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POPULATION ANALYSIS AND ROOSTING- AND
FEEDING-FLOCK BEHAVIOR OF BLACKBIRDS
DAMAGING SPROUTING RICE IN
SOUTHWESTERN LOUISIANA
I*4jL 1
RONALD
KRISTIN
F. LABISKY
E. BRUGGER
FLORIDA COOPERATIVE FISH AND WILDLIFE RESEARCH UNIT
TECHNICAL REPORT NO. 36
JUNE 1989
040pk
FINAL REPORT
Population Analysis and Roosting- and Feeding-flock Behavior
of Blackbirds Damaging Sprouting Rice
in Southwestern Louisiana
Ronald F. Labisky, Principal Investigator
and
Kristin E. Brugger, Associate Investigator
Department of Wildlife and Range Sciences
118 Newins Ziegler Hall
University of Florida
Gainesville, FL 32611-0304
Supported by
U. S. Department of Agriculture
Animal and Plant Health Inspection Service
Denver Wildlife Research Center
and
U. S. Department of the Interior
Fish and Wildlife Service
Florida Cooperative Fish and Wildlife Research Unit
Cooperative Agreement No. 14-16-0009-1544
Research Work Order No. 23
June 30, 1989
Citation:
R. F. Labisky, and K. E. Brugger. 1989. Population analysis and roosting- and
feeding-flock behavior of blackbirds damaging sprouting rice in
southwestern Louisiana. Florida Coop. Fish Wildl. Res. Unit, Tech. Rep.
No. 36. 77 pp.
EXECUTIVE SUMMARY
1. Rice is planted annually in southwestern Louisiana from mid-March
through late May. Blackbirds, primarily female red-winged blackbirds
(Aqelaius phoeniceus), cause localized damage to early-planted fields by
eating seed and pulling sprouts, possibly reducing or destroying stand
establishment. Financial losses ensue due to decreased yield, reduced grade
or price at the mill, and increased costs for replanting. There are no
studies that objectively quantify losses of the rice crop caused by blackbird
damage.
2. A 3-year field study, 1986-1988, was conducted during the early
planting season near one large blackbird roost, Millers Lake, Evangeline
Parish, Louisiana. The 3 objectives of the study were to: (1) identify the
geographic origin of red-winged blackbirds responsible for damage to rice
crops; (2) estimate size, composition, and stability of the Millers Lake
roost; and (3) describe feeding and movement patterns of flocks during the
damage period (planting season).
3. Migrant and nonmigrant redwings from across North America winter at
roosts in southwestern Louisiana. Northward migration begins in mid-February.
However, banding analyses revealed that some migrants (from as far away as
Michigan) may occur in southwestern Louisiana during planting season.
Morphometric data from a sample of approximately 500 red-winged blackbirds
collected at and surrounding Millers Lake during the spring season of 1986,
1987, and 1988 suggested that, after March 20 each year, >80% of adult males
and variable proportions (42-53% in 1986, 25-50% in 1987, and >80% in 1988) of
females were resident Louisiana birds.
4. The roost at Millers Lake declined in a negative exponential pattern
from about 15 million birds in mid-February to 20,000 in late April each year.
Female redwings were proportionally more abundant in March and April than were
cowbirds (Molothrus after starlings (Sturnus vulgaris), or common grackles
(Quiscalus quiscula). Two patterns of decline in roost size occurred during 6
years for which data were available: the roost, as of April 1, contained
either 500,000 or <100,000 birds. Turnover at the roost appeared to be high
during the planting season because of continued migration of females
throughout April.
5. The number of flocking birds observed feeding in fields declined with
advancing season, and was correlated with roost size. Mean flock size and
number of flocks observed per road survey also declined with season. Female
redwings were the most abundant group observed during road surveys. Sex
ratios of red-winged blackbirds observed per route were female-dominated in 24
of 28 surveys conducted during the 3 spring seasons of study. However,
second-year males were observed in male-dominated flocks, and thus, may
contribute to rice damage. There was no evidence of stability in flock
membership and only weak evidence of fidelity to feeding site.
6. Recommendations for mitigation of blackbird damage to spring-planted
rice depend on the severity and locations of damage. The lethal control of
blackbirds, although expensive, may be justified in specific rice fields
located near roosts. If large-scale population reduction is deemed necessary,
control efforts must focus on resident birds. The best recommendations are
those of the Louisiana State Agricultural Center (1987): delay rice planting
until after March 24 (or after April 1 for fields near large roosts); drill-
plant or water-plant only if a continuous water-flood of 2 to 6 inches can be
maintained; simultaneously plant as many fields as possible; and clear farms
of brush--a practice not conducive to the enhancement of many other wildlife
species. A critical need exists for a current cost-benefit analysis of crop
losses and damage control alternatives.
ACKNOWLEDGMENTS
Many people contributed to the work performed under this contract. Drs.
N. R. Holler and H. F. Percival, who developed the Cooperative Education
Agreement and research contract between the Denver Wildlife Research Center
(formerly under the U. S. Fish and Wildlife Service, now with U. S. Department
of Agriculture, Animal and Plant Health Inspection Service) and the Department
of Wildlife and Range Sciences, University of Florida, contributed time and
energy to the project. The Florida Cooperative Fish and Wildlife Research
Unit provided clerical and logistical support.
In Louisiana, E. A. Wilson (Louisiana State University Rice Research
Station, Crowley), E. A. LeBoeuf, and D. J. Leblanc (USDA/APHIS/ADC State
Office, Crowley and Baton Rouge) provided valuable advice and assisted with
logistical support throughout the project. The rice farmers of Evangeline
Parish, especially Mssrs. C. Fruge, and C. Veillon, both of Ville Platte,
allowed access not only to their fields, camps, docks, and homes, but also
provided information about blackbird damage to sprouting rice. Mr. G. Miller
(President, Millers Lake Association) and Mr. J. Shipp permitted access to
Millers Lake for collection of birds. Mr. A. Mire (LSU Agricultural Extension
Service, Ville Platte) provided background information about rice culture
practices and other agricultural activities in the Parish.
Assistance in field work and in collecting birds was provided by D.
Daneke and his retriever, Miss Kitty, and N. Dwyer, (Department of Wildlife
and Range Sciences, University of Florida), and C. Nelms and his retriever,
Jake, P. Lefebvre, R. Matteson, D. Decker, and J. Glahn (USDA/APHIS Denver
Wildlife Research Center). L. Whitehead typed drafts of the final report.
Jo Ann Monaco produced the final copy on the Word-Processor. Advice on
statistical analyses and software applications were provided by
S. Colbert, S. deSouza, W. Hubbard, J. Smallwood, H. Tiebout III, and E.
Warrag. M. L. Avery, R. A. Dolbeer, C. 0. Nelms, H. M. Tiebout III, and E. A.
Wilson reviewed drafts of the manuscript.
Sincere appreciation is extended to Drs. R. A. Dolbeer and M. L. Avery,
USDA/APHIS Denver Wildlife Research Center, for encouragement, support, and
technical advice.
TABLE OF CONTENTS
Executive summary
Acknowledgments . .
List of tables . .
List of figures . .
Chapter 1. Introduction .
Chapter 2. Overview of bird d
Rice farming .
Economy .
Annual sequence
Rice planting
Damage to sprout
Blackbird roosts
Chapter 3.
Chapter 4.
The study area: Mi
The study area
Millers Lake bla
Grain production
Migratory and nonn
blackbird populati
Introduction .
Methods . .
Banding data
Morphology .
Results . .
Banding data
Morphology .
Discussion .
Roost dynamics frc
Introduction .
Methods . .
Weather .
Roost size and
Spatial pattern
. . . . . iv
amage in planted rice in Louisiana .
of rice production . . .
ing rice and seedling protection .
. . . . . .
llers Lake as an area of special concern
Lckbird roost . . . .
migratory components of the red-winged
on . . . . .
m late winter through spring . .
position . . . .
of ro sting . . . .
s of roosting . . . .
Page
viii
. x
. 1
S3
S3
S3
S3
S5
S6
S8
9
S9
. 12
. 12
. 16
. 16
. 18
S18
. 19
. 21
. 21
. 22
S35
. 39
. 39
. 40
S40
40
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Page
Results . . . ... ....... 43
Spring weather patterns . . . .... 43
Roost size and composition . . . .... 44
Spatial pattern of roosting . . . .... 48
Other roosts . . . . .. ..... 48
Discussion . . . . ... .. ... . 50
Chapter 5. Foraging flock dynamics . . . .... 54
Introduction . . . . ... .... 54
Methods . . . . . . 54
Flock size, composition, and distribution . ... .54
Land-use patterns . . . . ... .. 55
Individual movements . . . .... 55
Statistical analyses . . . .... 56
Results . .. . ... . . 56
Flock size, composition, and distribution . ... .56
Land-use patterns . . . . ... .. 62
Individual movements . . . .... 62
Discussion . . . . . .. 66
Chapter 6. Summary and recommendations . . . ... 69
Literature Cited . . . . .. . . . 73
LIST OF TABLES
Page
Table 2-1.
Table 2-2.
Table 2-3.
Table 3-1.
Table 3-2.
Table 3-3.
Table 3-4.
Table 3-5.
Table 3-6.
Table 3-7.
Table 3-8.
Summary of results of 5 National Roost Surveys conducted
in winter by the U. S. Fish & Wildlife Service between
1962-63 and 1979-80 . . . . .
Summary of blackbirds and starlings estimated in annual
Christmas Bird Counts (CBC) conducted from Pine Prairie,
Louisiana, between 1974 and 1987 . . . .
Seasonal change in bird numbers and species composition
of blackbirds and starlings roosting at Millers Lake,
Evangeline Parish, Louisiana, during (1) winter 1974-75,
(2) spring 1979, and (3) fall 1979-spring 1980 . .
State of origin of red-winged blackbirds banded in North
America between May 1-September 30, 1924-85, and recovered
(banding station data excluded) in southwestern Louisiana
during 6 rice culture periods . . . .
State of origin of red-winged blackbirds banded in North
America between May 1-September 30, 1924-85, and recovered
(banding station data included) in southwestern Louisiana
during 6 rice culture periods . . . .
Yearly collections of red-winged blackbirds by age/sex
class and 10-day intervals, Evangeline Parish, Louisiana,
1986-1988 . . . . . . .
Means ( SD) of 10 traits of body size in second-year male
(SYM) red-winged blackbirds collected by shooting in
Evangeline Parish, Louisiana, during 6, 10-day collection
intervals, 1986-1988 . . . . .
. 10
. 13
S. 14
. 23
. 24
. 25
. 31
Means ( SD) of 10 traits of body size in second-year
female(SYF) red-winged blackbirds collected by shooting
in Evangeline Parish, Louisiana, during 6, 10-day
collection intervals, 1986-1988. . .... .. ..... 32
Means ( SD) of 10 traits of body size in after-second year
(ASY) male red-winged blackbirds collected by shooting in
Evangeline Parish, Louisiana, during 6, 10-day collection
intervals, 1986-1988 . . . . .
Means ( SD) of 10 traits of body size in after hatching-
year (AHY) female red-winged blackbirds collected by shooting
in Evangeline Parish, Louisiana, during 6, 10-day collection
intervals, 1986-1988 . . . . .
Percentage of nonresident red-winged blackbirds in the
Millers Lake area of Evangeline Parish, Louisiana, during 6,
10-day collection intervals in spring, 1986-1988 . .
. 33
. 34
. 36
viii
Table 4-1.
Table 4-2.
Table 4-3.
Table 5-1.
Table 5-2.
Table 5-3.
Table 5-4.
Table 5-5.
Table 5-6.
Table 5-7.
Estimates of roost size at Millers Lake, Evangeline Parish,
Louisiana, mid-February to late-April, 1986-88 . .
Multiple linear regression analyses relating log-
transformed roost size to Julian calendar date and 2 weather
variables (mean 3-day maximum air temperature [max] and
summed 3-day rainfall [rain]) at Millers Lake, Evangeline
Parish, Louisiana, for the period February 15-April 30,
1986-88 . . . . . .
Weekly estimates of percentage composition of starlings and
blackbirds in the Millers Lake roost, Evangeline Parish,
Louisiana, 1986-88 . . . . .
Total number of birds and flocks, and mean (SD) number of
birds per 3-min stop and flock, recorded along north and
south survey-routes, Evangeline Parish, Louisiana, 1986-
1988 . . . . . . .
Temporal distribution of flock sizes observed along north
and south survey-routes, Evangeline Parish, Louisiana, 1986-
1988. Percentage of total is given only for flocks with
<25 birds . . . . . .
Percentage species composition of birds observed along north
and south survey-routes, Evangeline Parish, Louisiana, 1986-
1988 . . . . . . .
Sex ratios of red-winged blackbirds observed along north
and south survey-routes, Evangeline Parish, Louisiana, 1986-
1988 . . . . . . .
Percentage of red-winged blackbird flocks engaged in
different activities (when first observed) in relation to
habitat types along north and south survey-routes, Evangeline
Parish, Louisiana, 1986-88 . . . .
Area (ha) of cropland by stage of rice production along the
north and south survey-routes, Evangeline Parish, Louisiana,
1987 . . . . . . .
Summary of movements of 6 radio-instrumented red-winged
blackbirds that were monitored between 17 and 28 March 1986
in Evangeline Parish, Louisiana . . . .
Page
. 45
. 47
. 49
. 57
. 59
. 60
. 61
. 63
. 64
. 65
LIST OF FIGURES
Page
Figure 2-1.
Figure 3-1.
Figure 3-2.
Figure 3-3.
Figure 3-4.
Figure 4-1.
The 8-parish region of southwestern Louisiana in which 75%
of the state's rice crop is produced annually. . . 4
Box-and-whisker plot of the measurement for wing chord by
year and 10-day collection interval for second-year male
(SYM) red-winged blackbirds collected near Millers Lake,
Evangeline Parish, Louisiana, 1986-88. . . ... 26
Box-and-whisker plot of the measurement for wing chord by
year and 10-day collection interval for second-year female
(SYF) red-winged blackbirds collected near Millers Lake,
Evangeline Parish, Louisiana, 1986-88. . . ... 27
Box-and-whisker plot of the measurement for wing chord by
year and 10-day collection interval for after second-year
male (ASY) red-winged blackbirds collected near Millers Lake,
Evangeline Parish, Louisiana, 1986-88. . . ... 28
Box-and-whisker plot of the measurement for wing chord by
year and 10-day collection interval for after hatching-year
female (AHY) red-winged blackbirds collected near Millers
Lake, Evangeline Parish, Louisiana, 1986-88. . ... 30
Negative exponential decline in size of the blackbird roost
during the period February 15-April 24, Millers Lake,
Evangeline Parish, Louisiana, 1986-1988. . . ... 46
CHAPTER 1: INTRODUCTION
Flocking blackbirds, primarily red-winged blackbirds (Aqelaius
phoeniceus), damage sprouting and ripening rice in the southern United States
by pulling new sprouts or removing ripening grains (Meanley 1971). Damage,
which is often localized in regions near blackbird roosts (Neff and Meanley
1957, Pierce 1970), may result in heavy financial losses (Holler et al. 1982).
Damage to spouting rice is particularly severe in early-planted rice in
southwestern Louisiana and east Texas (Kalmbach 1937, Besser 1973); hence,
there is a strong regional interest in development of techniques for
mitigation of bird damage during the early weeks of spring planting. Scare
devices, repellents, and toxicants have been used to reduce numbers of birds
in rice fields temporarily; however, a long-term solution is desired.
Blackbird population reduction has been proposed as a long-term method for
reducing damage to agricultural crops (Wright et al. 1980, Dolbeer 1986, Stehn
1989), and also has been suggested as a solution by local rice growers'
associations, but the information needed to develop a long-term bird
management program is sparse.
Before the effectiveness of alternative management strategies for reducing
damage to spring-planted rice by blackbirds southwestern Louisiana can be
evaluated, those components of the population responsible for damage--as well
as their patterns of roosting and flocking--must be identified. Answers to
the following questions would provide a strong foundation for development of a
blackbird management program in the region: (1) Are birds that damage rice in
spring of local or migratory origin, and if migratory, from what region do
birds that cause damage derive? (2) Are roosts stable in numbers and species
composition preceding and during the damage period? (3) Are feeding flocks
stable with respect to membership (individual birds)? (4) Are feeding flocks
consistent in their selection of feeding locations?
These questions provided the rationale for conducting a 3-year field study
of blackbird depredations on planted rice in southwestern Louisiana. The
study, conducted during the late winter and spring seasons of 1986, 1987, and
1988, had 3 major objectives:
(1) To determine the geographic source of populations of red-winged
blackbirds damaging sprouting rice in southwestern Louisiana;
(2) To determine the stability of roosting populations of blackbirds in
southwestern Louisiana preceding and during the period that sprouting
rice is susceptible to damage; and
(3) To determine the stability of feeding flocks of blackbirds damaging
sprouting rice.
