Management opportunities and techniques for roof and ground nesting black skimmers in Florida

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Management opportunities and techniques for roof and ground nesting black skimmers in Florida final performance report
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Coburn, Lara
Cobb, David
Gore, Jeff
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Florida Game and Fresh Water Fish Commission
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Tallahassee, Fla.
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Table of Contents
    Front Cover
        Page 1
        Page 2
    Main
        Page 3
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    Back Cover
        Page 29
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Full Text














Management Opportunities and Techniques
for Roof- and Ground-nesting Black
Skimmers in Florida



Lara Coburnm
David Cobb2
Jeff Gore3



'Department of Wildlife and Fisheries Science
210 Nagle Hall, Texas A & M University
College Station, TX 77843

2Florida Game and Fresh Water Fish Commission
Rt. 7, Box 3055, Quincy, FL 31351

3Florida Game and Fresh Water Fish Commission
6938 Highway 2321, Panama City, FL 32409-9338













Final Performance Report
1 July 1995-30 June 1996
Florida Game and Fresh Water Fish Commission
Wildlife and Habitat Management Project 7650

May 1997


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SCIENCE
LIBR*. I Y


Suggested citation:


Cobum, L., D. Cobb, and J. Gore. 1997. Management opportunities and
techniques for roof- and ground-nesting black skimmers in Florida. Florida
Game and Fresh Water Fish Comm. Final Perf. Rep., Tallahassee. 24 pp + ii.


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MANAGEMENT OPPORTUNITIES AND

TECHNIQUES FOR ROOF- AND GROUND-
NESTING BLACK SKIMMERS IN FLORIDA


LARA COBURN1
DAVID COBB2
JEFF GORE3

'Department of Wildlife and Fisheries Science, 210 Nagle Hall, Texas A & M University
College Station, TX 77843
2Florida Game and Fresh Water Fish Commission, Rt. 7, Box 3055, Quincy, FL, 31351
3Florida Game and Fresh Water Fish Commission, 6938 Highway 2321, Panama City, FL, 32409-9338

Abstract: The black skimmer is listed in Florida as a Species of Special Concern, and reproductive success
is extremely important to the recovery of the species. Factors affecting reproductive success include
predation, human disturbance, food availability, pollutants in the environment, habitat loss, and shifts in
nesting from ground to roof colony sites. In this report, we outline pertinent aspects of black skimmer life
history, summarize threats to their reproduction, and recommend management techniques to protect and
enhance nesting areas. New projects to implement and test management techniques and to evaluate their
impacts are also outlined. Although designed specifically for skimmer colonies, these recommendations
may also benefit other beach- and roof-nesting birds in Florida.



INTRODUCTION

The black skimmer (Rynchops niger) is listed in Florida as a Species of
Special Concern (Wood 1996), and reproductive success is extremely
important to the recovery of the species. Factors affecting reproductive
success include predation (Burger 1981a, Gochfeld 1981, Quinn 1989),
human disturbance (Gochfeld 1981), food availability (Tomkins 1951; Erwin
1977a,b), pollutants in the environment (Hays and Risebrough 1972, Gochfeld
1975, Burger et. al. 1994), and habitat loss (Downing 1973). Skimmers have
colonized alternative habitats (such as roofs) in conjunction with least
terns(Sterna antillarum), presumably in response to disturbance by humans,
predation, and a reduction in available habitat.

Cracked and crushed black skimmer eggs have been found in roof-nesting
colonies in Florida, suggesting that these colonies may not be as productive as
those on the ground (Greene and Kale 1976, Fisk 1978a, Gore 1987).
Skimmers in ground and roof colonies in northwest Florida have been found
to have low, but similar, reproductive rates (0.0-0.25 fledged chicks per nest).
Hatching rates were higher in ground-nesting colonies, but fledging rates were
higher in roof-nesting colonies (Coburn 1995).


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2 FLORIDA GAME AND FRESH WATER FISH COMMISSION FINAL PERFORMANCE REPORT


Although Gore (1991) suggested that black skimmer populations in
northwest Florida were stable, there has been a marked shift in nesting sites
(Downing 1973), which may have negatively impacted populations over time.
Active management of roof colonies may help to increase low overall
reproduction rates and positively impact state and regional breeding populations.

In this report, we outline pertinent aspects of black skimmer life history,
summarize threats to their reproduction, and recommend management techniques
to protect and enhance nesting areas. New projects to implement and test
management techniques and to evaluate their impacts are also outlined. Although
designed specifically for black skimmer colonies, these recommendations may
also benefit other beach- and roof-nesting birds in Florida.


