to sex based on Model 1 (Table 2-1).
Mean values (95% CI) are given, with M = male, F = female.............................25

2-4. Influence of social and environmental factors on apparent survival rate of acorn
woodpeckers (1972-2004). Regression coefficients (0) are given with 95% CI
(significant effects are shown in bold). SHM = survival of helper males; SHF
survival of helper females; SBM = survival of breeder males; SB = survival of
breeder fem ales. ...........................................................................25

2-5. Influence of social and environmental factors on transition probabilities of acorn
woodpeckers (1972-2004). Regression coefficients (0) are given with 95% CI
(significant effects are shown in bold). Symbols used are as follows: YT =
transition probability from juvenile to breeder (male); Y' = transition
probability from juvenile to breeder (female); Y' = transition probability from
juvenile to breeder (male); HB = transition probability from juvenile to breeder
(female). The p-values and 95% CI for TJH and THH were of equal magnitude
but opposite sign of JB and THB, respectively, for each sex ...............................26

2-6. Quasi-likelihood adjusted AIC differences (AQAICc) for five models of the
realized population growth of acorn woodpeckers (1972-2004) using Pradel's
reverse-time capture-mark-recapture model. Symbols are as follows: 0 =
survival, p = recapture, and X = realized population growth rate. Sex effect is
noted (s), time effect is noted (t), additive effect of sex and time is noted (s + t),
and a period (.) indicates the constant value of the parameter. .............................26

3-1. Fitness components and measures of fitness for female acorn woodpeckers
(1972-2004) that were banded as juveniles, were recorded at least once after









their first possible breeding season, and reproduced at least once. Means (SE)
and ranges are shown. A t-test was used to compare helpers (N = 80) and
potential breeders (N = 61), helpers and breeders that first reproduced at age 1 (a
=1; N = 44), and helpers and breeders that first reproduced at age 2 or older (a >
2; N = 17); P < 0.05 indicates statistical significance ................ ........ .......... 43

3-2. Correlation between fitness components and measures of fitness for female
acorn woodpeckers (1972-2004) that were banded as juveniles, were recorded at
least once after their first possible breeding season, and reproduced at least once.
The two measures of fitness were lifetime reproductive success (LRS) and
individual fitness (X). Age at first reproduction is represented as a. The Pearson
correlation coefficient is given above, and the p-value below (p < 0.05 indicates
a significant relationship between the two variables, N = 141). ...........................44















LIST OF FIGURES


Figure page

1. Annual variation in survival of acorn woodpeckers (1972-2004). Mean rates
(black lines) are shown with 95% confidence interval (gray shade) for male and
female helpers (SHM and SHF, respectively) and male and females breeders (SBM
and SBF, respectively). Estimates are based on Model 1 (Table 2-1) and are
compared to realized population growth rate (k). ...............................................27

2. Annual variation in transition probabilities of acorn woodpecker (1972-2004).
Mean rates (black line) are shown with 95% confidence interval (gray shade) for
the transition from juvenile to breeder stages for males (YT ) and females
(y ), and from helper to breeder stages for males ( B ) and females ( B).
Estimates are based on Model 1 (Table 2-1) and are compared to realized
population grow th rate ( )............................................... ..................................28















Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Science

DEMOGRAPHIC AND FITNESS CONSEQUENCES OF DELAYED DISPERSAL IN
THE COOPERATIVELY BREEDING ACORN WOODPECKER

By

Justyn T. Stahl

May 2006

Chair: Madan J. Oli
Major Department: Interdisciplinary Ecology

Cooperative breeding in birds occurs when more than two individuals provide

care for a single nest. In many species, the additional adults present are offspring from a

previous year who have delayed dispersal, and provision young who are non-descendant

kin. Delaying dispersal and age of first breeding can influence the demography and

fitness of individuals, but the fitness and demographic consequences of helping behavior

are poorly understood. Using long-term data (1972-2004), I examined the demographic

and fitness consequences of helping behavior in acorn woodpeckers Melanerpes

formicivorus. Using a multi-state capture-mark-recapture framework, I found that the

apparent survival of breeders was higher than helpers, and the survival of males was

higher than females. Juveniles were much more likely to become helpers rather than

breeders following fledging, and helpers were more likely to remain helpers rather than

becoming a breeder. Both survival and transition rates varied annually and were

positively influenced by the acorn crop. For fitness estimation, I focused only on









reproductive females who were banded as juveniles. Although helpers began

reproduction significantly later and lived significantly longer than breeders, there was no

significant difference in lifetime reproductive success or individual fitness between the

two groups. However, birds that successfully bred at age 1 without helping had a

significantly higher fitness than those who helped and successfully bred at age 2 or older.

My results suggest that delayed dispersal and reproduction in the acorn woodpecker lead

to a loss of fitness when conditions are favorable for successful reproduction. However, if

constraints in the environment prevent an individual from breeding at age 1, helping is a

viable option until reproduction is possible.














CHAPTER 1
INTRODUCTION

Cooperative breeding in birds occurs when more than two individuals provide

care for a single nest (Brown 1987). Why some individuals provide care for young who

are not their own has been of substantial interest to behavioral and population ecologists

alike (reviewed in Stacey & Koenig 1990; Cockburn 1998; Koenig & Dickinson 2004).

Recognized in 3.2% of the extant species of birds (Arnold & Owens 1998), cooperative

breeding can manifest itself in a number of ways. Multiple males can share a single

female mate (mate-sharing) and multiple females can lay eggs in the same nest (joint-

nesting). Offspring can disperse and attempt to breed independently when they become

sexually mature, or they can delay dispersal for a year or more and remain at home on

their natal territory. Dispersing individuals may return home at a later time, may breed on

their own, or may immigrate into another group of kin or non-kin. One of the most

interesting aspects common to many cooperative breeding systems is helping behavior,

which can generally be described as delayed reproduction past the point of sexual

maturity and provisioning of young that are not direct descendants.

It has been suggested that a shortage of a critical resource constrains the

availability of breeding territories and the ability of some individuals to reproduce

successfully. This idea, referred to as the ecological constraints hypothesis, has been

proposed as one cause of cooperative breeding (Koenig & Pitelka 1981; Emlen 1982).

Often, the ecological constraint is a shortage of reproductive vacancies and can be

associated with a lack of suitable nest cavities (Walters, Copeyon & Carter III 1992),









habitat (Woolfenden & Fitzpatrick 1984), or food-related resources (Koenig & Mumme

1987). Until a reproductive vacancy becomes available, helpers may remain on the natal

territory and help raise younger siblings.

The acorn woodpecker Melanerpesformicivorus is a cavity nesting species

common throughout the oak woodlands of western North and Central America, ranging

from Oregon south to Colombia. In California the acorn woodpecker practices

cooperative breeding, consisting of mate-sharing, joint-nesting and helping behavior,

although the most common reproductive group is a monogamous pair (Koenig &

Mumme 1987). Displaying one the most complex social systems of any vertebrate, the

acorn woodpecker has been studied continuously since 1971 at Hastings Natural History

Reservation in central coastal California (MacRoberts & MacRoberts 1976; Koenig &

Mumme 1987; Koenig, Haydock & Stanback 1998) and offers an opportunity to answer a

number of questions related to cooperative breeding and helping behavior. The majority

of demographic analyses regarding helping behavior were done several years ago

(summarized in Koenig & Mumme 1987). Since then, many years of additional field data

have been collected, and new techniques for estimating fitness and population dynamic

consequences of social behavior have been developed.

My objectives were to investigate demographic and fitness consequences of

helping behavior in the acorn woodpecker. In chapter 2 I investigated the effect of

cooperative breeding and helping behavior on survival, probability of breeding, and

realized population growth rate. I used multi-state and reverse-time capture-mark-

recapture (CMR) modeling approaches to estimate the aforementioned parameters, to









investigate the influence of helping on these parameters, and to test specific biological

hypotheses relevant to my study system.

In chapter 3 I investigated the effect of delayed dispersal (and thus delayed age of

first reproduction) to help at home on individual fitness of female acorn woodpeckers. I

estimated individual fitness using two methods: lifetime reproductive success (LRS), and

individual fitness ((X, McGraw & Caswell 1996). I compared fitness of females who

helped and those who dispersed as yearlings, allowing me to test for an effect of helping

and delayed dispersal on individual fitness. I also compared helpers and dispersers that

successfully bred at age 1 to investigate the fitness effects of variation in age of first

reproduction which arises from differing dispersal strategies. Finally, I examined the

effect of a number of social and environmental factors on individual fitness and its

components.

The application of recently developed capture-mark-recapture techniques (multi-

state models) and fitness estimation tools (k) to long term data from the acorn

woodpecker allowed me to address questions regarding cooperative breeding and helping

behavior that were not possible when similar studies were conducted decades ago.














CHAPTER 2
DEMOGRAPHIC CONSEQUENCES OF DELAYED DISPERSAL IN THE
COOPERATIVELY BREEDING ACORN WOODPECKER

Abstract

Acorn woodpeckers (Melanerpesformicivorus) often delay dispersal and

provision the offspring of close relatives. We investigated the demographic consequences

of helping behavior using a multi-state capture-mark-recapture approach in Program

MARK, and evaluated the influence of social and environmental factors on survival and

breeding probabilities of acorn woodpeckers. Breeders survived better than helpers, and

males survived better than females. Juveniles were much more likely to become helpers

rather than breeders following fledging, and helpers were more likely to remain helpers

rather than becoming a breeder. Both survival and transition rates varied annually. Group

size and composition had a significant influence on survival of breeder males, breeder

females and helper males. These social factors often had a negative influence on the

likelihood a female would attain a breeding position, while not significantly influencing

the same rates for males. The acorn crop positively influenced survival and probability of

breeding. Realized population growth rate varied annually and was positively affected by

both survival and transition probabilities. Survival of male breeders, which was strongly

influenced by the size of the acorn crop, generally had the greatest influence on

population growth rate.