This report is designed to (1) present an overview of rice farming
techniques and blackbird roost locations in southwestern Louisiana, focusing
specifically on one large winter blackbird roost in Evangeline Parish (Millers
Lake), (2) identify the component of the spring blackbird population in
southwestern Louisiana that causes damage to newly planted rice, (3)
characterize the size and composition of roosting blackbird populations in the
region, (4) describe the spatial and temporal distribution of flocks in rice
fields near Millers Lake in relation to rice farming practices, and (5) to
provide recommendations for mitigation of damage to newly planted rice by
blackbirds.
CHAPTER 2: OVERVIEW OF BIRD DAMAGE TO PLANTED RICE IN LOUISIANA
RICE FARMING
Economy
Rice production is a major industry in Louisiana. In 1987, approximately
100,000 t of rice (15% of the United States' production), valued at $200
million (Louisiana State University 1988), were produced on 210,000 ha. The
land area planted to rice and value of rice production in Louisiana peaked in
1981 at about 303,000 ha and $254 million, respectively (Fielder and Nelson
1984). Approximately 75% of the state's annual rice harvest is produced in 8
parishes (counties) of southwestern Louisiana (Fig. 2-1).
Annual Sequence of Rice Production
Planting dates are largely weather-dependent and can range from late
February through May; most fields, however, are planted between mid-March and
mid-April (P. Seilhan, Louisiana Coop. Ext. Serv., Crowley, pers. commun.).
Germination and seedling stand establishment require 1 to 4 weeks, depending
on environmental conditions; growth rates are notably depressed by cool
weather. Most varieties of medium- and long-grain rice planted in
southwestern Louisiana reach harvest maturity within 120 to 135 days from
planting; accordingly, the first harvest usually peaks in August. Regrowth
sometimes produces a second crop, called ratoon rice, which is harvested in
CHAPTER 2: OVERVIEW OF BIRD DAMAGE TO PLANTED RICE IN LOUISIANA
RICE FARMING
Economy
Rice production is a major industry in Louisiana. In 1987, approximately
100,000 t of rice (15% of the United States' production), valued at $200
million (Louisiana State University 1988), were produced on 210,000 ha. The
land area planted to rice and value of rice production in Louisiana peaked in
1981 at about 303,000 ha and $254 million, respectively (Fielder and Nelson
1984). Approximately 75% of the state's annual rice harvest is produced in 8
parishes (counties) of southwestern Louisiana (Fig. 2-1).
Annual Sequence of Rice Production
Planting dates are largely weather-dependent and can range from late
February through May; most fields, however, are planted between mid-March and
mid-April (P. Seilhan, Louisiana Coop. Ext. Serv., Crowley, pers. commun.).
Germination and seedling stand establishment require 1 to 4 weeks, depending
on environmental conditions; growth rates are notably depressed by cool
weather. Most varieties of medium- and long-grain rice planted in
southwestern Louisiana reach harvest maturity within 120 to 135 days from
planting; accordingly, the first harvest usually peaks in August. Regrowth
sometimes produces a second crop, called ratoon rice, which is harvested in
CHAPTER 2: OVERVIEW OF BIRD DAMAGE TO PLANTED RICE IN LOUISIANA
RICE FARMING
Economy
Rice production is a major industry in Louisiana. In 1987, approximately
100,000 t of rice (15% of the United States' production), valued at $200
million (Louisiana State University 1988), were produced on 210,000 ha. The
land area planted to rice and value of rice production in Louisiana peaked in
1981 at about 303,000 ha and $254 million, respectively (Fielder and Nelson
1984). Approximately 75% of the state's annual rice harvest is produced in 8
parishes (counties) of southwestern Louisiana (Fig. 2-1).
Annual Sequence of Rice Production
Planting dates are largely weather-dependent and can range from late
February through May; most fields, however, are planted between mid-March and
mid-April (P. Seilhan, Louisiana Coop. Ext. Serv., Crowley, pers. commun.).
Germination and seedling stand establishment require 1 to 4 weeks, depending
on environmental conditions; growth rates are notably depressed by cool
weather. Most varieties of medium- and long-grain rice planted in
southwestern Louisiana reach harvest maturity within 120 to 135 days from
planting; accordingly, the first harvest usually peaks in August. Regrowth
sometimes produces a second crop, called ratoon rice, which is harvested in
LOUISIANA
S : ;
US DOmntm o Cmmdre
I a*MU O Tr CNSUS
AD-400.16 (6/85)
Figure 2-1. The 8-parish region of southwestern Louisiana in which 75% of the
state's rice crop is produced annually. Millers Lake is located in the center
of Evangeline Parish.
U C f
or or f I 7 1 I 9W* W
I
--------
October. Many farmers believe that early planting results in early first
harvest, which, in turn increases the likelihood of a successful ratoon crop.
Recent research at the Louisiana State University Rice Experiment Station
has examined the potential for double cropping rice, by planting a second crop
in early August after first harvest, rather than allowing ratoon growth
(Dunand et al. 1985, Bollich et al. 1987). If this cropping
procedure proves successful with respect to yield/profitability, early spring
planting may be promoted in the future in order to facilitate the growing of a
successful second crop.
Rice Planting
Rice is planted either by water-seeding or dry-seeding, each with
variations. Rice may be sown aerially into flooded fields at rates of 100 to
170 kg/ha, or it may either be drilled in 18- to 25-cm rows or broadcast dry
at rates of 100 to 125 kg/ha. Water-seeded fields usually are drained
immediately after germination to promote "pegging" or root establishment.
Dry-seeded fields are flooded after seeding to promote germination, and then
drained for pegging. Some fields are "pin-point" flooded once seedlings have
rooted, i. e., after pegging, water is applied and the level is increased
slowly to match seedling growth. All fields, irrespective of planting method,
are usually flooded after stand establishment. Many farmers prefer water-
seeding to dry-seeding because the typically wet conditions of soil during
spring can delay field preparation required for dry-seeding. Also, the
preparation of the seed bed is less intensive and less costly for water- than
for dry-seeding. As an additional benefit, water-seeding suppresses weed
growth, especially that of red rice.
Damage to Sprouting Rice and Seedling Protection
After the water is drained from planted fields, the germinating seeds lie
on or near the soil surface, making them readily available to birds. Drilled
seed is less vulnerable than broadcast seed because a layer of soil covers the
grain. Stand establishment is affected by many factors, including weather
(temperature, wind, rain), disease, invertebrate pests, and vertebrate pests
--particularly blackbirds. Blackbirds damage rice plantings principally by
pulling the weakly rooted seedlings to obtain the grain. Seedlings are
vulnerable to blackbirds until the kernel is absorbed, which occurs when the
plant is about 8-cm tall or approximately 2-4 weeks of age (Wilson 1986).
Early planting exacerbates the bird-damage problem because, in addition to
resident blackbirds, large numbers of wintering migrant birds still remain in
the region, thus augmenting the abundance of blackbirds that feed in planted
rice. Goodloe (1983) reported that farmers who planted prior to mid-March
lost an average of 25% of the seed to birds. Seed losses >99% have been
reported in fields planted in mid-March, whereas fields planted during mid-
April may suffer less than 5% seed loss (Holler et al. 1986; Wilson et al.
1986, 1987, 1989).
Intensive efforts may be needed for field protection throughout the period
of stand establishment. Several methods are used to varying degrees to
prevent or reduce bird damage: gas-powered exploders are set in field edges;
field hands shoot "shell crackers" and live ammunition at flocks; scarecrows
are placed in fields; and chemical repellents are applied to seed or bait.
Toxic chemicals, such as Aldrin@, were once used on seed or bait as the
primary means to reduce blackbird damage in rice fields because of their low
cost and high effectiveness. However, the Environmental Protection Agency
(EPA) rescinded registration of such chemicals in 1975. Subsequent research
into toxicants and repellents has focused on Avitrol@ (Mott et al. 1982) and
Mesurol* (Mott et al. 1976, Holler et al. 1982). Effective repellency was
obtained with Methiocarb, but, currently, it is not registered for use on rice
seed. Since the mid-1970s, farmers primarily have used exploders and shooting
to attempt to protect fields from bird damage.
The loss of seedlings represents a potential for financial loss due to
reduced yield, and, if losses are severe, the need for replanting. Also if
stands of planted rice are thinned significantly by bird damage, openings that
allow weed growth often develop. Weed seeds (foreign material) may
contaminate harvested rice, thereby resulting in lower grades and prices at
the mill. If replanting is necessary, the harvest may be delayed, which may
result in a lower market price and a reduced probability for a ratoon crop.
The cost of replanting fields is estimated to be $100/ha for water-seeded
crops (Zacharias and McManus 1987). This value is lower than those presented
by Holler et al. (1982), who reported costs to replant fields heavily damaged
by blackbirds were $100-112/ha ($40-45/acre) for both water-planted and
drilled fields if no tilling were required, $125-137/ha ($50-55/acre) if light
tilling were required, and $150-162/ha ($60-65/acre) if heavy tilling were
required. These costs were based on estimates for owner-operated farms, with
basic costs of seed at $70/ha ($35/acre), and did not include possible costs
associated with losses in yield or grade due to late planting.
Unfortunately, little information is available to estimate overall
financial losses resulting from blackbird damage in southwestern Louisiana,
which is, in part, due to the lack of a cost-effective method to assess damage
in fields (Otis et al. 1983). However, based on crop insurance statistics for
1983, loss of seed rice to wildlife was minimal when compared with other
annual causes of rice loss, i. e., <2% of the total dollar amount of claims
(U. S. Dep. Agric., Fed. Crop Insurance Progr., unpubl. data).
Mail-questionnaires directed to local farmers have been used to estimate
losses of rice seed to blackbirds in Louisiana. In 1981, 63 of 71 (89%)
farmers responding to a survey in southwestern Louisiana reported that birds
were responsible for some damage to rice at one or more stages of development
(Goodloe 1983). Farmers reported losses of the newly planted seed that ranged
as high as 75% of the crop. Seed loss to birds was highest in Allen Parish,
averaging 27%; losses averaged <15% per parish in other parishes (Goodloe
1983). In a 1983 survey, 92% of 146 respondents from Vermillion Parish
reported blackbird depredation in the rice crop, and approximately 50%
reported that they suffered losses of sprouting rice to blackbirds severe
enough to require replanting (Linscombe 1983). Although losses were not
quantified, damage was localized and unevenly distributed among farms as
determined in a 1979 survey of 66 farmers in Acadia Parish (Naquin 1979).
These data are only suggestive inasmuch as responses may have been biased
towards those farmers receiving damage. However, if the pattern of localized
and uneven damage is valid, the cost of bird damage is unequally distributed
among farms/producers. Thus, differential spatial patterns of damage must be
considered in the development of effective control measures for reducing
blackbird damage to rice.
BLACKBIRD ROOSTS
In autumn, blackbirds and starlings across North America migrate south and
congregate in roosts, most of which form south of 38 N. latitude (Meanley
1971). Four species prevail in these roosts: common grackle (Quiscalus
quiscula); red-winged blackbird; brown-headed cowbird (Molothrus after ; and
European starling (Sturnus vulgaris). Seven National Roost Surveys (NRS) were
conducted by the U. S. Fish & Wildlife Service between 1958 and 1980 for the
purpose of mapping the locations and estimating the sizes of winter blackbird
and starling roosts. Although the data are difficult to compare among years
and locations, due to inconsistent observer efforts, the surveys provide
information on the location, size, and species composition of many roosts in
the eastern United States, and specifically in the Louisiana-Texas rice belt
(Table 2-1).
Common grackles and red-winged blackbirds are typically most abundant in
the eastern U. S., each comprising slightly less than 1/3 of individuals
identified to species (Table 2-1). Major roosts (> 1 million) were
concentrated along the lower Mississippi River, south of the confluence of the
Ohio and Missouri Rivers. Major roosts in the rice belt of southwestern
Louisiana and east Texas were found along coastal marshes of the Gulf of
Mexico and inland along the Mississippi River.
THE STUDY AREA: MILLERS LAKE AS AN AREA OF SPECIAL CONCERN
One roost, Millers Lake, near Ville Platte in Evangeline Parish (Fig. 2-
1), was chosen as an exemplar study roost because it is large, accessible,
close to rice fields, and is used year-round by roosting blackbirds. Hence,
birds are in the region throughout the rice-growing season. Bird damage to
rice fields has been reported from the region since the early 1970s.
The Study Area
The study area comprises Millers Lake and adjacent farmlands, principally
those to the south. Millers Lake is a 2,500 ha shallow, man-made impoundment,
with a maximum axis of 5 km. The lake is located at the boundary of the
European starling (Sturnus vulgaris). Seven National Roost Surveys (NRS) were
conducted by the U. S. Fish & Wildlife Service between 1958 and 1980 for the
purpose of mapping the locations and estimating the sizes of winter blackbird
and starling roosts. Although the data are difficult to compare among years
and locations, due to inconsistent observer efforts, the surveys provide
information on the location, size, and species composition of many roosts in
the eastern United States, and specifically in the Louisiana-Texas rice belt
(Table 2-1).
Common grackles and red-winged blackbirds are typically most abundant in
the eastern U. S., each comprising slightly less than 1/3 of individuals
identified to species (Table 2-1). Major roosts (> 1 million) were
concentrated along the lower Mississippi River, south of the confluence of the
Ohio and Missouri Rivers. Major roosts in the rice belt of southwestern
Louisiana and east Texas were found along coastal marshes of the Gulf of
Mexico and inland along the Mississippi River.
THE STUDY AREA: MILLERS LAKE AS AN AREA OF SPECIAL CONCERN
One roost, Millers Lake, near Ville Platte in Evangeline Parish (Fig. 2-
1), was chosen as an exemplar study roost because it is large, accessible,
close to rice fields, and is used year-round by roosting blackbirds. Hence,
birds are in the region throughout the rice-growing season. Bird damage to
rice fields has been reported from the region since the early 1970s.
The Study Area
The study area comprises Millers Lake and adjacent farmlands, principally
those to the south. Millers Lake is a 2,500 ha shallow, man-made impoundment,
with a maximum axis of 5 km. The lake is located at the boundary of the
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piedmont and coastal plain. Pine forests lie north, and agricultural fields
south of the lake. Lake water is derived from rainfall, seepage from the
rolling hills to the north, drainage from bayous to the west, and 2 wells.
Millers Lake originated as a shallow marsh that collected runoff from
rolling country to the north. The following history of the lake is reported
as described by Mr. and Mrs. Clifton Fruge, and Mr. Charles Veillon, residents
adjacent to the lake and members of the LaHaye and Miller families,
respectively. By the turn of the century, a levee was constructed in a joint
effort by the Miller and LaHaye families to retain water for flooding over a
large area for the production of rice, a crop introduced to the region in the
late 1800s. The lake was enlarged in the 1930s. And, in 1957, the U. S. Army
Corps of Engineers constructed a 2-m fortified levee to contain the lake
subsequent to hurricane-caused flooding. Two water wells were constructed on
the east shore of the lake after a drought in 1976, and have been used
occasionally to resupply water to the lake. Millers Lake provides irrigation
water to approximately 3,000 ha of rice fields south of the lake.
The lake is deepest at the southwest. Water depths are variable during
the year. In winter, water depths in the southwest portion of the lake range
from 2 to 3.5 m; in late spring, after rice fields are flooded, water depth
may be < 2 m. During drought periods, water depths have dropped to 1.5 m,
leaving exposed land inside the levees.
Millers Lake is in advanced stages of succession. Approximately 70% of
the lake is covered with woody vegetation, such as buttonbush (Cephalanthus
occidentalis) and tupelo (Nvssa aquatica). Thick mats of aquatic weeds grow
throughout the open water in the southwest portion of the lake. The Millers
Lake Association, the administrative body that oversees business related to
the lake, does not control woody vegetation in the lake--believing that it
reduces wave action, thereby controlling erosion and stabilizing the levees.
A detailed description of lake vegetation is provided by Ortego (1976).
Millers Lake Blackbird Roost
Millers Lake hosts a large winter blackbird and starling roost annually.
The roost first was included in National Roost Surveys in 1974-75 (Meanley
1976). Since 1974, the roost also has been included in the Pine Prairie Route
of the Audubon Christmas Bird Count; these counts yielded >100 million
blackbirds in 1986 and 1987 (Table 2-2), most of which were found at Millers
Lake (B. Ortego, Texas Parks and Wildlife, Jasmine, pers. commun).
Inconsistent observer effort or duplicate counting of birds may have
contributed to the high estimates reported in these Christmas Bird Counts.