BIOLOGY AND NESTING ECOLOGY

Skimmers winter on the southern shores of the Gulf of Mexico (including
Florida) and from western Mexico to Argentina and Chile (Clapp et al. 1983,
Spendelow and Patton 1988, Stevenson and Anderson 1994). Their breeding
range extends from Massachusetts to southern Florida on the Atlantic coast, from
southern Florida to the Yucatan Peninsula on the Gulf coast, and from southern
California to Ecuador on the Pacific coast (Loftin and Smith 1996). They also
breed locally near San Diego, California, and inland at the Salton Sea (American
Ornithologists Union 1983, Clapp et al. 1983, Spendelow and Patton 1988).

Skimmers typically arrive at their colony sites during April and May. In
Florida, they arrive during mid- to late April, whereas in Texas they begin
breeding during mid-March (Oberholser 1974, Stevenson and Anderson 1994).
The time of arrival at the colony site affects their ability to produce multiple
clutches (Bent 1921). Generally, skimmers nest near major ocean inlets or
shallow estuaries (Portnoy 1977, 1978; Therres et al. 1978; Buckley and Buckley
1980) on bare or sparsely vegetated sand beaches, natural barrier islands, sand
shoals, sandbars, and dredged material sites (Erwin 1977b, Barbour 1978,
Buckley and McCaffrey 1978, Chaney et al. 1978, Gochfeld 1978, Parnell et al.
1978, Blus and Stafford 1980, Loftin and Smith 1996). They have also been
found nesting at inland marsh sites, but these colonies tend to be smaller than
those on sandy sites (Portnoy 1977, 1978; Erwin 1980; Erwin et al. 1981). In
Florida, skimmers usually nest on open sand beaches, dredged material islands,
and berms along highways and causeways (Schreiber and Schreiber 1978). They
were first reported on roofs in south Florida in the 1970s (Greene and Kale 1976)
and in northwest Florida in 1986 (Gore 1987). They have also been found nesting
at inland sites (Langridge and Hunter 1986) and near lakes and rivers in the
central and southern regions of the state (Sprunt 1954, Barbour 1978).


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NESTING BLACK SKIMMERS IN FLORIDA-Cobum, Cobb, and Gore


Although skimmers prefer to nest in open unvegetated sites, they have
been found in, and appear to tolerate, a wide range of habitats, usually nesting
is the presence of terns (Sterna spp.) (Gochfeld 1978). This association may
be due to the greater aggressiveness of the terns towards intruders (Gochfeld
1978, Erwin 1979). Skimmers nest in association with common terns (S.
hirundo) and gull-billed terns (S. nilotica) on the Atlantic coast (Soots and
Parnell 1975; Gochfeld 1976, 1978; Erwin 1977b, 1979; Buckley and
McCaffrey 1978; Blus and Stafford 1980; Buckley and Buckley 1980) and
with gull-billed terns, least terns, and Forster's tears (S. forsteri) in Florida
(Fisk 1978a,b). Skimmers may also nest in colonies adjacent to those of
laughing gulls (Larus atricilla), sandwich terns (S. sandivicensis), and royal
terns (S. maxima), but there is usually no overlap between subcolonies (Blus
and Stafford 1980). In some cases, the presence of royal and sandwich terns
may cause black skimmers to desert a colony site.

Skimmer colony size is highly variable and can range from <10 pairs
(Coburn 1995) up to 1,000 pairs (Spendelow and Patton 1988). There is a
positive relationship between the number of terns and the number of skimmers
found at a colony site. Large black skimmer colonies are more likely to be
found in conjunction with large tern colonies (Gochfield 1978).

Skimmers are highly specialized tactile hunters and feed mainly on fish
and aquatic invertebrates (Erwin 1977a). They have a laterally flattened,
scissor-like bill, that is distinctive in that the lower mandible is longer than the
upper. Skimmers feed by flying in a straight line over the water with the lower
mandible below the surface. On contact with food, they snap their head down
and close their bill (Tomkins 1951). Skimmers feed mainly at night, but they
have also been seen wading into shallow pools during the day to pick up small
fish (Zusi 1962, Barbour 1978).

There is distinct sexual dimorphism, with males being approximately one-
fourth larger than females (Burger and Gochfeld 1990). Juveniles and non-
breeding adults are separable by the presence of a white band across the nape
of the neck. There is little known about the age at which they first breed, but
it is probably around 3 years. The oldest known black skimmer was 12 years
old (Kennard 1975), however, this is probably a underestimate because many
other larids live >20 years (Terres 1991).