Introduction

Social behavior influences survival, timing and probability of breeding,

reproduction, dispersal, and population growth rate in many species of birds and

mammals (Brown 1987; Stacey & Koenig 1990; Solomon & French 1996; Koenig &

Dickinson 2004). However, the population dynamic consequences of social behavior are

still not well understood. This is particularly true for cooperative breeding birds, in which

more than two individuals participate in providing care for a single nest (Brown 1987).

One common characteristic of many cooperative breeding systems is the presence

of helpers: individuals who delay dispersal, forego breeding and provide care for another

individual's offspring (Emlen 1982, 1995). In some species, individuals may return to

help a family member after dispersing and failing to breed as in the western bluebird

Sialia mexicana and long-tailed tit Acgiit,1h1 caudatus (Dickinson, Koenig & Pitelka

1996; MacColl & Hatchwell 2002). In others, birds nest independently but establish

territories near their parents and sometimes provision the young at both nests, as in

Galapagos mockingbirds Nesomimusparvulus and western bluebirds (Curry & Grant

1990; Dickinson et al. 1996). Delayed dispersers can in some cases also stay without

offering any help at the nest, as occurs in the Siberian Jay Perisorius infaustus (Ekman,

Sklepkovych & Tegelstrom 1994).

Delayed dispersal and helping behavior are often attributed to ecological

constraints (Koenig et al. 1992; Hatchwell & Komdeur 2000) with constraints in some

cases being so extreme as to cause helping behavior to be obligate (Boland, Heinsohn &

Cockburn 1997). Depending on the suite of environmental and social factors experienced

by an individual of dispersing age, the two possible strategies (disperse or stay) should

confer differing fitness advantages to the individual. For example, Covas et al. (2004)









showed that when unlimited food was provided to colonies of social weavers the

proportion of birds that helped decreased and more birds initiated independent breeding.

Similarly, Walters et al. (1992) experimentally showed that red-cockaded woodpecker

Picoides borealis helpers dispersed into suitable vacant territory when a key resource (a

suitable nest cavity) was available, but generally remained at home and helped until nest

cavities became available. Social factors, such as availability of mates, number of

breeders on natal territory and group size and composition, may also affect an

individual's dispersal decisions. When constraints on breeding are high, juveniles should

be more likely to stay at home and help rather than disperse and breed independently

(Koenig etal. 1992).

Individuals that provide help to relatives may increase their own inclusive fitness

indirectly due to the promotion of shared genetic material (Hamilton 1964; Griffin &

West 2002; Oli 2003). However, helping can have a positive or negative effect on the

direct fitness of an individual. Survival and the probability that an individual will breed

are two major components of fitness and population dynamics, and can be affected by

delaying dispersal and remaining on the natal territory past the point of sexual maturity.

Survival of non-breeders can be enhanced by familiarity with the natal territory, parental

nepotism, or by group augmentation (Ekman, Bylin & Tegelstrom 2000; Kokko,

Johnstone & Clutton-Brock 2001), while survival of breeders can be increased by the

load-lightening associated with the presence of helpers or group augmentation (Kokko et

al. 2001; Heinsohn 2004). By delaying immediate breeding, it is often possible to

increase the likelihood of successful breeding through territory or mate inheritance,

territory budding or by dispersing in coalitions of same-sex siblings (Woolfenden &









Fitzpatrick 1984; Hannon et al. 1985; Komdeur & Edelaar 2001; Dickinson & Hatchwell

2004). Changes in survival and the likelihood of breeding affect an individual's fitness

and the growth rate of a population, and thus population dynamics. An understanding of

how delayed dispersal and helping behavior influences probability of survival and

breeding, and of the influence of environmental and social factors on these rates, is

necessary for discerning the fitness and population dynamic consequences of helping

behavior in cooperative breeding systems.

Here we apply multi-state capture-mark-recapture (CMR) models (Williams,

Nichols & Conroy 2002) to 33 years of data to investigate the survival and population

dynamic consequences of delayed dispersal and helping in the cooperatively breeding

acorn woodpecker Melanerpesformicivorus. Specifically, we ask the following: Do non-

breeding helpers and breeders have different survival probabilities? Are there sex-specific

differences in survival (within helpers and breeders, and overall) or the probability of

becoming a breeder or helper after fledging? Which social and environmental factors

influence survival and probability of becoming a breeder? Finally, we used Pradel's

(1996) reverse-time CMR model to estimate and model the realized population growth

rate and investigate the effect of variation in survival and probability of breeding on the

growth rate.

Methods

Study Species

The acorn woodpecker is a cooperative breeding bird common in oak woodlands

of the Pacific coast. Breeding groups range from single monogamous pairs up to 15-

member mixed groups of breeders and non-breeding helpers of both sexes (Koenig &

Mumme 1987). Helpers are almost always offspring from prior years. Both breeders and









helpers contribute to territory defense, food storage, and care of young (Mumme, Koenig

& Pitelka 1990).

Exhibiting one of the most complex social systems of any vertebrate, the acorn

woodpecker is opportunistically polygynandrous, often practicing both mate-sharing and

joint-nesting within the same group (Koenig & Mumme 1987). Mate-sharing occurs

when 2-4 males, usually brothers or a father and son(s), compete for reproductive access

to 1-3 females, usually sisters or a mother and daughterss. Joint-nesting occurs when

multiple females synchronously lay eggs in a common nest. The latter occurs in 22% of

groups, while mate-sharing by males occurs in slightly over half (54%) of groups.

However, the most common breeding system, making up 23.5% of all groups, is a

monogamous pair (Koenig & Mumme 1987; Koenig & Stacey 1990). Incest is rare; a

genetic analysis by Haydock et al. (2001) found that only 14 of 400 (3.5%) offspring

apparently resulted from incestuous breeding. In general, the high costs of inbreeding

depression (estimated to be at least 1.2-1.8 lethal equivalents per individual) select

against helpers breeding with related group members (Koenig et al. 1998; Koenig,

Stanback & Haydock 1999). Helpers disappear at a higher rate than breeders, at least in

part because of dispersal outside the study area (Koenig et al. 2000).

The apparent limiting factor (or critical resource, cf. Walters (1990) for successful

breeding acorn woodpeckers in California is an acorn storage tree or granary (Koenig &

Mumme 1987). Members of the group drill and maintain holes in a tree throughout the

year, collecting acorn mast and storing it in the granary. Larger granary sizes allow the

food stores to last through the winter, enabling earlier nesting, larger clutches, and higher

nesting success in the spring (Koenig & Mumme 1987; Koenig & Stacey 1990).









Study Area

The study site is located on Hastings Natural History Reservation in the Santa

Lucia mountain range in Monterey County, California. Elevation ranges from 450 to

920m. Summers are hot and dry, while winters are cool and wet with infrequent snow.

Common trees in the study area include five species of oak: coast live oak (Quercus

agrifolia), canyon live oak (Q. chrysolepis), blue oak (Q. douglasii), black oak (Q.

kelloggii), and valley oak (Q. lobata), along with California sycamores (Platanus

racemosa), willows (Salix spp.), California buckeye (Aesculus californica), and madrone

(Arbutus menziesii). The plant communities in acorn woodpecker habitat include foothill

woodland, savanna-grassland, and riparian woodland. Acorn woodpeckers are most

frequently found in generally open areas with a dense ground cover of grasses and few

shrubs (Koenig & Mumme 1987).

Field Methods

Acorn woodpeckers at Hastings Reservation have been individually marked and

their survival and reproduction have been monitored since 1971 (MacRoberts &

MacRoberts 1976; Koenig & Mumme 1987; Koenig & Stacey 1990; Koenig et al. 1999).

Nestlings were banded between 20-25 days after hatching; fledging occurs at

approximately 30-32 days (Weathers, Koenig & Stanback 1990). Breeding status of

adults (helper or breeder) was determined indirectly based on the history and known

relatedness of birds. Generally, birds immigrating into a group were assumed to be

potential breeders, whereas offspring remaining from prior years were generally

classified as non-breeding helpers. However, when all breeders of the opposite sex

disappeared and were replaced by unrelated individuals, offspring were assumed to









inherit breeding status within their natal territory (Koenig & Mumme 1987; Haydock et

al. 2001). Further details on field methodology are given in Koenig & Mumme (1987).

Each fall starting in 1980, the acorn crop of the five Quercus species was

estimated. For each tree, the number of acorns counted in 15 s by each of two observers

was tabulated. Each count value was log-transformed and the mean value was calculated

and used as an index of the acorn crop (hereafter, acorn crop or AC) (Koenig et al.

1994a; Koenig et al. 1994b).

Group size and composition of each group was determined in the spring of each

year. The number of breeding males, breeding females, helper males and helper females

was determined for each group (details in Koenig & Mumme 1987).

Survival and Transition Probabilities: Estimation and Modeling

A multi-state CMR model was constructed separating birds into juvenile (J),

helper (H), and breeder (B) stages. A bird was considered a juvenile from banding (just

prior to fledging) until the following spring breeding season (1 March). Juvenile

woodpeckers had several potential options available to them. They could remain on the

natal territory past the point of sexual maturity and help, they could disperse to fill a

breeding vacancy (either within or outside of the study area), they could inherit breeding

status on their natal territory, or they could die. After becoming a helper, an individual

could die, continue to help, or become a breeder (either within the group, following the

death of the opposite-sex breeder, or by dispersing to another group to fill a reproductive

vacancy). Breeding birds could continue breeding (either in the same group or after

moving to another group), die, or (rarely) revert to becoming a helper.