Individual-observer censuses at Millers Lake, however, revealed that peak
winter populations of blackbirds, which occurred during January and February,
numbered about 15 million (Table 2-3). The species composition of the roost
changes during the winter, with red-winged blackbirds being relatively most
abundant early and late in the season, and cowbirds being relatively most
abundant in mid-winter (Table 2-3).
Grain Production
Historically, Evangeline Parish was a cotton-growing region. By the
1930s, rice had become a major cash crop. Five grains are currently produced
in Evangeline Parish: soybeans, rice, corn, sorghum, and wheat. Land area
planted to grains has decreased in the last decade from approximately 90,000
ha in the late 1970s-early 1980s to about 50,000 ha in 1987. This reduction
in cropland was due largely to a decreasing number of active farmers in the
parish and to the increasing participation by farmers in land conservation
reduces wave action, thereby controlling erosion and stabilizing the levees.
A detailed description of lake vegetation is provided by Ortego (1976).
Millers Lake Blackbird Roost
Millers Lake hosts a large winter blackbird and starling roost annually.
The roost first was included in National Roost Surveys in 1974-75 (Meanley
1976). Since 1974, the roost also has been included in the Pine Prairie Route
of the Audubon Christmas Bird Count; these counts yielded >100 million
blackbirds in 1986 and 1987 (Table 2-2), most of which were found at Millers
Lake (B. Ortego, Texas Parks and Wildlife, Jasmine, pers. commun).
Inconsistent observer effort or duplicate counting of birds may have
contributed to the high estimates reported in these Christmas Bird Counts.
Individual-observer censuses at Millers Lake, however, revealed that peak
winter populations of blackbirds, which occurred during January and February,
numbered about 15 million (Table 2-3). The species composition of the roost
changes during the winter, with red-winged blackbirds being relatively most
abundant early and late in the season, and cowbirds being relatively most
abundant in mid-winter (Table 2-3).
Grain Production
Historically, Evangeline Parish was a cotton-growing region. By the
1930s, rice had become a major cash crop. Five grains are currently produced
in Evangeline Parish: soybeans, rice, corn, sorghum, and wheat. Land area
planted to grains has decreased in the last decade from approximately 90,000
ha in the late 1970s-early 1980s to about 50,000 ha in 1987. This reduction
in cropland was due largely to a decreasing number of active farmers in the
parish and to the increasing participation by farmers in land conservation
Table 2-2. Summary of blackbirds and starlings estimated in annual Christmas
Bird Counts (CBC) conducted from Pine Prairie, Louisiana, between 1974 and
1987 (data from annual issues of American Birds). A single asterisk (*)
indicates the highest count per species in all CBC routes run in a given year
in the United States and Canada, as compiled by Burt L. Monroe, Jr., and
reported in annual issues of American Birds. Two asterisks (**) indicate the
all-time highest number of a species ever recorded in one count among all
counts since 1900 (Monroe 1986).
Total number
(millions)
Starlings Birds
No. of Number (millions) and in
Year observers Starlings Grackles Redwings Cowbirds blackbirds count
1974 9 0.024 0.023 0.060 0.353 0.460 0.467
1975 11 0.003 0.064 0.377 5.019* 9.563a 9.582
1976 11 0.007 0.018 1.120 4.022* 5.167 5.189
1977 10 0.001 0.015 0.123 0.363* 0.502 0.534
1978 17 0.027 0.036 0.259 2.303* 2.625 2.652
1979 12 0.025 0.028 5.655 16.610 22.318 22.352
1980 15 0.162 0.011 0.081 0.038 0.292 0.160
1981 14 0.031 0.011 2.331 5.950* 8.323 8.354
1982 14 2.395* 0.588 14.907" 19.806* 37.696 37.726
1983 8 0.256* 1.101* 16.032 30.031* 47.400 47.426
1984 18 0.312 0.096 24.520* 10.113* 35.041 35.078
1985 12 1.625 2.095 39.800 19.600 63.120 63.142
1986 9 10.010* 10.032 50.403 38.201* 108.646 108.687
1987 12 3.000" 27.509*" 53.148" 20.005* 103.662 103.697
'In 1975, 4.1 million unidentified blackbirds
Prairie Christmas Bird Count.
were included in the Pine
Table 2-3. Seasonal change in bird numbers and species composition of
blackbirds and starlings roosting at Millers Lake, Evangeline Parish,
Louisiana, during (1) winter 1974-75 (Ortego 1976), (2) spring 1979, and (3)
fall 1979-spring 1980 (Wilson 1986). Trace numbers of birds are indicated by
the letter T.
Percentage composition
Date Roost size Starlings Grackles Redwings Cowbirds
Winter 1974-75
Oct 12 45,000 10 0 90 0
Nov 25 550,000 5 10 20 65
Dec 18 1,000,000 T 15 20 65
Jan 18 15,000,000 T 5 20 75
Feb 08 15,000,000 T 5 20 75
Mar 26 500,000 T 5 40 55
Spring 1979
Feb 15 7,200,000 T 5 30 65
Feb 28 8,700,000 T 5 30 65
Mar 13 2,800,000 0 T 60 40
Mar 27 200,000 0 0 90 10
Apr 10 50,000 0 0 90 10
Apr 25 25,000 0 0 90 10
May 07 2,000 0 0 100 T
Fall 1979-Spring 1980
Aug 27 5,000 0 0 95 5
Sep 11 35,000 0 T 75 25
Sep 24 52,000 0 T 75 25
Oct 16 75,000 T T 80 20
Oct 30 100,000 T T 80 20
Nov 15 400,000 5 T 60 35
Nov 27 450,000 5 T 60 35
Dec 08 1,700,000 10 T 40 50
Dec 21 5,500,000 10 5 30 55
Jan 07 8,600,000 15 5 25 55
Jan 23 11,200,000 15 5 25 55
Feb 06 13,800,000 10 5 20 65
Feb 20 15,400,000 10 5 20 65
Mar 03 9,000,000 5 5 25 65
Mar 17 2,900,000 T T 60 40
Mar 30 1,100,000 T T 80 20
Apr 14 250,000 0 T 90 10
Apr 28 15,000 0 T 99 T
May 14 2,000 0 0 99 T
programs. The relative proportion of land planted to each grain crop changed
during the last decade; notably, the acreage of soybeans decreased from 78% to
48%, whereas that of rice increased from 20% to 40%. In 1987, the gross gate
value of the rice crop in Evangeline Parish was estimated to be $24.8 million
(Louisiana State University 1988).
CHAPTER 3: MIGRATORY AND NONMIGRATORY COMPONENTS OF THE RED-WINGED
BLACKBIRD POPULATION AT MILLERS LAKE: WINTER AND SPRING
INTRODUCTION
Winter and spring roosts in the Gulf coastal region of Louisiana and Texas
attract blackbirds that originate in breeding populations from Alberta to
Maine, as well as birds that are summer residents of states bordering the Gulf
of Mexico (Meanley et al. 1966). Roosts dissolve at the onset of breeding
season. Resident birds of Louisiana begin territory establishment in early to
mid-April (Kalmbach 1937, Wilson 1986); the breeding season in northern North
America begins in late April (Ohio: 20 April, Dolbeer 1976; Quebec: early May,
Greenwood and Weatherhead 1982). Hence, some birds that breed north of
Louisiana might remain in the Gulf-coastal region through April, joining
resident birds at roosts and in feeding flocks in newly planted rice fields.
Consequently, the blackbirds that are responsible for damage to planted rice
in southwestern Louisiana during March and April are probably a mixture of
residents from the Louisiana-Texas Gulf coast and nonresidents from northern
breeding localities. To design and assess effectiveness of blackbird
management programs, the components) of the mixed northern and southern
populations responsible for damage in the region must be identified. This
information also would add to the understanding of population dynamics, extent
of geographic mixing among populations, and general population movements by
red-winged blackbirds.
CHAPTER 3: MIGRATORY AND NONMIGRATORY COMPONENTS OF THE RED-WINGED
BLACKBIRD POPULATION AT MILLERS LAKE: WINTER AND SPRING
INTRODUCTION
Winter and spring roosts in the Gulf coastal region of Louisiana and Texas
attract blackbirds that originate in breeding populations from Alberta to
Maine, as well as birds that are summer residents of states bordering the Gulf
of Mexico (Meanley et al. 1966). Roosts dissolve at the onset of breeding
season. Resident birds of Louisiana begin territory establishment in early to
mid-April (Kalmbach 1937, Wilson 1986); the breeding season in northern North
America begins in late April (Ohio: 20 April, Dolbeer 1976; Quebec: early May,
Greenwood and Weatherhead 1982). Hence, some birds that breed north of
Louisiana might remain in the Gulf-coastal region through April, joining
resident birds at roosts and in feeding flocks in newly planted rice fields.
Consequently, the blackbirds that are responsible for damage to planted rice
in southwestern Louisiana during March and April are probably a mixture of
residents from the Louisiana-Texas Gulf coast and nonresidents from northern
breeding localities. To design and assess effectiveness of blackbird
management programs, the components) of the mixed northern and southern
populations responsible for damage in the region must be identified. This
information also would add to the understanding of population dynamics, extent
of geographic mixing among populations, and general population movements by
red-winged blackbirds.
Several methods can be used to infer geographic origin of a mixed
population of wintering birds. Banding data can reveal broad patterns in
wintering and breeding regions, although small sample sizes and limitations of
the data base constrain estimates of movement patterns by individuals and/or
populations within short time intervals. Support for identification of
geographic origin derived from banding data can be obtained by more detailed
methods that focus on traits that vary geographically among individuals. One
potential method is a study of geographic variation in endoparasite loads,
which previously has been employed to identify (1) geographic origins of
different populations of Canada goose (Branta canadensis) (Hanson et al. 1957)
and (2) migratory and nonmigratory populations of wood ducks (Aix sponsa)
(Thul et al. 1985). However, because of low rates of infection by
endoparasites in red-winged blackbirds during winter, the method was judged
inappropriate for this study (Brugger 1989).
A morphometric study appeared to be the most promising means to obtain a
second line of evidence, as a supplement to banding analyses, to estimate
geographic origin of redwings; this approach is characterized by a strong
empirical basis, relatively small sample size required for inferences, and low
cost. Redwings in the eastern and central United States have a gradual, but
regular dine of increasing size northward and westward from Florida (Power
1970, James et al. 1984). Morphometrics have been applied to a comparative
data base from redwings collected in central and southeastern U. S. for
analysis of general migratory movements of populations (James et al. 1984).
Additionally, the method has been applied to monthly collections of redwings
from southwestern Louisiana to identify components of the population
responsible for damage to planted rice. Using discriminant analyses, Wilson
(1986) found that the proportion of large-bodied redwings (assumed to be
migrants to Louisiana) obtained from monthly collections declined from
approximately 80% in January to 22% and 4% in March and April, respectively.
Wilson (1986) inferred from morphometric data and road surveys of flocks that
resident female redwings were responsible for most damage to planted rice.
Although Wilson (1986) identified resident redwings as the primary cause
of bird damage to rice, his sample sizes were small and his collections were
not repeated among years. Thus, there was no accounting for spatial and
temporal variation in sex- and age-related migration patterns that had been
documented from banding data, i.e., adult males moved shorter distances from
their breeding areas and stayed at wintering sites for a shorter time than did
adult females or birds of the year (Dolbeer 1978, 1982). Hence, the dynamics
of interpopulation mixing by redwings in southwestern Louisiana during spring
might be more complex than that described by Wilson (1986). The objective of
the current investigation was to refine the identification of geographic
source areas of redwings in southwestern Louisiana by a comparison of 2
independent methods: (1) analysis of banding data to define source populations
by ascertaining the summering (assumed to be breeding) areas of redwings found
in the rice growing region in Louisiana throughout all cultural phases of the
crop production; and (2) identification of general geographic source areas
from morphometric measurements of birds collected in Millers Lake roost and in
rice fields during 3 spring seasons, 1986-88.
METHODS
Banding Data
A copy of the entire 62-year (1924-1985) recovery-retrieval file for red-
winged blackbirds was provided by the U. S. Fish & Wildlife Service Bird
Banding Laboratory (BBL). The file was tabulated by dates and states where
migrants to Louisiana) obtained from monthly collections declined from
approximately 80% in January to 22% and 4% in March and April, respectively.
Wilson (1986) inferred from morphometric data and road surveys of flocks that
resident female redwings were responsible for most damage to planted rice.
Although Wilson (1986) identified resident redwings as the primary cause
of bird damage to rice, his sample sizes were small and his collections were
not repeated among years. Thus, there was no accounting for spatial and
temporal variation in sex- and age-related migration patterns that had been
documented from banding data, i.e., adult males moved shorter distances from
their breeding areas and stayed at wintering sites for a shorter time than did
adult females or birds of the year (Dolbeer 1978, 1982). Hence, the dynamics
of interpopulation mixing by redwings in southwestern Louisiana during spring
might be more complex than that described by Wilson (1986). The objective of
the current investigation was to refine the identification of geographic
source areas of redwings in southwestern Louisiana by a comparison of 2
independent methods: (1) analysis of banding data to define source populations
by ascertaining the summering (assumed to be breeding) areas of redwings found
in the rice growing region in Louisiana throughout all cultural phases of the
crop production; and (2) identification of general geographic source areas
from morphometric measurements of birds collected in Millers Lake roost and in
rice fields during 3 spring seasons, 1986-88.
METHODS
Banding Data
A copy of the entire 62-year (1924-1985) recovery-retrieval file for red-
winged blackbirds was provided by the U. S. Fish & Wildlife Service Bird
Banding Laboratory (BBL). The file was tabulated by dates and states where
recovered or banded, How Obtained and Status codes, and sex for birds banded
or recovered in (1) Louisiana and (2) southwestern Louisiana (the
area bounded by 29* and 31" N. latitude and 91 and 94 W. longitude) (Brugger
and Dolbeer, in press).
Morphology
Collections. Red-winged blackbirds were collected in and surrounding
Millers Lake during 10-day sampling intervals in March and April 1986 and
1987, and February, March, and April 1988. Three methods of collection were
used: (1) shotgun; (2) mist-nets; and (3) baited decoy traps, dove traps and
cannon nets. Live birds were weighed, measured, color marked for a study of
local movements, and released at the site of capture. Carcasses were weighed,
injected with 1 cc 70% ethanol in the esophagus to preserve gut contents, and
frozen for later measurement. Exact location, activity, and time of
collection were recorded for each bird collected.
Measurements. Nine parameters of external morphology were measured with
dial calipers (0.1 mm resolution): (1) bill length; (2) bill depth at center
of nostril; (3) lower bill width; (4) upper bill width; (5) wing chord; (6)
length of secondaries; (7) tail length; (8) tarsus length; and (9) central toe
length (Baldwin et al. 1931). Reliability of measurements was evaluated by
the repeated measuring of 10 individuals at 4 intervals during the collection
season; differences in measurements due to changes in technique over time were
not significant (P > 0.05).
Assumptions. Although the ages associated with the full sequence of molts
and plumages of redwings have not been established with certainty (James et
al. 1984), the assumption was made that all redwings collected from February
to April could be classified into 1 of 4 age/sex classes based on plumage
color: (1) second-year male (SYM), (2) second-year female (SYF), (3) after
second-year male (ASY), and (4) after hatching-year female (AHY). Also, it
was assumed that age and/or wear of feathers was identical on birds within
age/sex classes at this time of year, and that inter-year variation in body
sizes of local populations of birds was negligible.
Statistics and sampling. All statistical tests were performed at the a
priori alpha of 0.05. A one-way ANOVA of measurements by method of capture
was performed for birds collected in 1986 to determine if method of capture
resulted in bias of the collections by body size. Adult males that were
collected by shooting had longer bills and shorter tarsi than those captured
in mist-nets. AHY females collected from baited traps had shorter bills and
longer wings than those that were either shot or mist-netted. Thus, to avoid
biases associated with method of collection, only data for birds collected by
shooting were included in the analyses of morphometric measurements.
Data were tabulated by year, age/sex class, and 10-day interval of
collection (February 11-20, March 1-10, March 11-20, March 21-31, April 1-10,
and April 11-20). No collections were made the period February 21-28/29 in
any year. An exploratory analysis was performed by examining box-and-whisker-
plots (Tukey 1977, James et al. 1984) for each measurement within age/sex
class and 10-day collection interval; these plots present the median, middle
half, and range of the distribution of measures.
A 1-way ANOVA was used to test for differences in body mass and in each of
9 external measures within age/sex class and 10-day collection interval among
years. It was followed, where appropriate, by a Waller-Duncan multiple
comparison (Carmer and Swanson 1973).