Skimmers begin courtship (feeding, aerial chases, and displays) and
copulation soon after they arrive on the breeding grounds. Courtship and
copulation continue during nest creation, egg laying, and incubation. Eggs are
laid above the high water mark in an unlined scrape approximately 3.5 cm deep
and 10-15 cm in diameter (Terres 1991, Coburn 1995). Nests are generally


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4 FLORIDA GAME AND FRESH WATER FISH COMMISSION FINAL PERFORMANCE REPORT


widely spaced within the colony, with a nearest neighbor space of 198 187
cm (Burger and Gochfeld 1990). Mean inter-nest distance is greater in colonies
that contain few nests (542 + 520 cm; 6 nests) than in colonies which are larger
and contain more nests (121 65 cm; 35 nests) (Burger and Gochfeld 1990).

Egg laying begins in May and continues through July (Erwin 1977b). If
the first nest is destroyed early in the season, a second clutch will often be laid
at a new site within the colony. In comparison with other seabirds, skimmers
have an unusually large clutch of eggs (1-5). Clutches of more than 5 eggs
may result from 2 females laying in 1 nest (Erwin 1977b). The egg laying
period lasts from 4 to 8 days. Eggs are laid either on successive or alternate
days, but rarely on the same day. The length of the incubation period is 21-26
days, but there is disagreement as to which sex incubates (Burger 1981b,
Burger and Gochfeld 1990, Terres 1991).

Skimmers attend and guard their chicks until they fledge. Adult
aggression increases at hatching because small chicks are easily killed by
conspecifics and predators. Skimmers may kill small tern and black skimmer
chicks that wander into their territories (Safina and Burger 1983). Until they
fledge, chicks are fed partially digested and whole fish, primarily by the male
parent (Burger and Gochfeld 1990). Chicks can leave the nest after a few
days, but do not fledge until they are approximately 21-23 days old.

When an intruder approaches, young chicks crouch in the sand or near
vegetation. They create their own scrapes using the same sand-kicking
behavior used by adults to create nest scrapes (Bent 1921, Hays and
Donaldson 1970). In the event of a human intruder, adult skimmers may lead
their chicks several hundred meters away from the nest (Burger and Gochfeld
1990). For chicks, consequences of running from the nest can include being
eaten by predators or conspecifics, being subjected to aggression from
neighboring territorial adults, being separated from their parents for a time
sufficient to cause debilitation or starvation, and/or being exposed to the
effects of sun or rain (Gochfeld 1981).

Erwin (1977b) saw strong expressions of hatching asynchrony and sibling
dominance and found that 10 of 11 chicks fledged were first-hatched chicks.
This may be due to food limitations. Dorward (1962) suggested that sibling
aggression found in boobies and gannets (Sulidae) is related to severe food
limitation. Male chicks begin to grow more rapidly than females after about
11 days. Male chicks fledge at an average of 295 grams, while female chicks
fledge at 264 grams. Fledged chicks remain near the colony site until late
August or September (Erwin 1977b).


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NESTING BLACK SKIMMERS IN FLORIDA-Coburn, Cobb, and Gore


THREATS TO COLONIES AND REPRODUCTION

Habitat Loss

The most critical requirement of black skimmers is undisturbed nesting
sites because eggs, nestlings, and adults are highly vulnerable to disturbance
(Loftin and Smith 1996). However, beach-nesting habitat suitable for
skimmers is being lost at an alarming rate. Erosion has reduced the amount of
sand remaining above the high tide line (Downing 1973) and beach and dune
areas have been developed for residential purposes. Most of the remainder has
been developed into recreational areas with roads, parking lots, public
beaches, restaurants, and marinas. Due to the increased development of beach
habitats and the associated recreational use of undeveloped beaches, especially
during the breeding season (Barbour 1978), skimmers have begun nesting on
alternative substrates and in alternative habitats, including gravel covered
roofs, salt marshes, dredge material deposits, and causeways (Frohling 1965;
Downing 1973; Greene and Kale 1976; Fisk 1978a,b; Gore 1987; Burger and
Gochfeld 1990; Coburn 1995). In Florida, however, dredge material deposits
are often sandy areas frequently used for recreation, and causeways typically
support high levels of automobile traffic.