Following Williams et al. (2002), we define D" as the probability of being alive

and in state s at time i + 1, given that the bird was alive in state r at time i. The recapture

probability, p,, is the probability that a bird alive in state r at time i is captured or

observed after banding. Both D"' and p, assume that survival and transition between i

and i + 1 and capture at i depend only on the state at time i.

The parameter D"' is the product of both survival and probability of transition

between states (Eq. 17.30 of Williams et al. 2002):

D = S' ",

where S,, is the probability that an animal in state r at time i survives and remains in the

study population until time i + 1, and Y is the probability that an animal is in state s at

time i + 1, given that it was in state r at time i and survived until i + 1 and remained in the

study area (Williams et al. 2002). It is important to note that, due to the finite study area,

birds that disappeared could either have died or dispersed outside the study area.

Therefore, these estimates are apparent survival, and should not be interpreted as true

survival, especially in the case of helpers, a large fraction of which are known to disperse.

Similarly, transition probabilities for helpers becoming breeders are to an unknown extent

compromised by the probability of dispersal, as only the fate of individuals remaining in

the study area could be determined. In contrast, although breeders do sometimes disperse

(Haydock and Koenig, unpublished data), apparent survival for breeders is likely to

closely reflect actual survival. Additionally, for a given state, the transition probabilities

sum to 1 (i.e. ZY = 1).
s









In our analysis, we have three states, juvenile, helper and breeder. For example,

YJB is the probability that a juvenile bird in a given year will be a breeder one year later,

and YHH is the probability that a helper in one year will remain a helper the next year.

By definition, a juvenile bird at i cannot be a juvenile at i + 1, as it must become either a

helper or breeder. Therefore, JJ = 0. Additionally, birds cannot return to the juvenile

stage. Thus, HJ = YBJ = 0.

We excluded from analyses individuals that were banded as fledglings but did not

survive the winter. Because we included only those birds that survived to the first spring

following fledging, all juveniles included in the analyses became either a helper or a

breeder. Juvenile survival was thus set to 1.

We estimated the goodness of fit (GOF) for our multi-state model using Program

U-CARE (Choquet et al. 2003). With two sexes (male, female), three strata (juvenile,

helper, breeder) and 33 capture occasions, our data file was too large for Program U-

CARE to handle. Because of the large number of states in our model, we were unable to

use the standard Arnason-Schwarz (AS) multi-state model and instead used TEST 3G and

M of the Jolly-Move (JMV) model, as implemented in U-CARE. This is a reasonable

approach to GOF of the Arnason-Schwarz (AS) multi-state model, because the JMV

model is unlikely to show significantly greater fit to the data than the AS model (Cooch

& White 2005). Further details regarding JMV and AS models are available in Pradel,

Wintrebert & Gimenez (2003).

We implemented the multi-state model using Program MARK (White & Burnham

1999). For model comparison we used Akaike's Information Criterion (AIC), which

considers both the deviance of the model and the number of estimated parameters to









provide a parsimonious description of how well the models explained variation in the

data when compared to one another (Burnham & Anderson 2002; Williams et al. 2002).

A difference in AIC, values (AAICo) of less than two indicated that the models were

similarly supported by the data. A moderate difference in model support was shown when

2 < AAIC, < 7, and strong support when AAIC, > 10. We used the most parsimonious

model (lowest AICc) to examine survival and transition estimates.

Using the most parsimonious model, we investigated the effect of environmental

and social factors on survival and breeding probability by modeling the logits of survival

and transition rates as linear functions of the factors. Values of all environmental and

social factors were scaled such that value ranges from 0 to 1. The relationship between

the survival or transition rate and a covariate was considered significant if the 95%

confidence interval of the slope parameter of the linear model (0) did not include 0

(Williams et al. 2002).

Pradel's (1996) reverse-time CMR model, implemented in Program MARK, was

used to estimate and model the realized population growth rate (k). For model selection

and parameter estimation, we used the same methodology as for the multi-state model.

For the Pradel's model, our interest was in the overall growth rate of the population, and

the breeding status of birds (juvenile, breeder, helper) was therefore ignored. We used

Program RELEASE, implemented in Program MARK (White & Burnham 1999), to

estimate GOF for the general Pradel's model.

Results

Our dataset spanned 33 years (1972-2004) and included data for 1570 individual

acorn woodpeckers (845 males and 725 females). We included 1218 birds banded as

juveniles (690 males, 528 females), and 352 birds banded as adults (155 males, 197









female). Only juveniles that were present in the population during their first spring were

included in our analysis.

The fully time-dependent multi-state model of survival, recapture, and transition

probabilities, {SJ (s*t) SH (s*t) SB (s*t) PJ (s*t) PH (s*t) pB (s*t) TJB (s*t) YHB (s*t) YBB

(s*t)}, fit the data poorly (x2 105 = 219.9, P < 0.0001). The variance inflation factor, c, was

2.09, indicating over-dispersion (Lebreton et al. 1992). Therefore, we used calculated c to

aid in parameter estimation and model comparison.

The most parsimonious model (Model 1, Table 2-1) differed from the second and

third most parsimonious models (Model 2 and 3, respectively, Table 2-1) by AQAIC, of

0.25 and 1.13, respectively, indicating that these 3 models were practically identical.

Model 1 included an additive effect of sex and time in all parameters except recapture

probability of breeders and helpers, probability of transitioning from breeder to helper,

and by definition, all fixed parameters, which were constant across time and sex (Table 2-

1). Recapture (p) rates for helpers and breeders were equal and constant across time and

sex. The transition rate between breeder and helper was constant across time and sex. As

the most parsimonious model, we used Model 1 in further analyses.

Recapture probability (the probability of being seen again after banding, assuming

an individual is still alive) for helpers and breeders was high (0.958, 95% CI: 0.947,

0.975). Juvenile recapture was fixed to zero (Table 2-1). Survival rates showed temporal

variation and differed between sexes and breeding status. The apparent survival was

higher for males than females, and higher for breeders than helpers (Table 2-2). Apparent

breeder survival was approximately 10% higher than that of helper, and male survival

was 8% higher than female survival. The difference between male and female survival









was most pronounced among helpers, where males survived at a rate nearly 12% higher

than female helpers. Survival probability of helpers of both sexes exhibited greater

temporal variation than that of breeders (Figure 1).

Juvenile acorn woodpeckers were roughly 4-7 times more likely to become

helpers than breeders following fledging, but this differed between sexes. Juvenile males

had a higher probability than juvenile females of becoming a breeder in the study area in

the spring after fledging (Table 2-3). Additionally, helpers were approximately twice as

likely to remain helpers the following year as becoming breeders, with females having a

slightly higher probability of remaining helpers. Once an individual became a breeder,

the probability of it returning to the helping stage was very low and similar for the two

sexes (Table 2-3).

Using the most parsimonious model (Model 1, Table 2-1), we investigated the

effects of environmental and social factors on survival and transition probabilities.

Survival of male breeders (SBM) was positively influenced by the acorn crop, size of

storage facilities and group size. The effect of group size was due to both the number of

breeders and the number of helpers, since both positively influenced survival (Table 2-4).

Survival of female breeders (SBF) was positively influenced by acorn crop, storage holes

and group size. The group size effect on the survival of breeder females was due to

number of helpers in the group (especially helper males). Survival of male helpers (SHM)

was positively influenced by storage holes and group size, which was due to the number

of helpers (specifically helper males), but was unaffected by the annual acorn crop.

Survival of female helpers (SHF) was positively affected by only the acorn crop, showing

no influence of any social factor (Table 2-4).









The probability of a male breeding in his second year (the year following his

juvenile year, Y ), was not significantly influenced by any social or environmental

factor (Table 2-5). The same probability for females was negatively influenced by the

number of breeders in the group (both the number of breeding males and females), while

it was positively influenced by the acorn crop. Females, however, were less likely to

become breeders (WY) if there were more breeders in their natal group and more likely

to become a breeder if the acorn crop was good. Male helpers were more likely to

become breeders (Y ) when the acorn crop was good and less likely when there were

more helper females in the group. Female helpers were less likely to become breeders

(YHB) in larger groups (which was due to the number of helpers, specifically helper

females), and also more likely when the acorn crop was better (Table 2-5).

The fully time-dependent Pradel's model, { (s t) p (s t) X (s t)}, fit the data

poorly (x2 139= 475.4, P < 0.0001), and a variance inflation factor (c) of 3.42 was used to

aid parameter estimation and model comparison. The most parsimonious model included

constant capture probability (p), additive effect of sex and time on survival (0), and time

effect on realized population growth rate (X). A competing model that differed in QAIC,

by 1.23 (AQAICc) showed an additive effect of sex and time on X (Model 2, Table 2-6).

Because the AQAIC, was less than 2, there was no support for a difference between the

two models, indicating that the sex effect on X was negligible. Although there was no

significant difference between the two models, we chose the one yielding the smallest

QAIC, value (Model 1, Table 2-6), {O (s + t) p (.) X (t)}. The realized annual population

growth rate ranged over time from 0.598 (95% CI: 0.530, 0.674) to 1.547 (95% CI:

1.296, 1.846, Figure 1).