The discriminant analysis performed by Wilson (1986) to distinguish
northern migrants (termed nonresidents) from residents of southwestern
Louisiana yielded classification equations that were estimated to be highly
efficient in categorizing adult redwings. These equations were used to
estimate percentage of nonresidents by age/sex class and 10-day interval. A
second method of classification was based on the measure of wing chord, a
characteristic that Wilson (1986) found to be the singlemost important
discriminating variable. Birds were categorized as nonresidents when wing
chord exceeded 120, 115, and 97 mm, for ASY, SYM, and AHY birds, respectively
(Wilson 1986, Howell and van Rossem 1928). Second-year females, i.e., females
without color on the epaulets, were omitted from the analysis because no
comparable morphometric data from this age/sex class are available for
Lousiania--a result of poorly documented plumage characteristics for SYF.
RESULTS
Banding Data
[Note: This section contains only a summary of the analyses of the
banding data. A thorough treatise of the topic is available in Brugger and
Dolbeer (in press).]
Of 12,020 cases in the 62-year (1924-1985) recovery-retrieval file, 688
were either banded or recovered in Louisiana (610 banded in Louisiana and
recovered anywhere, and 655 recovered in Louisiana and banded anywhere). Of
these, 597 (86.7%) were year-round residents. Most records were obtained
prior to 1945.
Only 58 cases were randomly recovered (i.e., not biased by recovery at
banding operations) in southwestern Louisiana. In relation to rice-growing
seasons, resident birds comprised 34% (13 of 38) of records in winter, 67% (4
Louisiana yielded classification equations that were estimated to be highly
efficient in categorizing adult redwings. These equations were used to
estimate percentage of nonresidents by age/sex class and 10-day interval. A
second method of classification was based on the measure of wing chord, a
characteristic that Wilson (1986) found to be the singlemost important
discriminating variable. Birds were categorized as nonresidents when wing
chord exceeded 120, 115, and 97 mm, for ASY, SYM, and AHY birds, respectively
(Wilson 1986, Howell and van Rossem 1928). Second-year females, i.e., females
without color on the epaulets, were omitted from the analysis because no
comparable morphometric data from this age/sex class are available for
Lousiania--a result of poorly documented plumage characteristics for SYF.
RESULTS
Banding Data
[Note: This section contains only a summary of the analyses of the
banding data. A thorough treatise of the topic is available in Brugger and
Dolbeer (in press).]
Of 12,020 cases in the 62-year (1924-1985) recovery-retrieval file, 688
were either banded or recovered in Louisiana (610 banded in Louisiana and
recovered anywhere, and 655 recovered in Louisiana and banded anywhere). Of
these, 597 (86.7%) were year-round residents. Most records were obtained
prior to 1945.
Only 58 cases were randomly recovered (i.e., not biased by recovery at
banding operations) in southwestern Louisiana. In relation to rice-growing
seasons, resident birds comprised 34% (13 of 38) of records in winter, 67% (4
of 6) in planting, 100% (5 of 5) in growth, harvest, and ratoon crop, and 78%
(7 of 9) in ratoon harvest (Table 3-1).
Resident redwings comprised all of the 104 nonrandom recoveries (i.e.,
taken by banding operations) beyond the 58 recovered at banding stations. In
relation to rice-growing seasons, nonrandom recoveries suggested that resident
birds comprised 60% (37 of 62) of records in winter, 94% (31 of 33) in
planting, 100% (34 of 34) in first growth and harvest, and 94% (31 of 33) in
ratoon harvest (Table 3-2).
Migrant red-winged blackbirds that were recovered in Louisiana originated
in breeding populations from North and South Dakota, Minnesota, Iowa,
Missouri, Illinois, Michigan, New York south to Texas and Arkansas. Only 2
recoveries of migrants (1 female from Michigan and 1 male from Texas) were
made during the planting season.
Morphology
Exploratory analysis. Measurements were obtained from 488, 521, and 592
birds in 1986, 1987, and 1988, respectively (Table 3-3). A trend of
decreasing median wing-chord length with increasing collection date was
observed among years in SYM and SYF (Figs. 3-1 and 3-2), indicating a decline
in nonresident birds with seasonal advance. However, each year of study
yielded different patterns in median wing-chord length among collection
intervals for ASY and AHY. Median wing chord of ASY increased slightly from
mid-March to April in 1986, was relatively stable across all collection
intervals in 1987, and decreased from February to April in 1988, all evidence
suggesting a decline in the proportion of nonresidents after February (Fig. 3-
3). In 1986 and 1987, median wing chord of AHY decreased through March,
indicating a decline in proportion of nonresident birds; in 1987, it then
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Table 3-3. Yearly collections of red-winged blackbirds by age/sex class and
10-day intervals, Evangeline Parish, Louisiana, 1986-1988. All birds were
collected by shooting.
Age/sex Collection Year
class interval 1986 1987 1988
Second-year
male (SYM)
After second-
year male (ASY)
Second-year
female (SYF)
After hatching-
year female (AHY)
Feb 11-20
Mar 01-10
Mar 11-20
Mar 21-31
Apr 01-10
Apr 11-20
Feb 11-20
Mar 01-10
Mar 11-20
Mar 21-31
Apr 01-10
Apr 11-20
Feb 11-20
Mar 01-10
Mar 11-20
Mar 21-31
Apr 01-10
Apr 11-20
Feb 11-20
Mar 01-10
Mar 11-20
Mar 21-31
Apr 01-10
Apr 11-20
Total
0
3
37
26
8
2
0
2
39
42
19
6
0
0
16
17
0
0
0
1
141
93
24
18
488
0
7
9
7
16
1
0
3
42
17
65
6
0
10
28
26
9
5
0
29
101
79
55
6
521
130 t
120
S 110 n
a 100
90
1 2 3 4 5 6 1 2 3 4 5 6 1 2 3 4 5 6
1986 1987 1988
Collection Interval
Figure 3-1. Box-and-whisker plot of the measurement for wing chord by year
and 10-day collection interval for second-year male (SYM) red-winged
blackbirds collected near Millers Lake, Evangeline Parish, Louisiana, 1986-88.
Dotted horizontal line represents the separation point for migrants (above)
and residents (below). Collection intervals were: (1) February 10-20; (2)
March 1-10; (3) March 11-20; (4) March 21-31; (5) April 1-10; and (6) April
11-20. No collections were made during February 21-28/29 in any year.
110
100
90
1 2 3 4 5 6
1987
1 2 3 4
1988
Collection Interval
Figure 3-2. Box-and-whisker plot of the measurement for wing chord by year
and 10-day collection interval for second-year female (SYF) red-winged
blackbirds collected near Millers Lake, Evangeline Parish, Louisiana, 1986-88.
Collection intervals were: (1) February 10-20; (2) March 1-10; (3) March 11-
20; (4) March 21-31; (5) April 1-10; and (6) April 11-20. No collections were
made during February 21-28/29 in any year.
5 6
I i l i I
^I
"~b!e
120 ......... ..... ........... ..... ... ..... ... .......
S 110
0
Io
. 100
0M
90
1 2 3 4 5 6 1 2 3 4 5 6 1 2 3 4 5 6
1986 1987 1988
Collection Interval
Figure 3-3. Box-and-whisker plot of the measurement for wing chord by year
and 10-day collection interval for after second-year male (ASY) red-winged
blackbirds collected near Millers Lake, Evangeline Parish, Louisiana, 1986-88.
Dotted horizontal line represents the separation point for migrants (above)
and residents (below).Collection intervals were: (1) February 10-20; (2) March
1-10; (3) March 11-20; (4) March 21-31; (5) April 1-10; and (6) April 11-20.
No collections were made during February 21-28/29 in any year.
increased in mid-April, suggesting a temporary increase in the proportion of
nonresident AHY (Fig. 3-4). In 1988, median wing chord of AHY was relatively
stable during all collection intervals, suggesting constant proportions of
resident and nonresident birds throughout the spring planting season.
Analysis of variance. Few temporal trends in body size were detected by
ANOVA, coupled with multiple comparisons. Among SYM, mean body mass decreased
significantly with seasonal advance in 1987 (Table 3-4). Also, the mean
tarsus length of SYM increased with seasonal advance in 1986 (Table 3-4);
inasmuch as birds from more northerly regions of the continent are
characterized by longer tarsi, the latter suggested the presence of
nonresident SYM in the area during the late spring season of 1986.
Among SYF, mean body mass decreased from early March to early April in
1987, but then increased from early to mid-April; the increase in body mass of
birds in April suggested the probable influx of large-bodied nonresident SYF
(Table 3-5). Similarly, mean measures of bill depth at nares, wing chord,
length of secondaries, and length of tail among SYF in 1988 decreased in March
and increased in April (Table 3-5).
Among ASY males, mean body mass declined with advance of season each year
(Table 3-6). Mean bill length increased with seasonal advance in 1986 and
1987, wing chord decreased from February to March in 1988, and tarsus length
peaked in mid-March in 1986, all of which suggested a decline in the
proportion of nonresident adult males from late February through March (Table
3-6).
Among AHY females, all characters except tail length and tarsus length
varied among collection intervals in at least 1 year (Table 3-7). However,
only body mass varied significantly among collection intervals in each of the
3 years, decreasing significantly from February to April.
E
100
0 ..... ............. . .. ... .
o
90
C 90
1 2 3 4 5 6 1 2 3 4 5 6 1 2 3 4 5 6
1986 1987 1988
Collection Interval
Figure 3-4. Box-and-whisker plot of the measurement for wing chord by year
and 10-day collection interval for after hatching-year female (AHY) red-winged
blackbirds collected near Millers Lake, Evangeline Parish, Louisiana, 1986-88.
Dotted horizontal line represents the separation point for migrants (above)
and residents (below). Collection intervals were: (1) February 10-20; (2)
March 1-10; (3) March 11-20; (4) March 21-31; (5) April 1-10; and (6) April
11-20. No collections were made during February 21-28/29 in any year.
Table 3-4. Means ( SO) of 10 traits of body size in second-year male (SYM) red-winged blackbirds collected
by shooting in Evangeline Parish, Louisiana, during 6, 10-day collection intervals, 1986-1988. Traits are
body mass (MASS), bill length (BILL), bill depth at nares (BDN), bill width at gonys (WGON), bill width at
nares (WNAR), wing chord (CRD), length of secondary feathers (SEC), tail length (TAIL), tarsus length
(TARS), and length of central toe (CTOE). A dash (-) indicates that no birds were collected during the
interval.
Collection interval
Feb Mar Mar Mar Apr Apr
Trait Year 10-20 1-10 11-20 21-31 1-10 11-20
MASS (g) 1986 -66.0(1.0) 62.4(3.6) 61.4(3.6 62.1(2.7) 61.5(3.5)
1987 67.7(3.2) 62.7(4.5) 60.0(3.9) 58.5(3.3) 61.5 *
1988 61.7(7.4) 61.0(5.0) 56.4(2.6) 59.4(6.6) 59.0(2.6) 59.7(0.3)
BILL (mm) 1986 24.8(1.2) 23.8(0.6) 23.6(0.8) 24.3(0.9) 24.0(0.5)
1987 23.7(1.1) 23.6(1.0) 24.2(0.8) 24.0(0.8)
1988 23.4(1.1) 23.8(0.3) 24.0(0.5) 23.7(0.7) 23.5(1.2) 23.9(0.2)
BON (mm) 1986 10.9(0.3) 10.9(0.2) 11.0(0.4) 11.8(0.3) 11.2(0.7)
1987 11.9(0.8) 12.4(0.4) 11.8(0.5) 11.5(0.3)
1988 10.5(0.5) 10.4(0.4) 10.3(0.6) 10.2(0.5) 9.9(0.2) 10.0(0.1)
WGON (mm) 1986 10.0(0.4) 9.6(0.4) 9.4(0.4) 9.4(0.2) 9.4(0.1)
1987 9.6(0.3) 9.7(0.3) 9.5(0.1) 9.7(0.4) 0.2
1988 9.7(0.5) 9.6(0.3) 9.6(0.2) 9.6(0.4) 9.7(0.4) 9.4(0.1)
WNAR (mm) 1986 7.7(0.4) 7.7(0.3) 7.7(0.4) 7.6(0.1) 8.0(0.4)
1987 6.5(0.2) 6.6(0.3) 6.7(0.2) 6.6(0.2)
1988 6.7(0.3) 6.6(0.1) 6.6(0.3) 6.6(0.2) 6.5(0.4) 6.5(0.1)
CRD (mm) 1986 114.3(3.3) 111.9(3.5) 111.9(1.8) 109.7(5.4) 109.2(4.8)
1987 115.3(2.8) 113.0(3.2) 113.5(2.9) 111.7(3.2) 112.1
1988 114.4(4.7) 116.1(2.5) 112.7(3.3) 112.6(2.7) 112.6(1.9) 112.1(0.5)
SEC (mm) 1987 88.3(2.4) 89.4(4.0) 87.8(3.5) 87.2(3.7) 89.9
1988 87.8(3.6) 89.3(3.8) 89.2(4.2) 87.9(2.8) 86.8(2.4) 87.2(1.6)
TAIL (mm) 1986 86.5(1.3) 84.8(2.4) 87.5(3.6) 84.8(5.2) 86.6(2.9)
1987 85.6(2.8) 84.7(3.5) 84.5(2.8) 85.2(4.6)
1988 89.6(4.0) 90.0(3.1) 90.5(4.7) 88.3(4.1) 87.9(3.1) 92.0(8.6)
TARS (mm) 1986 27.5(0.5) 27.7(0.7) 27.8(1.6) 27.9(0.6) 28.2(0.7) *
1987 27.3(1.2) 27.5(1.2) 28.1(0.5) 27.5(0.9) 27.5
1988 28.1(1.1) 28.5(0.7) 28.0(0.8) 28.7(0.6) 27.9(0.4) 27.8(1.7)
CTOE (mm) 1987 19.0(0.4) 18.9(0.9) 19.4(0.6) 18.5(2.7) 19.5
1988 18.7(0.9) 18.7(0.8) 19.1(1.2) 19.1(0.7) 18.8(0.9) 19.0(0.4)
Significant differences among means within year, P < 0.05.
Table 3-5. Means ( SO) of 10 traits of body size in second year-female (SYF) red-winged blackbirds
collected by shooting in Evangeline Parish, Louisiana, during 6, 10-day collection intervals, 1986-1988.
Traits are body mass (MASS), bill length (BILL), bill depth at nares (BON), bill width at gonys (WGON), bill
width at nares (WNAR), wing chord (CRD), length of secondary feathers (SEC), tail length (TAIL), tarsus
length (TARS), and length of central toe (CTOE). A dash (-) indicates that no birds were collected during
the interval.
Collection interval
Feb Mar Mar Mar Apr Apr
Trait Year 10-20 1-10 11-20 21-31 1-10 11-20
MASS (g) 1987 44.7(5.8) 44.3(4.8) 40.5(3.3) 38.7(3.2) 46.8(6.9) *
1988 44.2(2.9) 41.2(3.8) 38.5(1.2) 40.3(3.9) 41.8(3.7) 41.5(5.4)
BILL (mn) 1987 20.2(0.5) 20.3(0.4) 20.3(0.5) 20.6(0.6) 20.4(0.6)
1988 20.4(0.9) 20.4(0.7) 20.2(0.7) 20.4(0.7) 20.4(0.8) 20.2(0.5)
BDN (mm) 1987 10.4(0.5) 10.5(0.5) 10.2(0.6) 10.0(0.6) 10.2(0.2)
1988 8.9(0.4) 9.0(0.3) 9.0(0.4) 8.6(0.3) 8.7(0.3) 8.8(0.3) *
WGON (mm) 1987 8.4(0.6) 8.6(0.5) 8.4(0.3) 8.2(0.4) 8.5(0.2)
1988 8.5(0.4) 8.5(0.3) 8.5(0.1) 8.4(0.3) 8.3(0.3) 8.4(0.3)
WNAR (mm) 1987 5.9(0.3) 5.9(0.2) 5.8(0.3) 6.0(0.3) 5.9(0.1)
1988 6.1(0.3) 5.9(0.2) 6.0(0.2) 5.9(0.1) 5.9(0.2) 5.9(0.1)
CRD (mm) 1987 98.3(4.1) 97.1(4.0) 96.1(3.0) 95.0(3.7) 94.2(3.1)
1988 96.5(2.9) 98.0(3.4) 95.9(2.9) 94.6(3.1) 95.6(2.9) 94.5(4.1) *
SEC (mn) 1987 73.7(3.5) 74.8(3.7) 74.2(2.5) 72.8(2.1) 73.0(1.9)
1988 75.1(2.3) 75.7(2.8) 75.0(2.5) 75.5(2.7) 73.4(2.1) 72.8(4.0) *
TAIL (mm) 1987 72.0(4.1) 72.0(3.6) 71.6(4.0) 70.3(2.7) 69.4(1.0)
1988 74.9(3.3) 75.7(3.5) 76.0(4.2) 73.7(3.3) 73.0(2.9) 74.3(2.0) *
TARS (mm) 1987 24.8(0.7) 24.9(0.8) 24.9(0.6) 24.9(0.8) 24.7(0.8)
1988 24.8(0.8) 25.3(1.0) 25.0(0.6) 25.1(0.7) 24.9(0.6) 24.6(0.4)
CTOE (mm) 1987 16.9(0.7) 17.0(0.4) 17.0(0.5) 16.7(0.7) 17.0(0.3)
1988 16.4(0.5) 16.5(0.5) 16.3(0.6) 16.7(0.5) 16.6(0.5) 16.7(0.1)
* Significant differences among means within year, P < 0.05.