This extremely heavy competition from humans for beach and dune
habitat has left most colonial seabirds access to only a few fragments of their
former nesting range. There are relatively few colonies of skimmers and terns
found on natural sites along the Atlantic coast (Gochfeld 1978). Perhaps
<20% of all least tern and black skimmers east of the Mississippi River nest
on natural sites (Downing 1973). Gore (1991) found a majority of black
skimmer colonies in northwest Florida on roofs.

Direct Human Disturbance

Human disturbance has been implicated as a cause of colony abandonment
and lowered reproductive success in several species of colonial nesting
waterbirds (Buckley and Buckley 1972, Nisbet and Drury 1972, Conover and
Miller 1978, Manuwal 1978), including black skimmers (Safina and Burger
1983, Potter 1992, Coburn 1995).

Recreational uses of beaches adjacent to black skimmer colonies pose a
major threat to colony success. Humans directly disturb colonies by
approaching, standing, or walking along the periphery; entering, walking, or
driving through the colony; and exploring and remaining in the colony for
recreational activities. The major impact of these activities on breeding






6 FLORIDA GAME AND FRESH WATER FISH COMMISSION FINAL PERFORMANCE REPORT




skimmers is the interruption of incubation, brooding, and feeding. This is
most detrimental during the hot portion of the day, as unattended eggs and
chicks can quickly overheat (Burger and Gochfeld 1990).

In addition to humans walking in or near colonies, other activities can
have a detrimental impact on the success of black skimmer colonies, including
the use of jet skis, boats, cars, and off-road vehicles near or in colonies (Burger
and Gochfeld 1990, Coburn 1995). Potter (1992) noted that 8 out of 11 dead
adult skimmers found on the JFK Causeway in Laguna Madre, Texas, were
killed by collisions with automobiles.

Deliberate attempts to harm adults, eggs, or chicks have been noted in
Massachusetts (Austin 1933, 1946), New Jersey (Burger and Gochfeld 1990),
and New York (Gochfeld 1974, Post and Gochfeld 1979), however, few
incidents of vandalism have been noted in black skimmer colonies in Florida.
Signs posted around colonies and the vigilance of beach residents probably
help to keep these incidents to a minimum.

Environmental Contamination

Eggshell thinning due to environmental contaminants has been associated
with nesting failure (Ratcliffe 1967). Thinning was not noticed in black
skimmer eggs in New York, New Jersey, South Carolina, or northwest Florida
(Blus and Stafford 1980, Burger and Gochfeld 1990, Coburn 1995), but has
been observed in Texas (White et al. 1984). From 1978 to 1981, White et al.
(1984) found DDE residues up to 51 ppm, with 35% of all eggs tested
containing >10 ppm. Eggshells were 4-12% thinner than normal, but residue
levels were not significantly correlated with shell thickness (White et al.
1984). These levels of contamination were, however, within the range of
values known to negatively affect reproduction in other avian species (L.
Stickel 1973, W. Stickel 1975).

Shell-less eggs can also be a result of contamination. Burger and
Gochfeld (1990) found a few shell-less eggs each year (0.5% of all eggs laid),
usually scattered around the colony rather than in nests. Tejning (1967)
showed that organomercurial contamination in hens produced an increase in
the number of shell-less eggs laid outside the nests.

Custer et al. (1983) and Nisbet and Reynolds (1984) provided evidence
that local variation in organochlorine levels in tern eggs and young were
related to local variation in residues in their food. King (1989) found DDE
and PCB in fish eaten by black skimmers in Galveston Bay, Texas.






NESTING BLACK SKIMMERS IN FLORIDA-Coburn, Cobb, and Gore


Data on metals in estuarine sediments in New York revealed ample
opportunity for them to enter the black skimmer food chain (Greig and
McGrath 1977). Mercury, cadmium, lead, and selenium have been found in
skimmers from Galveston Bay, Texas (King and Cromartin 1986). All
indications suggest that skimmers become contaminated through fish, their
primary food item.

An inconsequential number of oiling events have been reported for black
skimmers (Burger and Gochfeld 1990).

Food Availability

Lack (1954) hypothesized that food is the major limiting factor in bird
populations in general, and low black skimmer fledging success in Virginia
has been attributed to limited prey availability (Erwin 1977a). Custer and
Mitchell (1987) found evidence of contamination in dead young, but attributed
the majority of deaths to starvation. Large fish kills, due to high levels of
organochlorine contaminants, reduced the availability of food for chicks.

Accidents and Entanglements

Black skimmers have been observed entangled in fishing gear (Nickell
1964; L. Coburn, pers. obs.), kite strings, and plastic six-pack rings (Gochfeld
1973a,b). Skimmers have also been known to be entangled in natural
vegetation (Burger and Gochfeld 1990). Metal wire fences enclosing sites
have also caused injury to young that fly or run into them (Potter 1992).