We investigated the influence of the underlying survival and transition

probabilities on X, and found that all probabilities had a significant positive influence on

X. Survival of male breeders (SBM) generally had the greater influence on X (P = 2.023;

95% CI: 1.433, 2.612), followed by SBF ( = 1.718; 95% CI: 1.225, 2.212), SHM ( =

1.654; 95% CI: 1.810, 2.128) and SHF (3 = 1.450; 95% CI: 1.035, 1.865). Among

transition probabilities, yHB had the highest influence on (P = 0.443; 95% CI: 0.301,

0.585), followed by HB (3 = 0.442; 95% CI: 0.301, 0.582), V (3 = 0.369; 95% CI:

0.221, 0.517), and Y~ (3 = 0.334; 95% CI: 0.200, 0.469). The regression coefficients

and confidence intervals for the transition probabilities YT and HH were of equal

magnitude, but opposite sign of Y' and YHB, respectively for the corresponding sexes.

Discussion

In cooperatively breeding birds, there are costs as well as benefits associated with

a bird's decision to stay and help vs. disperse and attempt to breed independently (Brown

1987; Dickinson & Hatchwell 2004; Ekman et al. 2004). For example, survival of

philopatric individuals is often higher than that of dispersers due to factors such as

territory familiarity (Ekman et al. 2004). A bird's decision to disperse or stay and help

can also influence its probability of breeding. Survival and breeding probability are key

components of fitness, and the effect of dispersal decisions on these demographic

parameters can have important population dynamic consequences.

In our analyses, helpers had lower apparent survival than breeders, and thus

permanently disappeared with much greater frequency. However, this is clearly because

many disappearing helpers are dispersing to fill reproductive vacancies outside of the

study area (Koenig et al. 2000). Although breeder dispersal does occur (Haydock and









Koenig unpublished manuscript), apparent survival of breeders is relatively close to the

real survival rate due to the fact that once an individual begins breeding at a site, it

generally stays there until death or some catastrophic event occurs (Koenig & Mumme

1987). Of the 1570 individuals included in our analysis, 352 were banded as adults, the

majority of which were immigrants from outside of the study area. Assuming that

emigration equals immigration, 35.7% of helpers disappearing are filling breeding

vacancies outside of the study area (Koenig et al. 2000). Therefore, both the survival rate

and probability of attaining a breeding position for helpers were likely biased low. The

dispersal confound also exists for female acorn woodpeckers, who, like many avian

species, tend to disperse further than males (Greenwood 1980; Koenig et al. 2000).

Koenig and Mumme (1987), in a previous analysis of the Hastings population

(1973-1986), estimated annual survival of breeding males to be 0.824 and breeding

females to be 0.712. Our estimates (0.756 for breeding males and 0.714 for breeding

females) were similar to the extent that the earlier estimates fall within the 95%

confidence intervals (Table 2-2).

Koenig & Mumme (1987) hypothesized that the higher mortality of female

breeders is attributable to increased energetic expenditure by females associated with egg

laying and incubation, nest care, and territory defense. Additionally, the higher number of

males banded as juveniles that remained in the study area reflects male-biased natal

philopatry, while the higher number of females banded as adults (many of which

immigrated into the study area) reflects female-biased dispersal. This male-biased

philopatry is likely a major source of the apparently higher survival of helper male acorn

woodpeckers (Koenig et al. 2000).









In many systems, it is not possible to differentiate between individuals that

permanently emigrate from the study area and those that die. The confounding of

dispersal and survival is almost a universal problem (Koenig, Van Vuren & Hooge 1996).

Walters' (1990) work on red-cockaded woodpeckers has probably been the closest to

avoiding the problem in a cooperative breeder. Survival rates of red-cockaded

woodpeckers were similar to acorn woodpeckers: 0.76 for breeding males and 0.68 for

breeding females, but the survival of helper males was slightly higher than that of

breeders (0.78 compared to 0.76, Walters 1990)

Juvenile birds that survived their first breeding season were much more likely to

become a non-breeding helper than to attain a breeding position in the study area. Males

were almost twice as likely as females to become a breeder in the study area following

their first year. We offer two possible explanations for the higher breeding probability for

males. First, cobreeding among males is more common, and dispersing coalitions of

brothers were more successful at competing for breeding positions following a vacancy

(Hannon et al. 1985). Thus, a larger number of helper males will generally fill a given

number of vacancies than helper females. Secondly, male-biased philopatry typically

leads to more males remaining in the study area and attaining breeding positions than

females (Koenig et al. 2000).

A woodpecker that remained a helper for the year following fledging was roughly

twice as likely to become a breeder as a juvenile (compare YJB and HB for both sexes,

Table 2-3). But because the helper stage includes birds of varying age, the increased YHB

probability is likely an effect of age. Once an acorn woodpecker obtains a breeding

position, however, it rarely reverts to helper status (Haydock and Koenig unpublished









manuscript), in contrast to species in which "redirected helping" by failed breeders is

common, such as long-tailed tits and western bluebirds (Dickinson et al. 1996; MacColl

& Hatchwell 2002). Analysis of populations with redirected helping would likely yield

significant 'BH values.

Previous work has shown the acorn crop to be strongly correlated with

reproductive success, territory stability, and breeder survival in acorn woodpeckers

(Koenig & Mumme 1987; Koenig & Stacey 1990). Therefore it was not surprising that

the size of the acorn crop had a significant influence on survival and transition

probabilities of acorn woodpeckers. For breeders, acorn production increased annual

survival of both sexes. For helpers, however, only the survival of females was

significantly influenced by acorn production. In years with large acorn crops, juveniles

(females) and helpers (males and females) had a higher probability of attaining a

breeding position. Thus, more birds breed during years with bumper acorn crops, in much

the same way that Ural owls Strix uralensis take advantage of the vole cycle, and begin

breeding at different ages depending on food availability (Brommer, Pietiainen &

Kolunen 1998).

Using granary size (the number of storage holes available for acorn storage) as a

proxy for territory quality, Koenig & Mumme (1987) found no relationship between

territory quality and breeder survival. However, we found a positive influence of the

number of storage holes on the survival of breeders of both sexes, and male helper

survival. Larger granaries allow groups to take advantage of larger acorn crops, leading

to increased food availability in the winter (Koenig & Mumme 1987). Interestingly,

however, territory quality had no direct effect on the likelihood of a juvenile or helper









becoming a breeder. The dispersal decisions of western bluebirds are influenced by site

quality (Kraaijeveld & Dickinson 2001; Dickinson & McGowan 2005), but we found no

direct relationship between territory quality and dispersal. However, because territory

quality increases survival, it could indirectly increase the probability of breeding for

acorn woodpeckers because longer lived birds have more opportunities to breed. Long-

lived species place greater value on future reproduction (Stearns 1992), and this could be

one possible explanation for the high survival characteristic of cooperative breeding

species (Ekman et al. 2004).

Koenig & Mumme (1987) found that survival of male breeders increased with

group size, number of breeders, and number of helpers, but found no relationship

between the survival of female breeders and any group size or composition measurement.

We found that group size and composition had a strong positive effect on survival of both

sexes, with both the number of breeders and helpers positively influencing survival of

breeder males, and number of helpers positively influencing survival of breeder females.

Such increased survival in larger groups offers support for the group augmentation

hypothesis, which states that if larger groups confer increased survival, it benefits all

group members (especially breeders) to recruit and maintain larger groups (Brown 1994;

Kokko et al. 2001). The presence of helpers in a group can increase the survival of

breeders due to the reduction in breeder workload (Khan & Walters 2002; Heinsohn

2004), in addition to simply increasing group size and thus diluting the effects of

predation.

The realized population growth (k) fluctuated annually in a manner similar to the

survival probability of adult birds. Although the breeding probabilities had a significant









influence on X, their influence on X was generally less than those of survival probabilities,

consistent with the observation that growth rates of fairly long-lived birds were more

sensitive to survival parameters than reproductive parameters (Stahl & Oli in press).

Survival of breeder males, which had the strongest impact on the population growth rate,

was higher in years of larger acorn crops. This reinforces the importance of the role of the

acorn crop for group stability and individual survival (Hannon et al. 1987).

For delayed dispersal to be maintained, benefits of delayed dispersal must balance

or exceed associated costs (Ekman et al. 2004). This study suggests that acorn

woodpeckers that delay dispersal and help have a lower apparent survival than breeders.

Even if we consider that death and disappearance are not separable outcomes, and assume

that survival is equal (Walters 1990), helpers are unlikely to be fully compensated, at

least directly, for helping (Dickinson & Hatchwell 2004). The delayed start of

reproduction is likely to reduce direct fitness (McGraw & Caswell 1996; Oli, Hepp &

Kennamer 2002; Oli & Armitage 2003), but some other component of fitness may be

compensating individuals who follow this strategy. Helpers are certainly gaining indirect

fitness by assisting related breeders such that inclusive fitness of helpers is comparable to

those that disperse and breed (Hamilton 1964). MacColl & Hatchwell (2004), using Oli's

(2003) methodology for estimating inclusive fitness, showed that for long-tailed tits with

zero lifetime reproductive success (LRS), a non-zero fitness was possible through

helping. However, inclusive fitness consequences of cooperative breeding still remain

poorly understood. Earlier work by Koenig & Mumme (1987) found that males delaying

dispersal and helping may have a significantly longer reproductive lifespan and a higher

LRS than those who dispersed in their first year to begin breeding. Females that delay






23


dispersal and help, however, had a shorter reproductive lifespan and lower LRS than

first-year dispersers, with only the latter being significant (Koenig & Mumme 1987). A

thorough examination of the inclusive fitness consequences of delayed dispersal and

helping behavior is clearly desirable.