Table 3-6. Means (+ SD) of 10 traits of body size in after second-year (ASY) male red-winged blackbirds
collected by shooting in Evangeline Parish, Louisiana, during 6, 10-day collection intervals, 1986-1988.
Traits are body mass (MASS), bill length (BILL), bill depth at nares (BDN), bill width at gonys (WGON), bill
width at nares (WNAR), wing chord (CRD), length of secondary feathers (SEC), tail length (TAIL), tarsus
length (TARS), and length of central toe (CTOE). A dash (-) indicates that no birds were collected during
the interval.
Collection interval
Feb Mar Mar Mar Apr Apr
Trait Year 10-20 1-10 11-20 21-31 1-10 11-20
MASS (g) 1986 68.0(2.8) 61.3(3.5) 61.2(3.0) 62.7(2.7) 62.0(0.0) *
1987 68.3(4.6) 61.6(4.5) 61.2(1.8) 60.4(3.1) 62.0(3.7) *
1988 69.2(8.0) 60.5(2.1) 60.4(5.8) 60.1(3.6)
BILL (mm) 1986 24.3(0.1) 23.8(0.9) 24.9(1.0) 25.0(0.8) 25.1(0.5) *
1987 24.4(0.6) 24.3(1.3) 25.0(1.0) 24.9(0.8) 25.9(1.0) *
1988 23.7(0.8) 24.5(0.1) 24.4(0.9) 24.4(0.8)
BDN (mn) 1986 10.2(0.6) 10.1(0.4) 9.9(0.4) 9.9(0.5) 9.5(0.1)
1987 12.3(0.8) 11.7(0.5) 11.8(0.6) 11.6(0.5) 11.8(0.7)
1988 10.4(0.5) 10.1(0.4) 10.1(0.3) 10.0(0.4)
WGON (mm) 1986 9.7(0.1) 9.4(0.6) 9.4(0.4) 9.5(0.4) 9.7(0.3)
1987 9.6(0.5) 9.3(0.3) 9.4(0.2) 9.4(0.3) 9.5(0.4)
1988 9.8(0.4) 9.5(0.6) 9.5(0.3) 9.5(0.3)
WNAR (mm) 1986 7.3(0.4) 7.5(0.4) 7.7(0.5) 7.6(0.3) 7.9(0.4)
1987 6.4(0.3) 6.6(0.2) 6.5(0.3) 6.4(0.8) 6.3(0.1)
1988 6.7(0.3) 6.6(0.2) 6.5(0.1) 6.6(0.2)
CRD (mm) 1986 119.8(1.3) 112.6(10.6) 115.9(2.7) 116.8(5.3) 116.1(3.4)
1987 119.6(1.1) 117.7(4.3) 119.4(3.6) 117.8(3.3) 118.2(3.5)
1988 120.4(3.9) 117.3(3.8) 114.3(2.0) 116.6(2.2) *
SEC (mm) 1987 92.0(1.2) 93.6(4.0) 92.6(3.4) 91.8(2.6) 91.5(4.4)
1988 93.8(3.1) 91.7(1.5) 92.1(1.6) 90.4(2.4)
TAIL (mm) 1986 92.0(5.3) 92.2(4.5) 92.6(3.7) 92.8(1.9) 92.1(4.4)
1987 91.0(2.9) 90.6(4.5) 91.9(2.8) 91.7(3.6) 91.8(4.0)
1988 94.6(3.4) 96.6(3.4) 93.1(2.5) 93.9(3.4)
TARS (mm) 1986 25.5(4.0) 28.4(1.0) 27.9(0.9) 27.7(1.1) 27.0(0.6)*
1987 27.0(1.3) 28.2(1.2) 28.2(0.6) 27.9(0.8) 28.1(0.6)
1988 28.0(1.0) 27.7(0.4) 28.2(0.9) 28.2(0.8)
CTOE (mm) 1987 18.7(0.4) 19.0(0.9) 19.1(0.6) 19.1(0.7) 19.9(0.2)
1988 18.9(0.7) 17.9(0.4) 19.0(0.6) 18.8(0.8)
Significant differences among means within year, P < 0.05.
Table 3-7. Means (+ SD) of 10 traits of body size in after hatching-year (AHY) female red-winged blackbirds
collected by shooting in Evangeline Parish, Louisiana, during 6, 10-day collection intervals, 1986-1988.
Traits are body mass (MASS), bill length (BILL), bill depth at nares (BDN), bill width at gonys (WGON), bill
width at nares (WNAR), wing chord (CRD), length of secondary feathers (SEC), tail length (TAIL), tarsus
length (TARS), and length of central toe (CTOE). A dash (-) indicates that no birds were collected during
the interval.
Collection interval
Feb Mar Mar Mar Apr Apr
Trait Year 10-20 1-10 11-20 21-31 1-10 11-20
MASS (g) 1986 52.0 44.6(4.4) 42.0(3.7) 46.5(4.4) 43.7(5.5) *
1987 47.8(3.2) 45.3(3.9) 42.6(4.0) 40.8(3.7) 44.0(5.8) *
1988 46.0(3.6) 42.1(2.6) 40.3(3.3) 43.3(3.6) 42.6(5.8) 44.3(4.1) *
BILL (mm) 1986 21.1 20.4(0.7) 20.6(0.7) 20.8(0.6) 20.7(0.7)
1987 20.5(0.6) 20.5(0.5) 20.5(0.6) 20.7(0.7) 21.7(0.6) *
1988 20.3(0.7) 20.4(0.7) 20.5(0.6) 20.6(0.7) 20.7(0.6) 20.9(0.5)
BON (mm) 1986 9.0 8.8(0.3) 8.7(0.2) 8.6(0.3) 8.6(0.4)
1987 10.1(0.4) 10.4(0.5) 10.3(0.5) 10.1(0.3) 10.5(0.4) *
1988 9.1(0.9) 8.9(0.3) 9.0(0.3) 8.8(0.3) 9.0(0.4) 8.8(0.3)
WGON (mm) 1986 8.6 8.6(0.4) 8.4(0.4) 8.5(0.3) 8.5(0.5)
1987 8.4(0.2) 8.6(0.4) 8.4(0.3) 8.3(0.3) 8.4(0.3) *
1988 8.4(0.4) 8.4(0.3) 8.3(0.3) 8.5(0.3) 8.7(0.4) 8.4(0.3) *
WNAR (mm) 1986 6.9 6.9(0.4) 7.0(0.3) 7.2(0.4) 7.2(0.5)
1987 6.0(0.2) 6.0(0.2) 5.9(0.2) 5.9(0.2) 5.8(0.3)
1988 6.0(0.2) 5.9(0.2) 5.8(0.2) 6.0(0.3) 6.1(0.2) 6.0(0.3) *
CRD (mn) 1986 91.6 98.4(3.1) 97.4(4.0) 96.4(3.7) 96.6(3.8) *
1987 100.9(2.7) 99.4(3.0) 98.4(3.5) 98.7(3.2) 100.6(3.0) *
1988 99.1(2.8) 99.0(2.7) 99.8(3.1) 99.0(3.1) 99.5(3.1) 99.8(4.6)
SEC (mn) 1987 76.3(2.0) 76.9(2.3) 76.1(2.4) 75.6(2.6) 77.2(1.9) *
1988 76.3(3.2) 77.5(2.1) 77.3(2.9) 78.4(2.1) 76.1(2.0) 76.8(2.9) *
TAIL (mm) 1986 -76.2 75.6(3.7) 75.5(3.1) 74.5(3.7) 73.4(3.1)
1987 76.0(2.9) 75.4(2.4) 75.0(2.9) 75.3(2.4) 76.4(2.1)
1988 78.1(2.5) 78.3(2.6) 78.7(2.8) 78.1(2.9) 77.3(2.8) 80.1(3.6)
TARS (mm) 1986 -22.7 24.8(1.0) 24.5(1.2) 24.7(0.7) 24.4(0.6)
1987 24.8(0.9) 24.8(0.9) 24.7(0.9) 24.8(0.6) 25.1(0.7)
1988 25.0(0.7) 25.2(0.8) 25.2(0.8) 25.2(0.9) 25.4(0.8) 25.0(0.7)
CTOE (mm) 1987 -16.5(0.6) 17.1(0.5) 16.8(0.6) 17.0(0.6) 16.5(1.1)
1988 16.5(0.6) 16.3(0.6) 16.5(0.5) 16.6(0.6) 16.7(0.6) 17.0(0.7) *
Significant differences among means within year, P < 0.05.
Classification functions. As determined by Wilson's (1986) classification
equations, nonresident adult birds were present after the beginning of early
planting season each year (Table 3-8). However, there was great variation
among years and age/sex classes in the estimated percentages of nonresident
birds found near Millers Lake. During the main planting season (March 21
April 10), 0-17% of ASY and 15%-76% of AHY were classified as nonresidents.
Wing-chord data suggested that nonresident SYM left the region by April 10
each year, whereas some nonresident ASY remained through April. By wing chord
data, 40%-100% of AHY collected after March 20 were classified as nonresidents
(Table 3-8).
DISCUSSION
Two lines of evidence show that most redwings found in southwestern
Louisiana after mid-March were resident birds. Recoveries of banded birds,
compiled from 1924-1985, suggested that 67%-94% found in southwestern
Louisiana during March and April were resident birds. However, banding data
were insufficient to estimate age and sex-related patterns of migration.
Discriminant analysis of morphometric data suggested that >80% of adult male
redwings in the region were residents, and from 25%->80% of adult females
(42%-53% in 1986, 25%-50% in 1987, and >80% in 1988) were residents during
early planting season.
The estimates of the resident and nonresident components of the population
calculated from banding and morphometric data are in general agreement with
those of Wilson (1986), who reported that proportionally more residents were
present in April than in March. However, the current morphometric data
suggest a greater representation of nonresident birds, especially female
redwings, during the planting season than previously identified. The
Table 3-8. Percentage of nonresident red-winged blackbirds in the Millers
Lake area of Evangeline Parish, Louisiana, during 6, 10-day collection
intervals in spring, 1986-1988. Two methods for estimating migratory status
were used-- classification functions from discriminant analyses for adult
birds only (Wilson 1986), and simple wing-chord measurements for second-year
males and adult birds. A dash (-) indicates that no birds of an age/sex class
were collected during the interval. No classification function exists for
second-year males.
Age/sex Collection Classification function Wing chord
class interval 1986 1987 1988 1986 1987 1988
Second- Feb 10-20 57
year Mar 01-10 33 62 55
male Mar 11-20 22 22 16
(SYM) Mar 21-31 12 28 20
Apr 01-10 16 7 0
Apr 11-20 0 0 0
After Feb 10-20 0 59
second- Mar 01-10 4 0 0 50 50 0
year Mar 11-20 3 0 13 36 -
male Mar 21-31 17 0 0 12 50 0
(ASY) Apr 01-10 17 2 0 16 25 5
Apr 11-20 0 0 0 31 -
After Feb 10-20 32 78
hatching- Mar 01-10 81 23 0 93 79
year Mar 11-20 58 76 35 66 79 81
female Mar 21-31 38 76 15 46 70 71
(AHY) Apr 01-10 47 52 17 40 67 74
Apr 11-20 39 50 11 40 100 75
estimates also provide support for the observation of age- and sex-specific
migration patterns by red-winged blackbirds from wintering to breeding sites
(Dolbeer 1978, 1982). The among-year variation in the proportion of
nonresident female redwings in the area during the planting season suggests a
dynamic pattern of migration by females in southwestern Louisiana in late
winter and early spring.
Female redwings are implicated in causing the greatest damage to seeded
rice in the Millers Lake region of Evangeline Parish, as well as in other
parishes of southwestern Louisiana. Because of inter-year variation in the
proportion of nonresident female redwings, the component of the blackbird
population responsible for damage is dynamic. To illustrate, in 1986 and
1988, resident females primarily were responsible for damage, whereas, in
1987, migratory females principally were responsible for damage. Efforts to
mitigate bird damage to early planted rice are thus complicated by annual
change in the component of the population responsible for damage. The
lingering presence of migrant birds during some spring planting seasons, but
not others, also may result in increased seed loss during those years. A
conservative recommendation to avoid or reduce seed loss in either situation
is to delay planting until after migration is over.
The differences in estimates of the proportions of resident and
nonresident red-winged blackbirds derived both from banding data (biased
toward residents) and from morphometric data may reflect changes in
distribution patterns that are related to changes in agricultural practices
that occurred after World War II. Is it possible that prior to 1945, the
period in which most band recoveries were reported, there were not as many
migrant red-winged blackbirds remaining in Louisiana during spring. The
availability of food resources resulting from increased rice production in
recent decades may be the attractant that has precipitated an increase in the
numbers of migrant blackbirds that remain in the region after winter--or
during the planting season. Thus, the presence of these nonresident
blackbirds during the planting season, due to "delayed" migration, increases
the damage to spring-planted rice beyond the level normally caused by resident
blackbirds.
CHAPTER 4: ROOST DYNAMICS FROM LATE WINTER THROUGH SPRING
INTRODUCTION
The winter roost at Millers Lake has attracted large numbers of blackbirds
and starlings for several decades (Ortego 1976, Wilson 1986). Because of its
large size and close proximity to rice fields, the roost facilitates bird
damage to newly planted rice during spring. To develop a biologically-based
bird population management program for mitigating bird damage in the rice-
growing region, data that quantified roost dynamics--the changes in size and
species composition over time--at the Millers Lake roost in spring were
needed.
Factors that have influenced the size and stability of other blackbird and
starling roosts in the eastern United States offer some insight into the roost
dynamics at Millers Lake. Long-term variation in roost size simply may be
related to timing of migration or differential migratory patterns (White 1980,
Greenleaf 1982). Long-term stability in numbers and species in roosts may be
related to persistent mild air temperatures (Crebbs 1960, Meanley 1965, 1971,
Martin 1977, Cutright 1973). Short-term abundance of birds in roosts may be
related to (1) fluctuations in air temperature and precipitation (White 1980),
(2) food resource abundance (Coon 1974, Morrison and Caccamise 1985, Maccarone
1987), (3) roosting substrate availability (Weatherhead and Hoysack 1984), (4)
human intrusion (Greenleaf 1982), or (5) estimation techniques (White 1980).
CHAPTER 4: ROOST DYNAMICS FROM LATE WINTER THROUGH SPRING
INTRODUCTION
The winter roost at Millers Lake has attracted large numbers of blackbirds
and starlings for several decades (Ortego 1976, Wilson 1986). Because of its
large size and close proximity to rice fields, the roost facilitates bird
damage to newly planted rice during spring. To develop a biologically-based
bird population management program for mitigating bird damage in the rice-
growing region, data that quantified roost dynamics--the changes in size and
species composition over time--at the Millers Lake roost in spring were
needed.
Factors that have influenced the size and stability of other blackbird and
starling roosts in the eastern United States offer some insight into the roost
dynamics at Millers Lake. Long-term variation in roost size simply may be
related to timing of migration or differential migratory patterns (White 1980,
Greenleaf 1982). Long-term stability in numbers and species in roosts may be
related to persistent mild air temperatures (Crebbs 1960, Meanley 1965, 1971,
Martin 1977, Cutright 1973). Short-term abundance of birds in roosts may be
related to (1) fluctuations in air temperature and precipitation (White 1980),
(2) food resource abundance (Coon 1974, Morrison and Caccamise 1985, Maccarone
1987), (3) roosting substrate availability (Weatherhead and Hoysack 1984), (4)
human intrusion (Greenleaf 1982), or (5) estimation techniques (White 1980).
Time, weather, and resource abundance emerged as the factors most likely
to affect roost size, composition, and stability at Millers Lake as there is
little human intrusion in the lake after waterfowl season closes (mid-January)
and before fishing season begins (mid-March) and little change in roosting
substrate in the lake because of a laissez-faire management policy. Food
resource abundance could not be assessed in this study because of the large
area (5-6,000 km2) over which birds from Millers Lake might feed (Brugger
1988). Therefore, objectives of this facet of the investigation, conducted at
Millers Lake from late winter through the rice-planting season in the 3 years,
1986-88, were to: (1) enumerate roost size and species composition; (2)
develop a predictive model for bird abundance based on time and weather
variables; and (3) estimate stability of the roost immediately preceding and
during the damage period.