Weather and Flooding

Direct impacts of storms-particularly hurricanes-to birds include the
geographical displacement of individuals and increased mortality due to high
winds, heavy rains, storm surges, and flooding (Wiley and Wunderle 1993).
Bare sand washes and blows readily during storms and can force adults to
leave nests unattended (Downing 1973). Eggs can tolerate short periods of
inundation, but the major threat is the lethal chilling of embryos. Young chicks
are most vulnerable to overheating, but older chicks are more susceptible to
chilling from rain (Burger and Gochfeld 1990). Potter (1992) reported that
storm events were the primary cause of black skimmer nest failure on the JFK
Causeway over the Laguna Madre, Texas.

Indirect impacts include the loss of food supplies, nests, and nesting sites
(Wiley and Wunderle 1993). Weather and flooding have been implicated as
major reasons for black skimmer nest failure (Burger 1982, Morgan 1982,


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8 FLORIDA GAME AND FRESH WATER FISH COMMISSION FINAL PERFORMANCE REPORT


White et al. 1984, Potter 1992, Coburn 1995), and Florida is particularly
susceptible to severe storms and hurricanes during the summer. Colonies on
low-lying areas are easily inundated by storm surges and high tides associated
with these weather events.

Predation

Predation is a major threat to colonies and has been identified as a primary
factor in black skimmer colony abandonment (Burger 1982) and loss of eggs
and young in some colonies (DePue 1974, Morgan 1982, Coste and Skoruppa
1989, Burger and Gochfeld 1990, Coburn 1995). Typical mammalian
predators include foxes (Vulpes vulpes, Urocyon cinereoargenteus), coyotes
(Canis latrans), raccoons (Procyon lotor), and feral dogs and cats. Avian
predators impacting black skimmer colonies include owls (Strigidae),
laughing gulls, ruddy turnstones (Arenaria interpres), fish crows (Corvus
ossifragus), grackles (Quiscalus spp.), and great blue herons (Ardea herodias)
(Parkes et al. 1971, DePue 1974, Morgan 1982, Greene and Kale 1976, Potter
1992, Coburn 1995).

Cracking of Eggs on Roofs

In northwest Florida, Coburn (1995) found that roof-nesting black
skimmers had a lower hatching rate (8%) than those nesting on the ground
(68%). Roof nesting generally fails because the eggs develop cracks, caused
in part by the incubating adults. Black skimmer nests on roofs are
significantly shallower than those on the ground (1.7-2.0 cm and 3.4-4.9 cm,
respectively). The problem may be a lack of sides on the nest needed to
distribute the incubating bird's weight, or that the gravel or hard roof
concentrates the bird's weight onto a single point on the egg, causing it to
crack. Cracks can be monitored as they become larger, and eventually the egg
becomes crushed or the contents leak out (Coburn 1995). Cracked eggs have
not been seen in ground colonies, however, they have been documented in roof
colonies in various areas of Florida (Greene and Kale 1976, Gore 1991,
Coburn 1995), suggesting that it is not a localized problem.


MANAGEMENT AND PROTECTION OF GROUND COLONIES

The main threats to black skimmer ground colonies in Florida include
disturbance, predation, flooding and weather, vegetative succession, food
availability, and lack of suitable nesting habitat. The following actions can be
taken to minimize these threats, protect black skimmer ground colonies, and
maintain nesting sites.







NESTING BLACK SKIMMERS IN FLORIDA-Coburn, Cobb, and Gore


Limit Human Disturbance

Human disturbance of black skimmer ground colonies can be deterred
somewhat by posting signs around the periphery of the colony. Signs nailed
to wooden posts spaced 6-30 m apart (O'Meara and Gore 1988) should be
placed 100-200 m from the edge of the colony (Erwin 1989). Rope can be
strung between the signs as a further deterrent. Fencing provides added
protection by providing a physical barrier to humans and vehicles. Plastic
fencing attached to heavy metal rods can be placed 100-200 m around the
edge of the colony; wire fencing is not recommended as it may result in injury
to young birds (Potter 1992).

Signs, rope, and/or fencing should be in place at traditional nesting sites
prior to 1 April (i.e., before migrating terns and skimmers arrive at the site).
All protection activities should be completed either in the early morning or late
afternoon to minimize heat stress.