Table 2-1. Quasi-likelihood adjusted AIC differences (AQAICc) for five models of the survival, recapture, and transition probability in
relation to sex and status for acorn woodpeckers (1972-2004).

QAICc Number of
Number Model AQAICc Weight Parameters

1 S' (.) S (s+t) SB (s+t) pJ (.) pH = B (.) JB (s+t) HB (s+t) BH (.) 0 0.407 129

2 S (.) (s+t) SB (s+t) pJ (.) p(.) pB (.) JB (s+t) HB (s+t) BH (.) 0.25 0.360 130

3 SJ (.) H (s+t) SB (t) p (.) pH = (.) JB(s+t) yHB (s+t) BH (.) 1.13 0.231 128

4 SJ (.) S (t) SB (s+t) pJ (.) p ()= pB (.) (S+t) yHB (s+t) BH (.) 11.19 0.002 128

5 SJ (.) SH (s+t) SB (s+t) p (.) p (.) pB (.) JB (s+t) THB (S*t) B (.) 36.44 0.000 155
The symbols used in this table are defined as follows: S = survival; p = recapture rate; = transition rate; J =juvenile; H = helper; B =
breeder; TJB =juvenile to breeder transition (the effect was the same for juvenile to helper); THB = helper to breeder transition (the
effect was the same for remaining a helper); TBH = breeder to helper transition (the effect was the same for remaining a breeder); (t) =
time effect, no sex effect; (s+t) = additive effect of sex and time; (s*t) = interactive effect of sex and time; (.) = constant parameter, no
effect of sex or time. Parameters that had fixed values, and were thus constant (.) by definition, were as follows: SJ (1), pJ (0), "J (0),
yHJ (0), and TyBJ (0). The complementary parameter for each transition probability experienced the same effect (i.e. THB and THH both
experienced sex and time effects).









Table 2-2. Estimates of apparent survival rates (S) for male, female, breeder and helper
acorn woodpeckers (1972-2004) based on Model 1 (Table 2-1). Mean values
(95% CI) are given.
Male Female Overall
Helper 0.695 (0.452, 0.849) 0.577 (0.327, 0.771) 0.636 (0.389, 0.810)
Breeder 0.756 (0.587, 0.862) 0.714 (0.531, 0.835) 0.734 (0.559, 0.849)
Overall 0.725 (0.519, 0.855) 0.644 (0.427, 0.802) -


Table 2-3. Transition probabilities (') between juvenile, helper and breeder stages for
acorn woodpeckers (1972-2004) in relation to sex based on Model 1 (Table 2-
1). Mean values (95% CI) are given, with M = male, F = female.


Breeder
0.212 (0.067, 0.515)
0.130 (0.037, 0.392)
0.365 (0.158, 0.617)
0.344 (0.143, 0.601)
0.995 (0.987, 0.998)
0.995 (0.987, 0.998)


Helper
0.787 (0.485, 0.932)
0.870 (0.608, 0.963)
0.635 (0.383, 0.842)
0.656 (0.399, 0.857)
0.005 (0.002, 0.013)
0.005 (0.002, 0.013)


Table 2-4. Influence of social and environmental factors on apparent survival rate of
acorn woodpeckers (1972-2004). Regression coefficients (0) are given with
95% CI (significant effects are shown in bold). SH = survival of helper
males; SHF = survival of helper females; SB = survival of breeder males; SBF
= survival of breeder females.


Covariate
Group size

Breeders

Helpers

Breeder males

Breeder
females
Helper males

Helper females

Acorn crop

Storage
facilities


sHM
0.094
(0.010, 0.177)
0.069
(-0.030, 0.169)
0.088
(0.014,0.162)
0.057
(-0.040, 0.154)
0.085
(-0.014, 0.184)
0.076
(0.009,0.144)
0.064
(-0.011, 0.139)
0.376
(-0.079, 0.831)
0.191
(0.090,0.291)


sH"
-0.032
(-0.125, 0.061)
-0.055
(-0.161, 0.052)
0.010
(-0.070, 0.091)
-0.041
(-0.144, 0.062)
-0.056
(-0.159, 0.046)
0.016
(-0.068, 0.100)
-0.005
(-0.076, 0.067)
0.528
(0.047, 1.010)
0.062
(-0.041, 0.165)


SBM
0.205
(0.104, 0.306)
0.080
(0.002,0.158)
0.210
(0.099,0.322)
0.058
(-0.012, 0.127)
0.081
(-0.004, 0.166)
0.173
0.065, 0.282)
0.144
(0.046,0.243)
0.420
(0.013, 0.826)
0.135
(0.039,0.231)


sB"
0.146
(0.026, 0.267)
0.040
(-0.052, 0.132)
0.144
(0.022,0.267)
0.074
(-0.021, 0.168)
-0.016
(-0.099, 0.066)
0.124
(0.004,0.244)
0.088
(-0.022, 0.198)
1.04
(0.580, 1.498)
0.127
(0.008,0.247)


From:
Juvenile

Helper

Breeder









Table 2-5. Influence of social and environmental factors on transition probabilities of
acorn woodpeckers (1972-2004). Regression coefficients (0) are given with
95% CI (significant effects are shown in bold). Symbols used are as follows:
M = transition probability from juvenile to breeder (male); W = transition
probability from juvenile to breeder (female); Ym = transition probability
from juvenile to breeder (male); YHB = transition probability from juvenile to
breeder (female). The p-values and 95% CI for JH and WHH were of equal
magnitude but opposite sign of JB and THB, respectively, for each sex.


Covariate
Group size

Breeders

Helpers

Breeder
males
Breeder
females
Helper males

Helper
females
Acorn crop

Storage
facilities


B M
-0.008
(-0.260, 0.144)
-0.026
(-0.166, 0.114)
-0.037
(-0.182, 0.108)
0.019
(-0.102, 0.140)
-0.100
(-0.258, 0.059)
-0.033
(-0.163, 0.097)
-0.029
(-0.179, 0.121)
0.637
(-0.205, 1.478)
0.031
(-0.091, 0.152)


-0.052
(-0.263, 0.159)
-0.354
(-0.635, -0.074)
0.097
(-0.061, 0.256)
-0.293
(-0.553, -0.033)
-0.317
(-0.590, -0.044)
0.077
(-0.077, 0.231)
0.074
(-0.078, 0.226)
1.514
(0.439, 2.588)
-0.205
(-0.454, 0.043)


UHB
-0.079
(-0.176, 0.019)
-0.063
(-0.178, 0.051)
-0.068
(-0.155, 0.020)
-0.027
(-0.139, 0.084)
-0.101
(-0.217, 0.014)
-0.019
(-0.097, 0.060)
-0.111
(-0.202, -0.020)
1.009
(0.359, 1.659)
0.008
(-0.096, 0.112)


THB

-0.166
(-0.318, -0.015)
-0.101
(-0.263, 0.060)
-0.174
(-0.312, -0.035)
-0.069
(-0.225, 0.087)
-0.123
(-0.279, 0.033)
-0.174
(-0.315, -0.032)
-0.118
(-0.237, 0.002)
1.547
(0.699, 2.395)
-0.163
(-0.333, 0.008)


Table 2-6. Quasi-likelihood adjusted AIC differences (AQAICc) for five models of the
realized population growth of acorn woodpeckers (1972-2004) using Pradel's
reverse-time capture-mark-recapture model. Symbols are as follows: 0 =
survival, p = recapture, and X = realized population growth rate. Sex effect is
noted (s), time effect is noted (t), additive effect of sex and time is noted (s
t), and a period (.) indicates the constant value of the parameter.
QAICc
Number Model AQAICc Weights Parameters
1 (s t) p (.)X(t) 0 0.63 66
2 D (s t) p (.)0 (s t) 1.22 0.34 67
3 D (t) p (.) (t) 6.67 0.02 65
4 (t) p (.) (s +t) 8.72 0.01 66
5 P (s + t)p(.)0 (s) 104.35 0 36


\ I j I j I j I j











1.0
0.8

SHM 0.6
0.4 -
0.2
0.0
1.0
0.8
sHF 0.6
0.4
0.2
0.0 -
1.0
0.8 -

SBM 0.6
0.4
0.2
0.0
1.0
0.8 -

sBF 0.6
0.4
0.2
0.0
2.0

1.5 -

1.0

0.5


1972 1976 1980 1984 1988 1992 1996 2000 2004
YEAR

Figure 1: Annual variation in survival of acorn woodpeckers (1972-2004). Mean rates
(black lines) are shown with 95% confidence interval (gray shade) for male
and female helpers (SHM and SHF, respectively) and male and females breeders
(SBM and SBF, respectively). Estimates are based on Model 1 (Table 2-1) and
are compared to realized population growth rate (k).












1.0 -
0.8 -
JB 0.6 -
M
0.4 -
0.2 -
0.0 -
1.0
0.8

YJB 0.6
F 0.4

0.2
0.0
1.0
0.8

IHB 0.6
M 0.4-
0.2
0.0
1.0
0.8
B 0.6
wHB
F 0.4-
0.2
0.0
2.0

1.5

?, 1.0

0.5

0.0


1972 1976 1980 1984
YEAR


1988 1992 1996 2000 2004


Figure 2: Annual variation in transition probabilities of acorn woodpecker (1972-2004).
Mean rates (black line) are shown with 95% confidence interval (gray shade)
for the transition from juvenile to breeder stages for males (W ) and females
( ), and from helper to breeder stages for males (YHB ) and females (HB).
Estimates are based on Model 1 (Table 2-1) and are compared to realized
population growth rate (k).