METHODS
Weather
Records of daily minimum and maximum air temperatures and precipitation
were obtained from the Louisiana State University Weather Service Station at
Eunice (30 km south of Millers Lake) for the dates, February 1-April 30, 1986-
1988. Daily air temperatures and precipitation were plotted against time
(Julian date) to depict seasonal weather patterns.
Roost Size and Composition
Estimates of abundance and composition of birds roosting at Miller Lake
were made 1 evening per 7 days from mid-February to late-April, 1986, and 1
evening per 5 days from mid-February to April, 1987 and 1988. Because
Time, weather, and resource abundance emerged as the factors most likely
to affect roost size, composition, and stability at Millers Lake as there is
little human intrusion in the lake after waterfowl season closes (mid-January)
and before fishing season begins (mid-March) and little change in roosting
substrate in the lake because of a laissez-faire management policy. Food
resource abundance could not be assessed in this study because of the large
area (5-6,000 km2) over which birds from Millers Lake might feed (Brugger
1988). Therefore, objectives of this facet of the investigation, conducted at
Millers Lake from late winter through the rice-planting season in the 3 years,
1986-88, were to: (1) enumerate roost size and species composition; (2)
develop a predictive model for bird abundance based on time and weather
variables; and (3) estimate stability of the roost immediately preceding and
during the damage period.
METHODS
Weather
Records of daily minimum and maximum air temperatures and precipitation
were obtained from the Louisiana State University Weather Service Station at
Eunice (30 km south of Millers Lake) for the dates, February 1-April 30, 1986-
1988. Daily air temperatures and precipitation were plotted against time
(Julian date) to depict seasonal weather patterns.
Roost Size and Composition
Estimates of abundance and composition of birds roosting at Miller Lake
were made 1 evening per 7 days from mid-February to late-April, 1986, and 1
evening per 5 days from mid-February to April, 1987 and 1988. Because
Time, weather, and resource abundance emerged as the factors most likely
to affect roost size, composition, and stability at Millers Lake as there is
little human intrusion in the lake after waterfowl season closes (mid-January)
and before fishing season begins (mid-March) and little change in roosting
substrate in the lake because of a laissez-faire management policy. Food
resource abundance could not be assessed in this study because of the large
area (5-6,000 km2) over which birds from Millers Lake might feed (Brugger
1988). Therefore, objectives of this facet of the investigation, conducted at
Millers Lake from late winter through the rice-planting season in the 3 years,
1986-88, were to: (1) enumerate roost size and species composition; (2)
develop a predictive model for bird abundance based on time and weather
variables; and (3) estimate stability of the roost immediately preceding and
during the damage period.
METHODS
Weather
Records of daily minimum and maximum air temperatures and precipitation
were obtained from the Louisiana State University Weather Service Station at
Eunice (30 km south of Millers Lake) for the dates, February 1-April 30, 1986-
1988. Daily air temperatures and precipitation were plotted against time
(Julian date) to depict seasonal weather patterns.
Roost Size and Composition
Estimates of abundance and composition of birds roosting at Miller Lake
were made 1 evening per 7 days from mid-February to late-April, 1986, and 1
evening per 5 days from mid-February to April, 1987 and 1988. Because
relatively few birds were reported to return to the roost from the pinelands
lying north of the lake (Ortego 1976, Wilson 1986), only flightlines on the
southern fringe of the lake were censused. Two procedures for counting
blackbirds were used to estimate roost size.
Stationary count. Between 1630 and 1830 h, 1 or 2 observers, stationed
either at the south levee and/or on Rt. 376, estimated the number of birds
entering the lake from the south and southwest by means of block-count
methodology (Meanley 1965, Arbib 1972, Dolbeer et al. 1978). Counts were
conducted in 5-min segments. The first minute in five, birds flying to the
lake between the Lake road and the west treeline were estimated; the second
minute, the number of birds between the road and east treeline were estimated;
the third minute, species and sex of birds west of the road were estimated;
and the fourth minute, species and sex of birds east of the road were
estimated. No data were taken in the fifth minute. If 2 observers were
present at the same count site, each observer estimated birds in only one
compass direction (east or west). Estimates of number of birds per 5-min
segment were summed over the evening's counts and then multiplied by 5
(abundance recorded for only 1 min in each of two compass-direction counts per
5-min segment) to obtain an overall estimate of roost size. The total number
of 5-min segments per evening varied among counts depending on weather, time
of sunset, and flight patterns of birds.
Drive-and-count. In March and April 1988, the numbers of blackbirds
entering the roost in evening were estimated from counts taken along
standardized road transects (Ortego 1976, Wilson 1986). An observer drove the
8 km length of Rt. 376 at 48 km/h (30 mph) and block-counted numbers of birds
flying to the lake within a zone that extended 0.5 km on either side of the
road for each of 5, 1.6 km segments of the route. Each 1.6 km segment of the
route was driven in 2 min, once every 10 min. The total number of 10-min
segments driven per evening varied among evenings depending on weather, time
of sunset, and flight patterns of birds. Estimates were summed per 1.6 km
interval of road, and then multiplied by 5 to obtain an estimate of roost
size. Composition by sex and species could not be determined by this count
technique.
Assumptions inherent to both methods were: (1) equal competency among
observers in identifying and counting birds; (2) equal visibility of birds
throughout the sampling region; (3) equal numbers of birds in the sampling
space during each minute of the sampled and nonsampled times; and (4) equal
volumes of airspace sampled on each side of the road.
Species composition. In clear weather, evening flights of birds to the
roost occurred principally from 30 min pre-sunset to 30 min post-sunset.
During inclement weather (cold, rainy, cloudy), evening flights began as early
as 2 h before sunset. Under the latter conditions, usually due to the low
intensity of light, species and sex composition of birds entering the roost
could not be determined consistently by identifying individual birds in
flightlines, as recommended by Dolbeer et al. (1978). Female red-winged
blackbirds were especially difficult to distinguish from brown-headed cowbirds
in poor light. Information on the species and sex composition of blackbirds
at the roost derived from stationary counts was supplemented by 2 additional
methods. In 1986, species and sex composition of roosting birds were derived
from weekly samples collected in the northeast horn of Millers Lake. In 1987
and 1988, flocks, rather than individuals, entering the lake from the south
were identified to species and sex for 1 min of each 5 min-interval of the
stationary counts. In both cases, the spatial and temporal distributions of
species and sexes in the roost and in flightlines were assumed to be random.
Statistical analyses. Estimates of roost size compiled during 3 spring
planting seasons, 1986-1988, were combined for analysis. The subsequent decay
curve was fitted to polynomial and exponential equations (Rafferty and Norling
1985). A best fit was found by r2, the coefficient of determination. A logo,
transformation was then applied to estimates of roost size to linearize
values. Relationships among roost sizes, and date, air temperatures (mean
minimum and mean maximum for 3 days preceding the count), and precipitation
(sum of rainfall for 3 days preceding the count) were estimated with Pearson
correlation coefficients and multiple linear regressions at alpha=0.05. An
estimate of reliability of stationary-count and drive-and-count methods for
quantifying roost size was obtained by Spearman rank correlation.
Spatial Patterns of Roosting
Weekly surveys of Millers lake were made by boat during the interval
extending from 2 h pre-sunset to 0.5 h post-sunset to determine the
approximate locations of roosting birds. Early in 1986, it was determined
that birds roosted primarily in the northeastern portion of the lake during
March and April. Hence, after mid-March in subsequent years, only the
northeastern portion of the lake was surveyed by boat for locations of
roosting birds.
RESULTS
Spring Weather Patterns
Air temperatures and precipitation were different in each of the 3 spring
seasons in southwestern Louisiana. Freezes occurred approximately 4 days/mo
in February and 2 days/mo in March of each year. Dates of last freezes in
spring at Eunice, Louisiana, were: March 21-22, 1986; April 2-5, 1987; and
Statistical analyses. Estimates of roost size compiled during 3 spring
planting seasons, 1986-1988, were combined for analysis. The subsequent decay
curve was fitted to polynomial and exponential equations (Rafferty and Norling
1985). A best fit was found by r2, the coefficient of determination. A logo,
transformation was then applied to estimates of roost size to linearize
values. Relationships among roost sizes, and date, air temperatures (mean
minimum and mean maximum for 3 days preceding the count), and precipitation
(sum of rainfall for 3 days preceding the count) were estimated with Pearson
correlation coefficients and multiple linear regressions at alpha=0.05. An
estimate of reliability of stationary-count and drive-and-count methods for
quantifying roost size was obtained by Spearman rank correlation.
Spatial Patterns of Roosting
Weekly surveys of Millers lake were made by boat during the interval
extending from 2 h pre-sunset to 0.5 h post-sunset to determine the
approximate locations of roosting birds. Early in 1986, it was determined
that birds roosted primarily in the northeastern portion of the lake during
March and April. Hence, after mid-March in subsequent years, only the
northeastern portion of the lake was surveyed by boat for locations of
roosting birds.
RESULTS
Spring Weather Patterns
Air temperatures and precipitation were different in each of the 3 spring
seasons in southwestern Louisiana. Freezes occurred approximately 4 days/mo
in February and 2 days/mo in March of each year. Dates of last freezes in
spring at Eunice, Louisiana, were: March 21-22, 1986; April 2-5, 1987; and
Statistical analyses. Estimates of roost size compiled during 3 spring
planting seasons, 1986-1988, were combined for analysis. The subsequent decay
curve was fitted to polynomial and exponential equations (Rafferty and Norling
1985). A best fit was found by r2, the coefficient of determination. A logo,
transformation was then applied to estimates of roost size to linearize
values. Relationships among roost sizes, and date, air temperatures (mean
minimum and mean maximum for 3 days preceding the count), and precipitation
(sum of rainfall for 3 days preceding the count) were estimated with Pearson
correlation coefficients and multiple linear regressions at alpha=0.05. An
estimate of reliability of stationary-count and drive-and-count methods for
quantifying roost size was obtained by Spearman rank correlation.
Spatial Patterns of Roosting
Weekly surveys of Millers lake were made by boat during the interval
extending from 2 h pre-sunset to 0.5 h post-sunset to determine the
approximate locations of roosting birds. Early in 1986, it was determined
that birds roosted primarily in the northeastern portion of the lake during
March and April. Hence, after mid-March in subsequent years, only the
northeastern portion of the lake was surveyed by boat for locations of
roosting birds.
RESULTS
Spring Weather Patterns
Air temperatures and precipitation were different in each of the 3 spring
seasons in southwestern Louisiana. Freezes occurred approximately 4 days/mo
in February and 2 days/mo in March of each year. Dates of last freezes in
spring at Eunice, Louisiana, were: March 21-22, 1986; April 2-5, 1987; and
March 19-20, 1988. Conditions were dry in 1986, but relatively wet in 1988.
Measures of total rainfall (cm) per month in 1986, 1987, and 1988,
respectively, were: February--5.56, 16.33, 34.93; March--9.47, 17.75, 22.81;
and April--5.41, 11.48, 18.49.
Roost Size and Composition
In spring 1986, the number of blackbirds in the Millers Lake roost
decreased exponentially from approximately 15 million in February (E. A.
Wilson, Louisiana State Univ., Crowely, pers. commun.) to 15,000 in late April
(Table 4-1). A similar pattern of decline was observed in 1987 and 1988. The
decay curve of stationary-count roost estimates recorded for the period,
February-April, 1986-88 (Fig. 4-1), is described by the negative exponential
equation:
Roost size = 3,312,600,000 10(o05143*dae); (r2 = 0.83, P<0.05).
Mean 3-day minimum and maximum temperatures were correlated (r=0.82;
P<0.01); thus only one temperature variable (mean maximum) was used in
multiple regressions. Multiple regression of combined 3-year estimates of
roost size on date, mean 3-day maximum temperature, and summed 3-day rainfall
yielded only date as a significant explanatory variable (Table 4-2). Roost
size logol) and date were correlated among years (r= -0.92, P<0.01).
Temporary increases in roost size were noted after freezes or rains.
Although the drive-and-count method yielded slightly higher estimates than
did the stationary-count method in 5 of 6 paired counts, estimated roost size
derived by the two methods did not differ significantly (P>0.05) (Table 4-1).
Flightlines. Traditional evening flightlines were not observed at Miller
Lake. Instead, birds flew to the Millers Lake roost in wide swaths or waves,
Table 4-1. Estimates of roost size at Millers Lake, Evangeline Parish,
Louisiana, mid-February-late-April, 1986-88. Two methods of estimation were
used: stationary-count (no asterisk); or drive-and-count (*). Paired counts
in 1988, used to test reliability of the 2 methods, are linked by dotted
lines.
Estimated roost size
Date of
count 1986 1987 1988
14,900,000 a*
1,000,000 a*
700,000
500,000
374,000
608,000
Feb
Feb
Feb
Mar
Mar
Mar
Mar
Mar
Mar
Mar
Mar
Mar
Mar
Mar
Mar
Mar
Mar
Mar
Mar
Mar
Apr
Apr
Apr
Apr
Apr
Apr
Apr
Apr
Apr
Apr
Aor
110,000
22,000
2,000,000
2,000,000
2,500,000
950,000
85,000
45,000
38,000
20,000
58,000
30,000
11.000
18,000,000
6,000,000
450,000a*
:- 150,000
:- 513,000*
125,000a*
185,000*
117,500
73,500
78,000*
89,000*
68,000
20,000
24,000*
12,500
6,300*
"Drive-and-count
Rice Exp. Stn.,
estimates made by E. A. Wilson
Crowley, pers. commun.).
(Louisiana State Univ.,
bStationary-count estimate made by J. Glahn (USDA/APHIS/S&T,
Kentucky Field Stn., Bowling Green, pers. commun.).
cStationary-count estimate made by R. A. Dolbeer (USDA/APHIS/S&T,
Ohio Field Stn., Sandusky, pers. commun.).
434,500
a + 1986
1987
C' a 1988
4) 10-
N
0
O a
0
m 5-
+ 4W
30 60 90 120
February March April
Figure 4-1. Negative exponential decline in size of blackbird roost during
the period February 15-April 24, 1986-1988, Millers Lake, Evangeline Parish,
Louisiana.
Table 4-2. Multiple linear regression analyses relating log-transformed roost
size to Julian calendar date and 2 weather variables (mean 3-day maximum air
temperature [max] and summed 3-day rainfall [rain]) at Millers Lake,
Evangeline Parish, Louisiana, for the period February 15-April 30, 1986-88.
MLR model Variables Coefficient B-weight
(1) Size = date + max + rain Date -0.0491 -0.9845
Max 0.0202 0.0866
Rain -0.0018 -0.0034
MLR equation: Size = 9.00 0.05 date + 0.02 max
r 0.93, r2 = 0.86
(2) Size = date Date -0.0462 -.09253
MLR equation: Size = 9.21 0.05 date
r = 0.92, r2 = 0.85
covering much of the airspace south of the lake. Altitude of flight waves
ranged from <10 m to approximately 30-40 m. Major flightwaves originated from
the south and southwest of Millers Lake during February and March each year.
However, by late March, the flightwaves from the southwest diminished in size,
being replaced by those from the south and southeast.
Species composition. Species composition at Millers Lake varied weekly
form late winter to spring. The majority of common grackles, cowbirds,
starlings, and adult male redwings had abandoned the roost by late February.
Female redwings comprised 50%-99% of the birds in both roost counts and bird
collections during March and April (Table 4-3).
Spatial Pattern of Roosting
Observations of roosting blackbirds during mid-January to early March
indicated that birds settled on woody vegetation in the lake basin. From mid-
March through April, roosting birds were found only in the secluded northeast
portion of the lake, which is dominated by buttonbush. The spatial area used
by roosting birds diminished during late March and April, reflecting not only
a decline in roost size, but also a more selective use of available woody
vegetation.
Other Roosts
One other major roost, LaFleur Nursery, Grand Prairie, located within 50
km of Millers Lake, comprises 50 ha of mature palms, oaks, and ornamentals
that provide a dense woody substrate for roosting. Other, more distant
roosts, included: Barry's Nursery, Grand Coteau; the coastal marshes south of
Port Arthur; the marshes south of Gueydan; and Bundick Lake, Beauregard
Parish. Roosts occurring more proximate to Millers Lake, previously
documented in the National Roost Surveys, e.g., Cheneyville, Meeker, and
covering much of the airspace south of the lake. Altitude of flight waves
ranged from <10 m to approximately 30-40 m. Major flightwaves originated from
the south and southwest of Millers Lake during February and March each year.
However, by late March, the flightwaves from the southwest diminished in size,
being replaced by those from the south and southeast.
Species composition. Species composition at Millers Lake varied weekly
form late winter to spring. The majority of common grackles, cowbirds,
starlings, and adult male redwings had abandoned the roost by late February.
Female redwings comprised 50%-99% of the birds in both roost counts and bird
collections during March and April (Table 4-3).