Fences, rope, signs, and posts should be maintained throughout the
season, as strong storms and high winds can damage them. All items should
be removed from the sites at the end of the breeding season, usually in late
August. Signs are available from the regional nongame wildlife biologist in
each regional office of the Florida Game and Fresh Water Fish Commission
(see Appendix A).

Sites should be monitored for human intrusion periodically during the
breeding season. If human disturbance becomes and remains a problem,
enforcement of state and federal laws protecting black skimmers and their nesting
sites may be necessary. Educating beachgoers, both residents and tourists, is also
effective in deterring humans from entering or otherwise disturbing colonies.
People seem to be less likely to disturb colonies once they learn why it is
important to minimize disturbance of the birds (L. Coburn, pers. obs.).

Another important consideration is limiting disturbance by bird watchers,
nature photographers, bird banders, and biologists. If it is necessary for
biologists or other investigators to monitor colonies, visits should be made in
the early morning or late afternoon in order to limit exposure of eggs and chicks
to the weather. Visits should be limited in duration (<30 minutes if possible) to
minimize the amount of time each bird is kept off its nest. Investigators should
also wear drab colored clothing, avoid looking directly at the birds, and
approach the colony in an oblique manner (Burger and Gochfeld 1981).


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10 FLORIDA GAME AND FRESH WATER FISH COMMISSION FINAL PERFORMANCE REPORT


Exclude and Control Predators

Black skimmers rely heavily on aggressive terns to thwart potential
predators, however, predation from mammals and birds continues to be a
problem in ground colonies (Coburn 1995). Fencing may be used to partially
excluded mammalian predators from colonies. This may deter some large
predators such as dogs, foxes, and coyotes. Electric fencing has been
suggested to control smaller mammalian predators and those that can climb
plastic fences (O'Meara and Gore 1988), however, this may injure or kill
young skimmers or terns that accidentally run or fly into it.

Domestic animals, such as dogs and cats, should be kept off the beaches as
they will chase adults and eat eggs and chicks. State and federal regulations
should be consulted before initiating any program to trap or kill feral cats or
dogs and other wild animals responsible for the destruction of nests or colonies.

Most avian predation occurs after an initial human disturbance, when
adults leave the nest to attack the intruder. This behavioral response leaves
nests, chicks, and eggs unattended (Austin 1929). The key to limiting avian
predation is limiting human disturbance to the colony. Some avian predators
(e.g., laughing gulls) can be partially controlled by limiting vegetative
succession and decreasing the amount of available nesting habitat for species
other than skimmers and terns. Burger and Gochfield (1990) suggested that
when gulls first begin to nest at or near a black skimmer or tern colony, it is
important to eliminate the nest, thereby discouraging further nesting by gulls.
If the nest is left intact and is successful, 1 pair can serve as a nucleus,
attracting dozens or even hundreds of pairs within 2 or 3 years (Burger and
Gochfeld 1990). Again, state and federal regulations should be consulted
before any action is taken.

Minimize the Effects of Weather and Flooding

Little can be done to prevent storms or flooding from damaging colony
sites and destroying chicks and embryos. However, the extent of the damage
can be mitigated somewhat by limiting human disturbance so adults do not
leave nests unattended. Also, wooden pallets or boards can be placed on the
site to provide shelter for chicks. These shelters can also provide shade for
chicks during the hottest portion of the day.

Platforms can be built on low-lying sites that are frequently over washed by
storm surges and high tides. This would entail placing wooden pallets end to end
over the colony site and covering the pallets with clean dredge material. Sand
could also be placed on the site without pallets, but the pallets may help anchor






NESTING BLACK SKIMMERS IN FLORIDA-Cobum, Cobb, and Gore


the sand. Elevated platforms can also be used. Decoys may help to insure that
terns and skimmers nest on the elevated site. Any work should be completed and
decoys should be in place prior to 1 April in order to attract returning birds.

Control Vegetation

Skimmers prefer to nest in open, unvegetated sites (Spendelow and Patton
1988) but will nest in a wide range of habitats in order to nest with terns
(Gochfeld 1978). Vegetation should, therefore, be managed to accommodate
the species of tern that skimmers nest with, primarily least terns. Least terns
prefer to nest on sites that have approximately 4% vegetative cover (Burger
1984) found in dispersed clumps (Thompson and Slack 1982).