CHAPTER 3
FITNESS CONSEQUENCES OF DELAYED REPRODUCTION IN A
COOPERATIVE BREEDER: DOES HELPING HELP?



Abstract

Cooperative breeding in birds occurs when more than two individuals provide care

for a single nest. In many species, the additional adults present are offspring from a

previous year who have delayed dispersal, and provision young who are non-descendant

kin. Delaying dispersal and age of first breeding can potentially influence fitness, but

fitness consequences of helping behavior are poorly understood. Using long-term data

(1972 2004), we examined the fitness consequences of helping behavior in acorn

woodpeckers Melanerpesformicivorus. Although helpers began reproduction

significantly later and lived significantly longer than breeders, there was no significant

difference in lifetime reproductive success or individual fitness between the two groups.

However, birds that successfully bred at age 1 without helping had a significantly higher

fitness than those who helped and successfully bred at age 2 or older. Our results suggest

that delayed dispersal and reproduction in the acorn woodpecker leads to a loss of fitness

when conditions are favorable for successful reproduction. However, if constraints in the

environment prevent an individual from breeding at age 1, helping is a viable option until

reproduction is possible.









Introduction

Cooperative breeding in birds occurs when more than two individuals provide

care for a single nest (Brown 1987). Frequently, helpers are offspring from a previous

nest who have delayed dispersal, and provide care for their siblings from a later clutch

(Cockburn 1998). Previous studies of the possible costs and benefits of helping indicate

that fitness benefits of helping are often lower than loss of fitness from not breeding

independently (Reyer 1984; Woolfenden & Fitzpatrick 1984; Komdeur & Edelaar 2001;

MacColl & Hatchwell 2002; Dickinson & Hatchwell 2004). One likely reason for the

reduced fitness is a delayed onset of reproduction, which can profoundly influence fitness

(Cole 1954; Lewontin 1965; McGraw & Caswell 1996). Early reproduction generally

allows an individual to begin propagating its genes sooner than delayed reproduction

(e.g., Oli et al. 2002; Oli & Armitage 2003), but doing so can have detrimental effects if

survival, growth or reproductive potential is compromised (e.g., Pyle et al. 1997).

If delayed age of first reproduction reduced fitness, why would an individual

delay reproduction and help? One explanation is that constraints in the environment limit

individuals' ability to successfully breed immediately upon reaching sexual maturity;

helping is a strategy that enables some birds to make the best of a bad situation (Koenig

& Pitelka 1981; Emlen 1982; Koenig et al. 1992). In many cooperatively breeding birds,

a critical resource (such as suitable habitat, a food related resource or a breeding cavity)

is often necessary for successful breeding, and the absence of this resource can lead to a

shortage of breeding positions (Woolfenden & Fitzpatrick 1984; Koenig & Mumme

1987; Walters et al. 1992).

The acorn woodpecker Melanerpesformicivorus is a cavity nesting species and a

common inhabitant of the oak woodlands of California (Koenig & Mumme 1987; Koenig









et al. 1995). Practicing opportunistic polygynandry in addition to helping behavior, the

cooperatively breeding acorn woodpecker exhibits one of the most complex social

systems of any vertebrate, with group size ranging from a single pair to a mixed group of

up to 15 breeders and helpers. Territorial inheritance is rare, yet many individuals delay

dispersal for one year or more and help on their natal territory (Koenig & Mumme 1987).

Using long-term (1972-2004) life-history data, we investigated the fitness

consequences of helping behavior in female acorn woodpeckers. Specifically, we

addressed the following questions: (1) Does helping (and delayed age of first

reproduction) reduce fitness? Specifically, do helpers have lower fitness compared to

birds that disperse and breed without helping? (2) What social and environmental factors

influence fitness? (3) If helpers have lower fitness than breeders, why does helping

persist?

Methods

Study Species and Study Site

The acorn woodpecker is a cooperatively breeding, cavity nesting species that

ranges throughout the oak woodlands of California and possesses one of the most

complex social systems of any vertebrate (Koenig & Mumme 1987). The reproductive

system of this species is opportunistic polygynandry, a mix of mate-sharing (multiple

males compete for mating opportunities with one or more females) and joint-nesting

(multiple females breed with one or more males and lay their eggs in a single nest).

Group size and composition range from a monogamous pair to a 15-member mixed group

of breeders and non-breeding helpers of both sexes (Koenig & Mumme 1987). Both

breeders and helpers contribute to territory defense, food storage, and care of young

(Mumme et al. 1990).









In parts of their range, acorn woodpeckers construct granaries, or storage trees, in

which they store acorn mast, food that supplements their diet of sap and flying insects

(MacRoberts & MacRoberts 1976; Koenig & Mumme 1987). These granaries are a

critical resource, allowing territories to be maintained year-round and may serve as an

alternative to winter fattening when food is more scarce (Koenig et al. 2005). Groups

with acorn stores remaining in spring experience earlier nests and higher reproductive

success than those without stored acorn mast (Koenig & Mumme 1987; Koenig & Stacey

1990).

This study took place at Hastings Natural History Reservation in central coastal

California. Plant communities at Hastings include foothill woodland, savanna-grassland,

and riparian woodland. Common trees in the study area include five species of oak: coast

live oak (Quercus agrifolia), canyon live oak (Q. chrysolepis), blue oak (Q. douglasii),

black oak (Q. kelloggii), and valley oak (Q. lobata), along with California sycamores

(Platanus racemosa), willows (Salix spp.), California buckeye (Aesculus californica),

and madrone (Arbutus menziesii). Acorn woodpeckers are mainly found in open areas,

nesting in oaks and sycamores, with a dense ground cover of grasses, and few shrubs

(Koenig & Mumme 1987).

Field Methods

Intensive research on acorn woodpeckers at Hastings Reservation began in 1971

(MacRoberts & MacRoberts 1976) and continues to date (Koenig & Mumme 1987;

Koenig et al. 2000; Haydock & Koenig 2003); data collected during 1972-2004 were

used in this study. The majority of natural cavity nests (located 2 20m up in trees) were

located and checked, and groups were censused to determine membership. Birds were

individually marked with leg bands as nestlings 20-25 days after hatching or as soon as









possible if immigrating as adults. Group size and composition (number of breeders and

helpers) were determined by observation and the known relationship of group members

based on banding record (Koenig et al. 1998; Koenig et al. 1999). Further details on field

methodology can be found in Koenig & Mumme (1987).

Each fall starting in 1980, the acorn crop of the five Quercus species was

estimated. For approximately 250 oak trees, the number of acorns counted in 15 s by each

of two observers was recorded. Each count value was log-transformed and the mean

value was calculated and used as an index of annual acorn crop (hereafter, acorn crop or

AC) (Koenig et al. 1994a; Koenig et al. 1994b).

Estimation of Fitness

Although there is a significant amount of reproductive skew among breeding male

acorn woodpeckers (Haydock & Koenig 2003), the egg destruction behavior of joint-

nesting females enforces hatching synchrony and leads to equal parentage of fledglings

by breeding females (Mumme, Koenig & Pitelka 1983; Haydock et al. 2001). Therefore,

to estimate the number of young fledged per female, we divided the number of young

fledged for a group by the number of breeding females in the group. Survivorship was

determined by repeated visits to each group, and by monitoring the territory until all

group members were seen (Koenig & Mumme 1987). For the purposes of this analysis,

because we only focus on individuals who successfully reproduced at least once in their

lives, birds disappearing were treated as dead.

We used two measures of individual fitness. First, we calculated the lifetime

reproductive success (LRS) as the total number of offspring produced by a female during

her lifetime (Clutton-Brock 1988; Newton 1989). A second measure of individual fitness

(k) was estimated using the methods of McGraw & Caswell (1996). This fitness measure









represents the rate at which an individual's genes are propagated if the vital rates remain

the same (McGraw & Caswell 1996). A population projection matrix for each individual

was constructed, with age-specific annual fertility rates (estimated as one half times the

number of offspring produced by a female per year) on the first row of the matrix, and

survival probability of 1 along the lower sub-diagonal until age of last reproduction. The

dominant eigenvalue of the matrix is the estimate of individual fitness (k). All fitness

calculations were done using MATLAB.

Statistical Analysis

We used a t-test to compare mean fitness of individuals that helped as yearlings to

those that did not. Because some individuals may attain a breeding position at age 1 but

fail to breed, we also used t-tests to compare the fitness of helpers to individuals who

dispersed and bred successfully at age 1, and to those who dispersed at age 1 but did not

successfully reproduce until age 2 or older.

Effect of Social and Environmental Factors on Fitness

We investigated the influence of social and environmental factors on both LRS

and X. Social factors included group size (the number of adults in the group), the number

of male, female and total helpers, and the number of male, female and total breeders.

Environmental factors included the acorn crop of the previous fall (potentially still

present in the spring), the acorn crop of the current fall, and the number of storage holes

in the group's granary (an estimate of territory quality). We collected data for each social

and environmental variable in three ways: the year the individual was born, the year the

individual began breeding and the average over the individual's lifespan (Koenig &

Mumme 1987).









We used step-wise variable selection procedure in multiple linear regression to

identify variables significantly influencing LRS or X (System) 2002).Then, using only

those variables selected by the stepwise variable selection procedure, we fitted a multiple

linear regression with LRS (or X) as response variable and the aforementioned social and

environmental variables as predictors.