Spatial Pattern of Roosting
Observations of roosting blackbirds during mid-January to early March
indicated that birds settled on woody vegetation in the lake basin. From mid-
March through April, roosting birds were found only in the secluded northeast
portion of the lake, which is dominated by buttonbush. The spatial area used
by roosting birds diminished during late March and April, reflecting not only
a decline in roost size, but also a more selective use of available woody
vegetation.
Other Roosts
One other major roost, LaFleur Nursery, Grand Prairie, located within 50
km of Millers Lake, comprises 50 ha of mature palms, oaks, and ornamentals
that provide a dense woody substrate for roosting. Other, more distant
roosts, included: Barry's Nursery, Grand Coteau; the coastal marshes south of
Port Arthur; the marshes south of Gueydan; and Bundick Lake, Beauregard
Parish. Roosts occurring more proximate to Millers Lake, previously
documented in the National Roost Surveys, e.g., Cheneyville, Meeker, and
Table 4-3. Weekly estimates of percentage composition of starlings and
blackbirds in the Millers Lake roost, Evangeline Parish, Louisiana, 1986-88.
Five dominant species are listed: European starling (EUST); red-winged
blackbird (RWBL); brown-headed cowbird (BHCO); common grackle (COGR); and
great-tailed grackle (GTGR). Only red-winged blackbirds were identified to
sex. Methods for obtaining estimates were either counts (CNT) or collections
(CLCT).
RWBL Method
Sample
Date EUST Female Male BHCO COGR GTGR size CNT CLCT
1986
Mar 15 5 55 15 5 20 0 60 x
Mar 22 4 75 10 5 5 1 84 x
Mar 29 0 85 1 5 0 5 78 x
Apr 04 0 90 7 1 1 1 52 x
Apr 11 0 90 7 2 0 1 57 x
1987
Mar 08 5 70 10 5 10 0 1,000 x
Mar 15 0 95 2 2 1 0 1,000 x
Mar 22 0 50 40 5 4 1 800 x
Mar 29 0 95 4 1 0 0 300 x
Apr 04 0 95 3 1 0 1 500 x
Apr 11 0 95 5 0 0 0 500 x
Apr 18 0 99 1 0 0 0 300 x
1988
Feb 15 5 30 30 20 15 0 2,000 x
Mar 15 1 50 30 10 9 0 1,000 x
Mar 22 0 70 20 5 4 1 500 x
Mar 29 0 80 15 4 0 1 500 x
Apr 04 0 90 5 4 0 1 500 x
Apr 11 0 95 3 1 0 1 200 x
LeCompte (Meanley et al. 1976), were not active in 1986, 1987, and 1988. No
roosts were reported from Chicot State Park, 10 km northeast of Millers Lake.
The history of the LaFleur Nursery roost is reported as described by Danny
LaFleur (LaFleur Nursery, Grand Prairie, Louisiana, pers. commun., March 17,
1988):
"The roost has been in existence about 15 years. At first the roost was
made up of mostly robins. About 3 years ago, the roost was estimated to
be about 1 million birds in winter. In the last 3 years, blackbirds
increased in abundance, causing damage to oaks (leaf loss) and palms (leaf
shed). The damage is a nuisance, but does not cause major financial
loss."
In 1988, the LaFleur Nursery roost contained about 1 million birds on
February 19. Although no specific effort was attempted to estimate species
composition at the LaFleur Nursery, the most abundant species were common
grackles and red-winged blackbirds, in that order, with lesser numbers of
robins (Turdus miqratorius), great-tailed grackles, and starlings. On March
17, approximately 250,000 birds were present, with a similar qualitative
species composition. No birds were observed roosting at the nursery on April
12, 1988.
DISCUSSION
The task of estimating roost size at Millers Lake is fraught with
difficulties because of the large size of the lake and the great abundance of
birds at the roost. Techniques used in this study were somewhat inadequate to
estimate the size and species composition of the roost accurately because of
insufficient numbers of observers available to cover the entire area of
Millers Lake. Although differences were not significant, the drive-and-count
method consistently yielded higher estimates than did the stationary-count
method. However, the 2 techniques for estimating roost size yielded values
that were correlated, suggesting internal consistency or repeatability of the
estimates. Both methods possibly underestimated roost size.
Roost size at Millers Lake between mid-February and late April was
correlated with date of the year, and reflected migration patterns related to
onset of breeding activity by species and age classes. Seasonally, both sexes
of starlings, cowbirds, and grackles, along with adult male redwings,
abandoned the roost before adult female and immature male redwings. The
decline began at least 6 weeks prior to breeding season for starlings (April
1-July 20: Kessel 1957, Collins and de Vos 1966), and 8 weeks prior to the
breeding season for red-winged blackbirds and common grackles (April 20-July
20: Erskine 1971, Dolbeer 1976), and brown-headed cowbirds (April 20-June 20:
Scott and Middleton 1968).
In late March, flightwaves were reduced to flightlines and shifted in
location from the southwest to the south and southeast. This late-March shift
in flight pattern might be a reflection of variable planting practices in the
different regions south of the lake, variable distances to feeding sites, or
variable roosting locations in the lake.
The size of the population of roosting blackbirds at Millers Lake during
planting season is dynamic. The roost declined in size with seasonal advance.
Implications to management are clear. If planting could be delayed until
after the roost size declined below a predetermined level, e.g., 50,000 birds,
the likelihood of bird damage would be reduced (Wilson et al. 1989).
Another question important to bird population management remains to be
answered: do blackbirds exhibit fidelity to the roost or is there a shift by
individuals among roosts during the planting season? Circumstantial evidence
suggests that turnover in the Millers Lake roost is high during the early
planting season because (1) there is a large proportion of nonresident birds
at the roost, (2) spring migration occurs throughout April, and (3) Millers
Lake is an isolated, but centrally located, roost that is used by migrants.
Numerous large roosts exist at a distance from Millers Lake (e.g., in the
Louisiana and Texas coastal marshes, approximately 100-500 km to the south and
southwest); however, only one large roost was located within 50 km, or daily
flight range, of Millers Lake. If blackbirds and starlings migrating from
more southerly wintering areas travel along the Louisiana-Texas coast and then
across to the Mississippi River, Millers Lake might serve as a short-term
roost during the spring migration. Accordingly, turnover in the roost might
be expected to be high throughout the early planting period, at least until
migration is completed.
In summary, there are two elements of roost dynamics that influence
potential efforts to implement control measures for reducing the abundance of
blackbirds causing damage to sprouting rice. The first element is the timing
and rate of roost dissolution, as it reflects the total number of blackbirds
to which control measures must be directed at any given point in time. The
second element is the rate of roost turnover. To illustrate, even though a
given roost might exhibit a 5% net loss in numbers daily, the concurrent
change in the composition of individuals in the roost could range from very
low to very high. Thus, if, for example, lethal control measures were
directed at birds causing significant crop damage in a specific locality, the
relief might be short duration because "new" birds in the roost might be
attracted to the same locality for foraging. Thus, given the exponential
decline in roost size with advancing season, delayed planting appears to be
the most cost-effective and reliable recommendation for reducing seed losses
of rice to blackbirds.
CHAPTER 5: FORAGING FLOCK DYNAMICS
INTRODUCTION
Blackbirds wintering at Millers Lake make daily trips from the roost to
forage for grain and insects in surrounding fields, farmyards, and woodlots.
Flocks, which range in size from 2-10,000 birds, often fly to foraging
locations as far as 50 km south and southwest of the roost (Ortego 1976). To
predict location, timing, and intensity of bird damage, baseline data were
needed concerning flock dynamics and movement patterns of blackbirds in the
rice-growing region near Millers Lake. Objectives were to describe: (1) the
size and composition of flocks observed foraging in rice fields; (2) the
spatial and temporal distributions of flocks in the agricultural landscape;
and (3) the daily flocking and movement patterns by individuals.
METHODS
Flocks Size, Composition, and Distribution
Morning flights of birds from the roost were followed to estimate flight
direction and destinations of foraging flocks during January 7-10 and March 6-
10, 1986, and March 6-9, 1987. In addition, 2 survey routes, 30 km in length,
were established to assess the spatial distribution of flocks as well as the
temporal changes in flock size and composition. One route (north), located
from 0-15 km directly south of Millers Lake, was surveyed during March and
April 1986-88, and also during 14-19 February 1988. The second route, located
CHAPTER 5: FORAGING FLOCK DYNAMICS
INTRODUCTION
Blackbirds wintering at Millers Lake make daily trips from the roost to
forage for grain and insects in surrounding fields, farmyards, and woodlots.
Flocks, which range in size from 2-10,000 birds, often fly to foraging
locations as far as 50 km south and southwest of the roost (Ortego 1976). To
predict location, timing, and intensity of bird damage, baseline data were
needed concerning flock dynamics and movement patterns of blackbirds in the
rice-growing region near Millers Lake. Objectives were to describe: (1) the
size and composition of flocks observed foraging in rice fields; (2) the
spatial and temporal distributions of flocks in the agricultural landscape;
and (3) the daily flocking and movement patterns by individuals.
METHODS
Flocks Size, Composition, and Distribution
Morning flights of birds from the roost were followed to estimate flight
direction and destinations of foraging flocks during January 7-10 and March 6-
10, 1986, and March 6-9, 1987. In addition, 2 survey routes, 30 km in length,
were established to assess the spatial distribution of flocks as well as the
temporal changes in flock size and composition. One route (north), located
from 0-15 km directly south of Millers Lake, was surveyed during March and
April 1986-88, and also during 14-19 February 1988. The second route, located
CHAPTER 5: FORAGING FLOCK DYNAMICS
INTRODUCTION
Blackbirds wintering at Millers Lake make daily trips from the roost to
forage for grain and insects in surrounding fields, farmyards, and woodlots.
Flocks, which range in size from 2-10,000 birds, often fly to foraging
locations as far as 50 km south and southwest of the roost (Ortego 1976). To
predict location, timing, and intensity of bird damage, baseline data were
needed concerning flock dynamics and movement patterns of blackbirds in the
rice-growing region near Millers Lake. Objectives were to describe: (1) the
size and composition of flocks observed foraging in rice fields; (2) the
spatial and temporal distributions of flocks in the agricultural landscape;
and (3) the daily flocking and movement patterns by individuals.
METHODS
Flocks Size, Composition, and Distribution
Morning flights of birds from the roost were followed to estimate flight
direction and destinations of foraging flocks during January 7-10 and March 6-
10, 1986, and March 6-9, 1987. In addition, 2 survey routes, 30 km in length,
were established to assess the spatial distribution of flocks as well as the
temporal changes in flock size and composition. One route (north), located
from 0-15 km directly south of Millers Lake, was surveyed during March and
April 1986-88, and also during 14-19 February 1988. The second route, located
CHAPTER 5: FORAGING FLOCK DYNAMICS
INTRODUCTION
Blackbirds wintering at Millers Lake make daily trips from the roost to
forage for grain and insects in surrounding fields, farmyards, and woodlots.
Flocks, which range in size from 2-10,000 birds, often fly to foraging
locations as far as 50 km south and southwest of the roost (Ortego 1976). To
predict location, timing, and intensity of bird damage, baseline data were
needed concerning flock dynamics and movement patterns of blackbirds in the
rice-growing region near Millers Lake. Objectives were to describe: (1) the
size and composition of flocks observed foraging in rice fields; (2) the
spatial and temporal distributions of flocks in the agricultural landscape;
and (3) the daily flocking and movement patterns by individuals.
METHODS
Flocks Size, Composition, and Distribution
Morning flights of birds from the roost were followed to estimate flight
direction and destinations of foraging flocks during January 7-10 and March 6-
10, 1986, and March 6-9, 1987. In addition, 2 survey routes, 30 km in length,
were established to assess the spatial distribution of flocks as well as the
temporal changes in flock size and composition. One route (north), located
from 0-15 km directly south of Millers Lake, was surveyed during March and
April 1986-88, and also during 14-19 February 1988. The second route, located
30-55 km southwest of the lake, was surveyed during March and April 1987. The
surveys were begun 30 min after sunrise and completed within 3 h. In 1987,
the 2 routes were censused on the same days.
Blackbirds and starlings observed within 0.4 km of the road were counted
for 3-min periods at each of 20 stops, located at 1.6-km intervals along each
30-km route. Estimates of flock size, composition, activity, and habitat (for
foraging flocks only) were recorded on a standard checksheet.
Land-use Patterns
In 1987, land-use patterns along both survey routes were quantified twice
each month by mapping all land-use activities within 0.4 km of either side of
routes. Five categories of land-use were designated: out-of-rice production;
disked seedbed; water-planted rice; drill-planted rice; and seedling stand.
Individual Movements
Telemetry. During the period March 17-19, 1986, 6 red-winged blackbirds
(1 after second-year male, 2 second-year males, and 3 after hatching-year
females) were each outfitted with a SM 1 transmitter, 0.312 Hg power cell, and
whip antenna (AVM Instrument Co., Livermore, Calif.). Potential battery life
was 9-14 days. All birds were captured at the same locations east of Millers
Lake. Attempts were made to monitor the roost location and morning feeding
locations of instrumented birds; however, birds often could not be located
because signal range of the transmitter was limited to <0.05 km.
Color marking. In March 1986, 62 individuals from 5 flocks were captured
by mist-netting or rocket-netting. Individual birds were measured, marked
with colored, plastic-leg streamers (Guarino 1968), and released en masse by
30-55 km southwest of the lake, was surveyed during March and April 1987. The
surveys were begun 30 min after sunrise and completed within 3 h. In 1987,
the 2 routes were censused on the same days.
Blackbirds and starlings observed within 0.4 km of the road were counted
for 3-min periods at each of 20 stops, located at 1.6-km intervals along each
30-km route. Estimates of flock size, composition, activity, and habitat (for
foraging flocks only) were recorded on a standard checksheet.
Land-use Patterns
In 1987, land-use patterns along both survey routes were quantified twice
each month by mapping all land-use activities within 0.4 km of either side of
routes. Five categories of land-use were designated: out-of-rice production;
disked seedbed; water-planted rice; drill-planted rice; and seedling stand.
Individual Movements
Telemetry. During the period March 17-19, 1986, 6 red-winged blackbirds
(1 after second-year male, 2 second-year males, and 3 after hatching-year
females) were each outfitted with a SM 1 transmitter, 0.312 Hg power cell, and
whip antenna (AVM Instrument Co., Livermore, Calif.). Potential battery life
was 9-14 days. All birds were captured at the same locations east of Millers
Lake. Attempts were made to monitor the roost location and morning feeding
locations of instrumented birds; however, birds often could not be located
because signal range of the transmitter was limited to <0.05 km.
Color marking. In March 1986, 62 individuals from 5 flocks were captured
by mist-netting or rocket-netting. Individual birds were measured, marked
with colored, plastic-leg streamers (Guarino 1968), and released en masse by
flock. Different colors were used to designate individuals from different
flocks (orange, 13 individuals; green, 9; red, 14; pink, 17; white, 9).
Reports of color-marked birds were solicited from farmers and biologists in
the region.
Statistical Analyses
Spearman rank-correlation was used to estimate the relationship between
logl-transformed roost size and the number of flocking birds observed on road
surveys. Wilcoxon's matched pairs t-test was used to test for differences in
total number of flocking birds, mean flock size, and number of flocks between
north and south survey routes in 1987. Multiple linear regressions were used
to estimate relationships between flock occurrence and roost size, distance to
roost, time, weather, and land-use categories.
RESULTS
Flock Size, Composition, and Distribution
In January and March, morning flights of flocks were followed as far as 60
km southwest of Miller Lake before they were lost to view. Local residents
believe that, in mid-winter, birds may forage as far away as Lake Charles (90
km from the roost) (V. Fruge and J. Ardoin, Ville Platte, Louisiana, pers.
commun.).
Individual 30-km road surveys, conducted during the 3 years, 1986-1988,
yielded counts of blackbirds and starlings that ranged from 182 to 15,229
(Table 5-1). The total number of birds observed per survey declined with
advance of the season. The number of birds per route was significantly
correlated with roost size in 1987 (north: r2=0.71, P=0.03; south r2=0.81,
flock. Different colors were used to designate individuals from different
flocks (orange, 13 individuals; green, 9; red, 14; pink, 17; white, 9).
Reports of color-marked birds were solicited from farmers and biologists in
the region.
Statistical Analyses
Spearman rank-correlation was used to estimate the relationship between
logl-transformed roost size and the number of flocking birds observed on road
surveys. Wilcoxon's matched pairs t-test was used to test for differences in
total number of flocking birds, mean flock size, and number of flocks between
north and south survey routes in 1987. Multiple linear regressions were used
to estimate relationships between flock occurrence and roost size, distance to
roost, time, weather, and land-use categories.