Removing vegetation is preferable to killing it without removal.
Vegetation can be removed manually or with roto-tillers, bulldozers, tractors,
or plows (O'Meara and Gore 1988). Vegetation can also be covered with
dredge material. Methods of killing vegetation include spraying with salt
water or with Ureabore, a highly concentrated salt compound (Kotliar and
Burger 1984). Herbicides can also be used, but their use should be limited
because of the threat of environmental contamination. Any efforts to control,
remove, or kill vegetation should be completed at traditional nesting sites prior
to 1 April in order to minimize disturbance to returning terns and skimmers.

Protect and Maintain Feeding Sites

Most black skimmer ground colonies are within close proximity to estuaries,
bays, the Gulf of Mexico, or the Atlantic Ocean. Food is probably not a limiting
factor in these cases, however, environmental contamination may result in fish
kills or sublethal contamination of fish. These situations have not been noted in
Florida to date, but close monitoring of the food supply is necessary. Due to the
size of the water bodies used as feeding sites by black skimmers, measures to
protect these areas may not be feasible. Periodic monitoring of fish and black
skimmer egg contents may, however, help to identify trends and elucidate
possible reasons for reproductive failures if they occur.

Provide Additional Habitat

Nesting habitat can be created for least terns and black skimmers (Loftin
and Smith 1996). Dredged material can be deposited on existing beaches or
barrier islands when vegetative succession has occurred. This may be ideal if
the site has been used previously, is relatively free of mammalian predators,
and has low levels of human disturbance. Dredge material can also be
deposited offshore to create islands of new habitat. The sand should be clean


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12 FLORIDA GAME AND FRESH WATER FISH COMMISSION FINAL PERFORMANCE REPORT


and can be a mixture of sand and shell; clay and silt should be avoided as eggs
can become glued to the finer particles in wet weather (Thompson and Slack
1982). Small amounts of shell debris may serve to increase the site's
attractiveness to terns (O'Meara and Gore 1988) and skimmers. Dredge spoil
islands should be built high enough to avoid being washed over by high tides
and should also be large enough to provide some temporal stability, as they will
erode over time due to wave action and storms. The islands should have a slight
slope to allow for drainage, but if the slope is too great, heavy rains and high
winds will erode the sand quickly, possibly covering nests and eggs and burying
chicks. Shade for chicks can be provided with wooden pallets. Created sites
should be posted to deter human disturbance and the islands should be
monitored for mammalian predators and to control vegetative succession.

Decoys have been used successfully to attract terns and skimmers to
suitable nesting habitat, both new or unused (Slaydon 1981, Kress 1983,
Kotliar and Burger 1984). Decoys can be made from styrofoam or wood, with
a dowel placed in the underside of the model for mounting in the sand. Decoys
representing both species should be used on the site and placed in pairs or
singly about 1.5 m apart (O'Meara and Gore 1988). All decoys should be in
place prior to 1 April and can be removed once birds colonize the site.
Nonaggressive sounds of nesting terns and skimmers can be played to further
attract birds to the site (Kress 1983). Although decoys have been used
successfully in the first year at 2 sites in northwest Florida (Florida Game and
Fresh Water Fish Commission, unpubl. data), decoys and recordings may have
to be used for several years before terns and black skimmers colonize a site
(Kotliar and Burger 1984).


MANAGEMENT AND PROTECTION OF ROOF COLONIES

The primary cause of reproductive failure of black skimmers on roofs is
the cracking of eggs (Greene and Kale 1976, Gore 1987, Cobum 1995).
Other factors impacting roof colonies include human disturbance, predators,
weather and flooding, and food availability. Several actions can be taken to
limit the effects of these factors and to increase the reproductive success of
skimmers on roofs.

Limit Disturbance

Due to the nature of their location, roof colonies are not subject to the
amount of intense disturbance ground colonies endure. Maintenance workers
and biologists are typically the only humans present in roof colonies.
Unnecessary visits should be avoided throughout the breeding season, and, if







NESTING BLACK SKIMMERS IN FLORIDA-Cobun, Cobb, and Gore


necessary, visits should be made early in the morning and limited in duration.
Eggs on roofs are well camouflaged and difficult to see, thus, care should be
used to avoid stepping on eggs or young chicks. Mobile chicks will run from
the nests when a human enters the colony, and will often inadvertently run off
the side of the roof. Therefore, if walking on the roof, care should be used to
avoid running chicks. Chicks that fall off roofs are generally able to survive
and should be returned to the colony.

Exclude Predators

Roofs are generally inaccessible to mammalian predators, but cats and
raccoons may be able to climb onto them. Maintenance workers should be
consulted, and if mammalian predators are found on roofs they should be
trapped and destroyed. Preventive measures should then be taken to preclude
access to roofs by other mammalian predators that may locate the colony.
Avian predators that can impact roof colonies are usually chased away by
terns, but wooden pallets can be placed in the colony to provide cover for
chicks. Human disturbance should be minimized to prevent providing
predators with the opportunity to attack unattended eggs and chicks.