Results

We had lifetime survival and reproductive data for 498 females that were banded

as juveniles and were resighted at least once during or after their first potential breeding

season. We excluded individuals who were still alive (last seen in 2004) and those that

never successfully reproduced within the study area. Therefore, all analyses were based

on 141 females (61 potential breeders and 80 potential helpers).

On average, female acorn woodpeckers lived 4.35 0.25yrs, began breeding

between at age 2.39 + 0.11, reproduced 2.55 0.19 times, and had a mean post-

maturation survival (numbers of years survived beyond the age of first reproduction) of

1.96 0.22yrs (Table 3-1). The mean LRS was 6.90 + 0.65 and mean individual fitness

(k) was 1.27 0.04. The longest lived individual (female 787) in our study lived 16yr,

and had the largest LRS (37.75), although not the highest X (1.54, compared to the

maximum of 2.62). This individual did not help in its first year, and first successfully

reproduced at age 1.

Lifetime reproductive success and fitness were strongly correlated with each

other, and both fitness measures were strongly correlated with lifespan, post-maturation

survival, lifetime number of reproductive events and mean number of young fledged

(Table 3-2). Neither measure of fitness was significantly related to age at first

reproduction (a). Only two fitness components showed a significant and positive









relationship with a: lifespan and mean young fledged. Lifespan and reproductive lifespan

were positively correlated with one another, as well as with number of reproductive

events, and mean young fledged.

Comparing the fitness components and fitness of females that delayed dispersal

and helped for at least one year (helpers; n = 80) to those that dispersed and attempted to

breed at age 1 (potential breeders; n = 61), we found the following (Table 3-1). Helpers

began breeding at around age 3, which was significantly delayed compared to potential

breeders. Helpers outlived potential breeders, living about 1.5 yr longer. There was no

significant difference between helpers and potential breeders in terms of post-maturation

survival, number of reproductive events, mean young fledged per reproductive event,

lifetime reproductive success or individual fitness (k).

Because some individuals disperse but fail to successfully reproduce at age 1, we

also compared helpers to those individuals who successfully reproduced at age 1

("breeders," n = 44). As expected, helpers began reproduction significantly later (3.09

compared to 1.0, p < 0.05) but lived longer than the breeders (5.08 compared to 3.20, p <

0.05). Birds that began breeding at age 1 had a significantly larger individual fitness (k)

than helpers (1.43 compared to 1.21, p < 0.05). Neither LRS nor any other component of

fitness differed significantly between the two groups.

Comparing helpers to those potential breeders who dispersed and acquired a

breeding position but failed to successfully reproduce until age 2 or older, the only

measure of fitness or fitness component that differed significantly between the two

groups was lifespan, with helpers living 1.2 yr longer (5.08 compared to 3.88, p < 0.05).

Neither measure of fitness, nor any other component of fitness differed. Dispersers who









bred at age 1 had higher fitness than those who dispersed at age 1 but failed to breed until

age 2 (1.43 and 1.18, respectively, p < 0.05).

Of 29 candidate social and environmental variables considered, the stepwise

variable selection procedure selected five variables as having significant influence on X:

the acorn crop the year before birth (AC), the granary size on the territory when the

individual was born (GBirth), granary size the year the individual began breeding

(GBreed), the average granary size over the individual's lifetime (GLife), and the number

of breeding females in the group the year the individual began breeding (BF). A multiple

linear regression model with these variables included explained 44.3% of the variation in

X (X = 1.80094 0.25147AC 0.00012GBirth 0.0027GBreed + 0.000518GLife -

0.19782BF; R2 = 0.443, p <0.0009). None of the social or environmental variables

considered significantly influenced LRS.

Discussion

Two of the most interesting characteristics of cooperative breeding systems are

offspring retention and helping behavior. Instead of dispersing when they are capable of

breeding independently, many individuals stay at home for 1 yr or more and help to raise

siblings. Helping is generally considered a route to lower fitness, and helpers attempt to,

but rarely do, recoup the loss of fitness due to foregoing reproduction and remaining at

home (Dickinson & Hatchwell 2004).

The acorn woodpecker follows one of two routes upon reaching sexual maturity.

Individuals can delay breeding for one or more years and help raise non-descendant kin,

the path of so-called helpers. Alternatively, individuals can disperse and attempt to breed

as soon as they are sexually mature at age 1 (potential breeders). In this study, helpers

had a delayed age of first reproduction and a longer lifespan but neither measure of









fitness, nor any other component of fitness differed significantly between helpers and

potential breeders. However, it is important to note that some individuals disperse and

gain a dominant position in a group, but for some reason (e.g. no mate, poor acorn crop,

predation) fail to successfully fledge any young at age 1. Thus, we compared fitness and

its components of birds who successfully reproduced at age 1 ("breeders") to those who

delayed dispersal and helped; this comparison should better elucidate the fitness

consequences of helping and delayed of first reproduction. Females that successfully

reproduced at age 1 had higher fitness (k) compared to those who stayed home and

helped. In other words, dispersing and breeding is advantageous if the individual can

successfully reproduce at age 1, otherwise there is no difference in fitness for helpers or

breeders. There was no difference in LRS between the two groups. However, LRS may

not adequately quantify fitness because it only considers the amount of reproduction and

ignores the timing of reproduction, which can also substantially influence fitness

(McGraw & Caswell 1996; Oli et al. 2002). Consequently, inferences based on X may be

more appropriate (McGraw & Caswell 1996; Oli et al. 2002; Oli & Armitage 2003).

One obvious consequence of helping is that reproduction is delayed. Early work

proposed that a delayed age of first reproduction could reduce fitness (Cole 1954;

Lewontin 1965). Recent comparative life history studies of birds and mammals found

that Cole's prediction strictly holds in species characterized by early maturity and high

reproductive rates (Oli & Dobson 2003; Stahl & Oli in press); this conclusion is

consistent with findings of several empirical studies that have examined the effect of age

at first reproduction on fitness. In the wood duck, for example, individuals beginning

reproduction at an earlier age had a greater individual fitness than those who delayed age









of first reproduction (Oli et al. 2002), indicating that fitness benefits of early reproduction

outweigh associated costs (e.g. Reznick 1985). In the goshawk, a species characterized by

late age of first reproduction and low fecundity, individuals who delayed maturity had

higher fitness than those who began breeding earlier (Kruger 2005).

Using LRS as a measure of fitness, Koenig & Mumme (1987) and Koenig &

Stacey (1990) found no significant difference in fitness between acorn woodpeckers of

either sex that helped and those who reproduced without helping. Consistent with

findings of Koenig & Mumme (1987) and Koenig & Stacey (1990), we found no

significant difference in LRS between helpers and breeders. When X was used as a

measure of fitness, however, we found that delayed reproduction due to helping is costly

in acorn woodpeckers, with significantly lower fitness for females that helped for one or

more years compared to those who bred as yearlings.

If helping is a route to lower fitness, why has it been maintained in the system?

We offer three possible explanations, which are not mutually exclusive: (1) ecological

conditions prevent some individuals from beginning to breed at sexual maturity, (2)

females are more successful at dispersing and attaining a breeding position with a sister,

and (3) inclusive fitness benefits.

Ecological constraints have been proposed as the cause of cooperative breeding,

especially offspring retention (Koenig & Pitelka 1981; Emlen 1982). In cooperative

breeding systems, a critical resource is often necessary for a family unit to exist. In the

case of acorn woodpeckers, the critical resource is the granary (Koenig & Mumme 1987).

Without a tree in which to store acorns, a family unit may not persist and therefore

ecological constraints may influence delayed dispersal, helping behavior and an average









age of first reproduction over a year past the point of sexual maturity. Acorn

woodpeckers invest a considerable amount of time constructing and maintaining their

granary much in the same way red-cockaded woodpeckers Picoides borealis must invest

considerable effort creating cavities in live trees (leading to delayed dispersal and

reproduction in a number of males Walters et al. 1992). In red-cockaded woodpeckers,

males typically begin reproduction between 2 3yrs of age (for comparison, as female

helpers are rare Walters 1990). Another cooperative breeder that has been intensively

studied, the Florida scrub-jay Aphelocoma coerulescens generally breeds for the first time

at age 2, with only 3 females breeding at the age of sexual maturity (lyr) in 18 years of

study (Woolfenden & Fitzpatrick 1990). Individuals remaining at home must wait for a

breeding vacancy to become available, and it can be advantageous to disperse with

siblings and compete as a unit for vacancies (Hannon et al. 1985).

Up to 30 helpers from surrounding territories vigorously compete for reproductive

vacancies; the winners of such "power struggles" are often larger groups consisting of

same-sex sibling units (Koenig 1981; Hannon et al. 1985). Therefore, sisters (or brothers)

dispersing together were more likely to attain a breeding position when one became

available, and remaining at home past the point of sexual maturity may be advantageous

if it increases a female's chances of successfully dispersing with and competing alongside

a sister for a breeding vacancy. Joint dispersal leads to approximately one half of all

joint-nesting occasions (Koenig & Mumme 1987; Mumme, Koenig & Pitelka 1988).

Because helpers in this system were nearly always related to at least one of the

breeders (most often both breeders are the helper's parents) and therefore the young they

provision, helpers are most certainly accruing indirect fitness benefits (Koenig &









Mumme 1987). Helper presence has been shown to increase reproductive success and

survival of group members (Koenig & Mumme 1987; Stahl, Koenig & Oli unpublished

manuscript). Therefore the inclusive fitness totals of helpers are most certainly likely to

be higher than our direct fitness estimates, and may lead to helping being advantageous in

some or all situations.