RESULTS
Flock Size, Composition, and Distribution
In January and March, morning flights of flocks were followed as far as 60
km southwest of Miller Lake before they were lost to view. Local residents
believe that, in mid-winter, birds may forage as far away as Lake Charles (90
km from the roost) (V. Fruge and J. Ardoin, Ville Platte, Louisiana, pers.
commun.).
Individual 30-km road surveys, conducted during the 3 years, 1986-1988,
yielded counts of blackbirds and starlings that ranged from 182 to 15,229
(Table 5-1). The total number of birds observed per survey declined with
advance of the season. The number of birds per route was significantly
correlated with roost size in 1987 (north: r2=0.71, P=0.03; south r2=0.81,
flock. Different colors were used to designate individuals from different
flocks (orange, 13 individuals; green, 9; red, 14; pink, 17; white, 9).
Reports of color-marked birds were solicited from farmers and biologists in
the region.
Statistical Analyses
Spearman rank-correlation was used to estimate the relationship between
logl-transformed roost size and the number of flocking birds observed on road
surveys. Wilcoxon's matched pairs t-test was used to test for differences in
total number of flocking birds, mean flock size, and number of flocks between
north and south survey routes in 1987. Multiple linear regressions were used
to estimate relationships between flock occurrence and roost size, distance to
roost, time, weather, and land-use categories.
RESULTS
Flock Size, Composition, and Distribution
In January and March, morning flights of flocks were followed as far as 60
km southwest of Miller Lake before they were lost to view. Local residents
believe that, in mid-winter, birds may forage as far away as Lake Charles (90
km from the roost) (V. Fruge and J. Ardoin, Ville Platte, Louisiana, pers.
commun.).
Individual 30-km road surveys, conducted during the 3 years, 1986-1988,
yielded counts of blackbirds and starlings that ranged from 182 to 15,229
(Table 5-1). The total number of birds observed per survey declined with
advance of the season. The number of birds per route was significantly
correlated with roost size in 1987 (north: r2=0.71, P=0.03; south r2=0.81,
Table 5-1. Total number of birds and flocks, and mean (SD)
per 3-min stop and flock, recorded along north and south (s)
Evangeline Parish, Louisiana, 1986-1988.
number of birds
survey-routes,
Air
temp. Total number Mean number (+SD)
Date (0C) Birds Flocks Stop Flock
1986
Mar
Mar
Mar
Mar
Apr
Apr
Apr
Apr
1987
Mar
Mar
Mar
Mar
Apr
Apr
Apr
Mar
Mar
Mar
Mar
Apr
Apr
Apr
1988
Feb
Feb
Mar
Mar
Mar
Mar
Apr
Apr
Apr
95 (156)
562 (1,183)
164 (236)
15.5
-1.0
3.3
5.0
11.7
16.7
10.0
8.8
11.0
13.0
14.3
12.5
10.0
19.0
18.0
10.0
13.3
16.6
13.3
8.3
18.3
18.8
7.2
8.3
2.7
14.4
14.4
21.6
18.3
12.2
11.1
2,029
11,361
3,847
2,584
1,395
1,968
1,071
1,338
15,229
4,687
5,361
5,824
1,053
723
704
3,870
3,516
671
1,177
1,263
361
182
3,056
1,966
1,179
4,011
790
1,229
1,135
1,392
703
64 (102)
295 (155)
102 (134)
82
53
58
26
29
276
77
155
155
28
19
24
62
63
10
37
21
7
3
51
41
36
152
33
50
52
35
29
(170)
(117)
(74)
(38)
(67)
(763)
(106)
(380)
(254)
(26)
(29)
(53)
(122)
(136)
(23)
(84)
(75)
(10)
(2)
(79)
(67)
(67)
(409)
(33)
(62)
(53)
(61)
(64)
118
56
78
41
54
762
245
271
294
47
31
30
188
174
31
56
55
13
4
152
97
57
198
38
58
52
60
31
(314)
(122)
(127)
(55)
(113)
(1,452)
(356)
(671)
(537)
(61)
(60)
(122)
(268)
(359)
(45)
(116)
(130)
(16)
(8)
(227)
(182)
(102)
(500)
(63)
(87)
(80)
(117)
(69)
P=0.01) and 1988 (north r2=0.75, P=0.009), but less so in 1986 (north:
r2=0.71, P=0.055).
Numbers of flocks observed per survey and mean flock size declined with
advance of the season (Table 5-1). Large flocks (>500) were not observed
after the first week of April (Table 5-2). Flock size and the number of birds
per stop were highly variable among surveys.
The number of flocks observed per survey and mean flock size also declined
with increasing distance from Miller Lake. In the 1987 paired surveys, the
total number of birds observed on the north survey route (<15 km of the roost)
was greater than on the south route (>30 km from the roost) (t=2.19, P<0.02).
Similarly, mean flock size was greater on the north surveys than on the south
surveys (t=2.34, P<0.01) (Table 5-1).
Red-winged blackbirds comprised the majority of identifiable birds in each
survey (Table 5-3). Great-tailed and common grackles, brown-headed cowbirds,
and starlings were observed infrequently along the north route. However,
great-tailed grackles were observed frequently along the south route in late
March and early April.
The sex ratio among red-winged blackbirds observed per route was female-
dominated in 6 of 8 surveys in 1986, 11 of 14 surveys in 1987, and 7 of 9
surveys in 1988 (Table 5-4). The preponderance of female red-winged
blackbirds was most prominent in the latter half of March in all years.
Flocks near Millers Lake (north route) were female-dominated in late March and
early April, whereas those more distant (south route) were male-dominated
throughout March and April.
There was high variability in the activities of flocks among surveys
within and among years: 4-81% of flocks were in flight; 0-55% were foraging in
Table 5-2. Temporal distribution of flock sizes observed along north and south
(s) survey-routes, Evangeline Parish, Louisiana, 1986-1988. Percentage of
total is given only for flocks with <25 birds.
Number of birds in flock
Number
Date of Flocks <25 (%) 25-99 100-199 200-499 500-999 1,000+
1986
Mar
Mar
Mar
Mar
Apr
Apr
Apr
Apr
1987
Mar
Mar
Mar
Mar
Apr
Apr
Apr
Mar
Mar
Mar
Mar
Apr
Apr
Apr
1988
Feb
Feb
Mar
Mar
Mar
Mar
Apr
Apr
Apr
(47)
(26)
(59)
(60)
(76)
(52)
(77)
(81)
(38)
(40)
(51)
(47)
(62)
(79)
(80)
(56)
(62)
(95)
(70)
(85)
(95)
(100)
(55)
(67)
(65)
(56)
(48)
(43)
(40)
(66)
(71)
Table 5-3. Percentage species composition of birds observed along north and
south (s) survey-routes, Evangeline Parish, Louisiana, 1986-1988.
Percentages species composition
Red- Brown- Great-
winged headed Common tailed European
Date blackbird cowbird crackle crackle starling Unknown
1986
Mar
Mar
Mar
Mar
Apr
Apr
Apr
Apr
1987
Mar
Mar
Mar
Mar
Apr
Apr
Apr
Mar
Mar
Mar
Mar
Apr
Apr
Apr
1988
Feb
Feb
Mar
Mar
Mar
Mar
Apr
Apr
Apr
09s
16s
23s
29s
06s
13s
21s
Table 5-4. Sex ratios of red-winged blackbirds observed along north and south
(s) survey-routes, Evangeline Parish, Louisiana, 1986-1988.
Percentage Sex
identified ratio Dominant sex in flocks
Date to sex M:F Male Female None
1986
Mar
Mar
Mar
Mar
Apr
Apr
Apr
Apr
1987
Mar
Mar
Mar
Mar
Apr
Apr
Apr
Mar
Mar
Mar
Mar
Apr
Apr
Apr
1988
Feb
Feb
Mar
Mar
Mar
Mar
Apr
Apr
Apr
95
100
85
91
78
98
97
96
1.2:1
1:3.8
1:6.7
1:7.3
1:2.2
1:1.5
1.1:1
1:2.1
1:2.3
1:3.5
1:5.7
1:8.0
1:3.8
1:2.6
1:2.6
1:2.3
1:9.1
1:1.4
1:2.2
1.1:1
3:1
6:1
9.3:1
3.9:1
1:1.7
1:11.1
1:3.4
1:5.3
1:4.7
1:3.7
1:1.9
4
2
0
2
3
2
4
5
3
10
4
2
2
6
5
10
7
11
3
15
11
9
8
18
8
5
4
6
2
5
8
stubble or cultivated rice fields; and 0-63%.were observed loafing in trees or
shrubs near rice fields (Table 5-5).
Land-use Patterns
Rice-field preparation in the region south of Millers Lake began during
the first week of March each year. In 1986 and 1987, several experimental
research fields were planted earlier than normal, with planting being
completed by 16 March (Wilson et al. 1986, 1987, 1989). The earliest rice
plantings by producers occurred on March 9, 1987, and March 17, 1988.
In 1987, approximately 93% of land along the north route and 83% along the
south route had been planted to rice by the third week of April. Land close
to Millers Lake was prepared earlier than that more distant from the lake; on
March 23, 39,000 ha were in rice cultivation along the north route as compared
to 18,500 ha along the south route (Table 5-6).
Individual Movements
Telemetry. Radio-tagged birds were detected no farther than 8 km from
their capture site during 11 days of tracking from March 17-28, 1986 (Table 5-
7). The adult male remained alone near an irrigation canal 1 km east of
Millers Lake for the first 7 days and nights, after which time he returned to
the roost. Then, his signal was lost for 3 days. He was last located
roosting in the eastern portion of the lake on the night of March 28.
One second-year male was located in feeding flocks east of Millers Lake on
the mornings of March 20 and 21. Thereafter, no daytime locations were
obtained. The other second-year male was located daily with flocks foraging
1-8 km south and southwest of the roost. Both second-year males returned to
Millers Lake nightly to roost.
stubble or cultivated rice fields; and 0-63%.were observed loafing in trees or
shrubs near rice fields (Table 5-5).
Land-use Patterns
Rice-field preparation in the region south of Millers Lake began during
the first week of March each year. In 1986 and 1987, several experimental
research fields were planted earlier than normal, with planting being
completed by 16 March (Wilson et al. 1986, 1987, 1989). The earliest rice
plantings by producers occurred on March 9, 1987, and March 17, 1988.
In 1987, approximately 93% of land along the north route and 83% along the
south route had been planted to rice by the third week of April. Land close
to Millers Lake was prepared earlier than that more distant from the lake; on
March 23, 39,000 ha were in rice cultivation along the north route as compared
to 18,500 ha along the south route (Table 5-6).
Individual Movements
Telemetry. Radio-tagged birds were detected no farther than 8 km from
their capture site during 11 days of tracking from March 17-28, 1986 (Table 5-
7). The adult male remained alone near an irrigation canal 1 km east of
Millers Lake for the first 7 days and nights, after which time he returned to
the roost. Then, his signal was lost for 3 days. He was last located
roosting in the eastern portion of the lake on the night of March 28.
One second-year male was located in feeding flocks east of Millers Lake on
the mornings of March 20 and 21. Thereafter, no daytime locations were
obtained. The other second-year male was located daily with flocks foraging
1-8 km south and southwest of the roost. Both second-year males returned to
Millers Lake nightly to roost.
Table 5-5. Percentage of red-winged blackbird flocks engaged in different
activities (when first observed) in relation to habitat types along north and
south (s) survey-routes, Evangeline Parish, Louisiana, 1986-88.
Number Loafing Foraging Flying
Date of flocks (Shrubs) (Stubble Cultivated Other) (All)
1986
Mar
Mar
Mar
Mar
Apr
Apr
Apr
Apr
1987
Mar
Mar
Mar
Mar
Apr
Apr
Apr
Mar
Mar
Mar
Mar
Apr
Apr
Apr
1988
Feb
Feb
Mar
Mar
Mar
Mar
Apr
Apr
Apr
Table 5-6. Area (ha) of cropland by stage of rice production along the north
and south survey-routes, Evangeline Parish, Louisiana, 1987. The total area
surveyed along each route was 12.8 km2; approximately 93%of cropland along
the north route and 85% along the south route was committed to the production
of rice in 1987.
Stage of rice production
Out-of- Flood- Drill- Seedling
Date Route production Tilled planted planted stand
Mar 10 N 91,630 22,610 4,760 0 0
S 103,550 545 4,905 0 0
Mar 23 N 79,730 15,470 23,800 0 0
S 90,470 0 18,530 0 0
Apr 6 N 58,310 32,130 11,900 8,330 8,330
S 63,220 27,250 13,080 327 5,123
Apr 21 N 47,600 29,750 11,900 3,570 26,180
S 52,000 29,430 7,630 320 19,620
Table 5-7. Summary of movements of 6 radio-instrumented red-winged blackbirds
that were monitored between 17 and 28 March 1986 in Evangeline Parish,
Louisiana. All birds were captured 1 km east of Millers Lake. Evening roost
locations were determined between 1800 and 2000 hours, and daytime feeding
locations between 0700 and 0900 hours.
Maximum
distance Number of Found
Dates from capture Days Nights Nights with
Sex Age" monitored site (km) found found in roost flocks
M ASY Mar 17-28 1 7 8 2 no
M SY Mar 19-25 2 2 3 3 yes
M SY Mar 19-28 8 4 7 7 yes
F AHY Mar 17-24 5 4 6 6 yes
F AHY Mar 17-23 6 5 2 1 no
F AHY Mar 17-24 2 3 4 4 yes
aASY = after second-year; SY = second-year; AHY = after hatching-year.
b The first date denotes date of capture/radio-instrumentation.
Radio contact with the 3 adult females was maintained between March 17-24.
One bird fed daily with flocks east of the lake and roosted in the
northeastern portion of the lake each night. The second female was recorded
roosting 50 m east of the lake the first night, then spent subsequent days and
nights in fallow fields <2 km from the lake; this bird appeared to be
solitary. The third female was located 3 mornings in feeding flocks east of
the lake and 4 nights in the Millers Lake roost.
Color marking. Color-marked birds from 4 flocks were sighted 16 times (10
orange, 4 green, 1 red, 1 pink), always as the only tagged bird in a flock,
during the 3 weeks, March 24-April 13, 1986. One flock color (white) was
never resighted.
DISCUSSION
Sex-segregated flocks of red-winged blackbirds remained active in rice
fields of Evangeline Parish throughout the spring (March-April) planting
seasons. Female-dominated flocks of redwings were observed most frequently
along the north route and were the primary cause of bird damage to newly
planted rice in this area. Although the total number of birds and mean flock
size declined with seasonal advance after mid-March, flocks of 25-500 birds
were active, causing damage to unprotected fields, along the north route until
the third week of April.
Large flocks (>1,000 birds) were observed along the north route in the
mid-March, but not in February or April. On January 7-10, 1986, March 5-8,
1986, and February 15-19, 1988, large flocks were observed in Jefferson Davis
and Acadia parishes, 40-80 km south and southwest of Millers Lake, but not in
Evangeline Parish. If the birds observed in Jefferson Davis and Acadia
parishes were birds that roosted at Millers Lake, their choice of feeding
sites might reflect local food-resource availability. During the winter,
local food-resource depression proceeds outward from Millers Lake as the
millions of birds feeding on waste grain and weed seeds create a ring of
depleted feeding sites around the lake; thus, by late winter, birds must fly
greater distances from the lake to locate food. Because fields near Millers
Lake are prepared for rice planting earlier than more distant fields, birds
are attracted to the closer and richer feeding sites diskedd fields) during
March. This shift to foraging sites near the lake results not only in
selective gleaning of weed seeds, insect larvae, and waste grains from fields,
but also possibly in extending residence time of flocking birds that might
otherwise migrate or begin territory establishment.
Daily movements of red-winged blackbirds in this rice-growing region are
poorly understood; however, there is weak evidence suggesting fidelity to
foraging areas. The radio-telemetry findings suggest that redwings, like
starlings, may forage in local areas, or daily activity centers (Bray et al.
1975, Martin 1977, Morrison and Caccamise 1985), for several days before
moving to a different locality. However, radio-instrumented birds may have
moved more often or farther than was detected because radio signals were often
lost and tracking efforts were discontinuous. Alternatively, the short radius
of movement identified in this study might reflect poor adjustment by birds to
transmitters (Gessaman and Nagy 1988, Wanless et al. 1988). Observations of
orange-tagged birds were made repeatedly in fields within a 1-km radius,
suggesting fidelity to feeding sites.
There was no evidence for stability of flock membership during the spring
planting season in the Millers Lake region of southwestern Louisiana. None of
the 62 color-marked birds was resighted in the company of another marked
individual. More importantly, flock size decreased with distance from the
roost, a finding consistent with a radial model of flock dispersal, i.e.,
large flocks break up as they disperse from the roost. The model is distance-
based and suggests a dynamic pattern of membership of birds in the flocks
feeding in the vicinity of the roost.
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