Stacey & Ligon (1987; 1991) showed that acorn woodpecker helpers remaining

on high quality territories had higher LRS and survivorship. Both the acorn crop and

territory quality positively influence survival, probability of breeding and reproductive

success in acorn woodpeckers (Koenig & Mumme 1987; Stahl et al. unpublished

manuscript), and this study has shown direct effects on individual fitness were also

apparent. Using granary size (number of storage holes) as a proxy for territory quality, we

found that birds breeding on higher quality territories over their lifetime had higher

fitness. The most important social effect on fitness was that joint-nesting females had

lower X than females nesting alone. This result is consistent with that of Mumme et al.

(1988) who found that the lifetime reproductive success of joint-nesting females was less

than or equal to that of singly nesting individuals, despite increased survival and territory

dominance resulting from co-breeding.

Three caveats must be mentioned. First, the results presented in this manuscript

are only for females, and may not apply to males. Males are the more common helping

sex and co-breed more often than females joint-nest (Koenig & Mumme 1987). Unlike

females, parentage in males often significant skewed (Haydock et al. 2001; Haydock &

Koenig 2003). Second, breeders typically occupy a territory continuously once they begin

breeding, and are therefore more likely to remain in the study area than helpers (Haydock









& Koenig unpublished manuscript; Stahl et al. unpublished manuscript). Because of this

we only have fitness estimates for those helpers who remained in the study area, and the

fitness of those leaving the study area to fill breeding vacancies could possibly be

different than those remaining in the study site (Koenig et al. 2000). Third, we have only

considered the direct fitness of individuals and our estimates must therefore be seen as

conservative. Indirect fitness benefits could possibly make up for the cost of delayed

dispersal. Future work should employ the methodology of Oli (2003) to accurately

quantify the inclusive fitness that Hamilton (1964) suggested.

In conclusion, helping in the acorn woodpecker is a route to lower fitness, but

only if dispersers successfully reproduce at age 1. If the prospects of successful

reproduction look grim, staying at home can be worthwhile. Females breeding on

territories with larger granaries had higher fitness. The only obvious social factor that

affected fitness was that joint-nesting females suffered reduced fitness. At least in the

acorn woodpecker, helping is maintained due to constraints placed on individuals in

terms of limited reproductive vacancies. Helpers are possibly attempting to make due by

accruing inclusive fitness benefits and/or waiting to disperse with siblings, which

increases the likelihood of attaining a breeding position (Hannon et al. 1985).












Table 3-1. Fitness components and measures of fitness for female acorn woodpeckers (1972-2004) that were banded as juveniles, were
recorded at least once after their first possible breeding season, and reproduced at least once. Means (SE) and ranges are
shown. A t-test was used to compare helpers (N = 80) and potential breeders (N = 61), helpers and breeders that first
reproduced at age 1 (a =1; N = 44), and helpers and breeders that first reproduced at age 2 or older (a > 2; N = 17); P <
0.05 indicates statistical significance.
Helpers vs. Potential Breeders Helpers vs. Breeders
Fitness All females Helper Potential P a = 1 P a > 2 P
Measure/Component Breeders
Age at first reproduction 2.39 (0.11) 3.09 (0.14) 1.48 (0.11) <0.05 1.00 (0) <0.05 2.70 (0.16) 0.08
(a)
Lifespan 4.35 (0.25) 5.08 (0.32) 3.39 (0.36) <0.05 3.20 (0.47) <0.05 3.88 (0.43) <0.05
Post-maturation survival 1.96 (0.22) 1.99 (0.28) 1.92 (0.36) 0.88 2.20 (0.47) 0.67 1.18(0.43) 0.21
Reproductive events 2.55 (0.19) 2.58 (0.24) 2.52 (0.30) 0.89 2.77 (0.39) 0.65 1.88 (0.36) 0.21
Lifetime reproductive 6.90 (0.65) 7.21 (0.84) 6.50 (1.01) 0.59 7.21 (1.31) 1.00 4.66 (1.24) 0.19
success (LRS)
Individual fitness (k) 1.27(0.04) 1.21 (0.03) 1.36(0.07) 0.06 1.43(0.10) <0.05 1.18(0.05) 0.72
Mean young fledged per 2.42 (0.09) 2.55 (0.13) 2.26 (0.12) 0.10 2.23 (0.15) 0.13 2.31 (0.18) 0.41
event












Table 3-2. Correlation between fitness components and measures of fitness for female acorn woodpeckers (1972-2004) that were
banded as juveniles, were recorded at least once after their first possible breeding season, and reproduced at least once. The
two measures of fitness were lifetime reproductive success (LRS) and individual fitness (k). Age at first reproduction is
represented as a. The Pearson correlation coefficient is given above, and the p-value below (p < 0.05 indicates a significant
relationship between the two variables, N = 141).
LRS x a Lifespan Post-maturation survival Reproductive events
X 0.618
<.001


0.058 -0.146
0.494 0.083

0.837 0.390 0.441
<.001 <.001 <.001


Lifespan


Post-maturation survival


Number of reproductive events


Mean young fledged per event


0.903 0.509 -0.015 0.891
<.001 <.001 0.856 <.001

0.921 0.531 -0.037 0.857
<.001 <.001 0.665 <.001


0.674
<.001


0.325
<.001


0.423
<.001


0.974
<.001

0.306
<0.001


0.305
<0.001














CHAPTER 4
CONCLUSIONS

Group living can affect the fitness and population dynamics of social animals. In

cooperative breeding birds, some individuals delay reproduction and assist in raising

young that are not their own direct descendants, whereas others disperse and attempt to

breed independently when they attain sexual maturity (Cockburn 1998). Direct benefits

of delayed dispersal and/or helping behavior can include increased survival (e.g., Ekman

et al. 2000) and territory acquisition via budding (Woolfenden & Fitzpatrick 1984;

Komdeur & Edelaar 2001). An individual can gain indirect fitness by helping aids kin

(Hamilton 1964). However, because delayed reproduction is a characteristic of helping,

this behavior can also have fitness costs. Despite decades of research, the fitness and

population dynamic consequences of cooperative breeding are poorly understood.

In this thesis I investigated both the fitness and demographic consequences of

cooperative breeding and the strategy of delayed dispersal and reproduction for helpers in

the cooperatively breeding acorn woodpecker. Applying recently developed techniques

for estimating survival and fitness to a long-term data set, my results were based on

robust calculations.

In chapter 2, I investigated the demographic and population dynamic

consequences of helping behavior and cooperative breeding and found that survival and

breeding probabilities of acorn woodpeckers differed depending on sex, status and year.

The apparent survival (the probability of surviving and remaining in the study area) of

helpers was lower than breeders. However, this difference was likely biased towards









breeders as they are more likely to remain in the study area; many helpers that

disappeared were filling reproductive vacancies outside the study area. Males (both

helpers and breeders) survived at a greater rate than females. Juvenile males, when

compared to females, were nearly twice as likely to become breeders immediately after

their first year of life. For both sexes, though, the most common strategy was to delay

dispersal and help for at least one year.

A number of social and environmental factors influenced the survival and

breeding probabilities. Acorn woodpeckers were more likely to survive and also had a

higher probability of becoming a breeder in years when the acorn crop was high. Larger

granaries (i.e., better quality territories) also positively influenced survival, but

surprisingly had no effect on breeding probability. Group size and composition both

increased survival of group members. The realized population growth rate (k) varied

annually and was most influenced by survival, followed by breeding probabilities.

Because the acorn crop positively influences survival and probability of attaining a

breeding position, the changes in realized growth rate reflect the variation in the annual

acorn crop.

In chapter 3, I examined the fitness consequences of helping behavior. Although

individual fitness of helpers did not differ from that of potential breeders (individuals that

dispersed as yearlings without helping), female acorn woodpeckers that successfully

reproduced at age 1 had a significantly higher individual fitness (k) than those that helped

and delayed age of first breeding until age 2 or later. This difference was due to a delayed

onset of reproduction for helpers. Furthermore, yearling dispersers who did not









successfully reproduce until age 2 or later did not have a significantly greater fitness than

helpers (who also did not begin breeding until age 2 or later).

Four factors significantly influenced X: the acorn crop prior to birth, two measures

of territory quality (lifetime average, and year of first reproductive event), and the

number of co-breeding females in the group when the individual began breeding.

Females living on higher quality territories showed higher fitness, but individuals who

shared reproduction with other females suffered a fitness loss.

In conclusion, my study has shown that helping in acorn woodpeckers generally is

a route to lower fitness if dispersers successfully reproduce as yearlings. However, if

successful reproduction is not possible staying at home is the next best strategy. In years

with large acorn crops, more birds are able to acquire breeding positions within a group

and fitness is increased on territories with larger granaries in which to store these acorns.

When the acorn crop fails, the likelihood of breeding is significantly decreased and

staying at home is suitable in the face of such constraints. These inferences are made

based only on direct fitness, and the results may differ if inclusive fitness were

considered. Therefore, inclusive fitness calculations should be a main target for future

research in this and other cooperative breeding systems.
















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BIOGRAPHICAL SKETCH

Justyn Stahl was born on September 13, 1980, in Alton, Illinois. He is the oldest

son of Ralph and Bette Stahl. After graduating from Alton High School in 1998, he

moved to Miami, Florida, to attend the University of Miami where he received his

bachelor's degree in biology in 2002. He then spent two field seasons working on

prothonotary warblers in Southern Illinois. He moved to Gainesville, Florida, and the

University of Florida in the fall of 2003.