Minimize the Effects of Weather and Flooding

Heavy rains can flood nests, leaving eggs and young chicks in standing
water. Eggs can float away in high water or can be blown out of nests by high
winds. A slight pitch (i.e., 5') in the roof may help provide drainage and
decrease the amount of standing water. Roofs on new buildings should have
a slight pitch and sufficient drainage routes incorporated into their design.

Roofs provide chicks with some protection from storms. During severe
weather or during the hot portion of the day, chicks that are able to leave the
nest can crouch in the corer of the parapets or by the side of roof structures
to avoid wind, rain, and the sun. Wooden pallets can also be added in the
middle of roofs to provide shade.

Maintain Feeding Sites

Black skimmer roof nesting sites are found at varying distances from
large bodies of water, and skimmers may be feeding at smaller sites closer to
the roof. It is possible to watch birds leaving the colony and follow them to
find their feeding sites. Smaller water bodies usually have lower numbers of
fish and may be more susceptible to contamination. However, these smaller
bodies of water are easier to monitor and manage than larger sites. Checking
fish and eggs for contaminants may be worthwhile, and protection of feeding
sites may be necessary.


____






14 FLORIDA GAME AND FRESH WATER FISH COMMISSION FINAL PERFORMANCE REPORT


Increase Hatching Success

Increasing Nest Depth.-Eggshell cracking, the primary cause of reduced
hatching success in roof-nesting skimmers, is probably a result of 2 factors:
attributes of the gravel and the shallowness of nests. Experiments to increase
nest depth on roofs has shown some evidence of increased hatching success
(Cobum 1995). Nest depth has been shown to have a significant positive
relationship with hatching success, and a uniform thicker layer of gravel may
provide the needed support for the brooding adults' body weight. An increase
of 2.5 cm of gravel, creating an average depth of at least 4.0 cm on the roof,
may help improve hatching. Construction gravel is not expensive and the main
cost would be in transporting gravel to the site and placing it on the roof.

Building owners and managers should be consulted about augmenting the
gravel cover on their roofs. Several owners in northwest Florida have been
cooperative. The gravel should be added prior to 1 April to insure that when
the birds arrive no humans or equipment will be on the roof, causing the birds
to colonize elsewhere. The gravel should be of the same type and color as is
already in place.

Decrease Gravel Size.-The most significant problem related to the
gravel substrate is individual particles becoming embedded in the underlying
tar and protruding up into the nest. Very shallow nests with loose gravel under
the eggs also characteristically produce cracked eggs. The effect of the larger
gravel is somewhat mitigated in deeper nests in which a depression can be
made for the eggs and the edges of the nest can support the body weight of the
adult (Coburn 1995). Although it does not appear that gravel size is the most
important factor in the cracking of the eggs, this attribute is probably
important. Gravel typically found on roofs varies in size from 4.9 mm to 16
mm in diameter (Cobum 1995). When adding gravel to roofs to increase nest
depth, it may be prudent to use a smaller size (i.e., <4.9 mm).

Prevent Flooding.-In order to improve drainage, a slight pitch or slope
can be added to a new roof during construction. However, this will not help
those birds nesting on preexisting roofs. If roofs on which skimmers and terns
nest are renovated, adding a slight pitch (i.e., 50) could significantly improve
nesting conditions. Well-constructed roofs do not collect standing water; roof
designs or repairs that prevent roof leaking will also reduce the chance of
flooding of skimmer nests.

Investigate the Possibility of Environmental Contamination.-
Eggshell thinning should be investigated as a possible cause for cracking
eggs at each location where cracks have been noticed. Cracked eggs from






NESTING BLACK SKIMMERS IN FLORIDA-Coburm, Cobb, and Gore


roofs can be compared with uncracked eggs from other sites and to historical
data on black skimmer shell thickness. If a significant thinning has
occurred, every effort should be made to protect feeding sites and reduce
contamination. Site protection is often difficult, however, so initial efforts
should be to alleviate substrate problems

Increase Fledging Success

Fence Roof Edges, Drain Pipes, and Vents.-Young chicks often fall off
the sides of roofs, causing injury or death from the fall or from starvation.
Fencing >15 cm in height made from plastic screen, plastic coated wire, or
galvanized hardware cloth can be placed around roof edges that are not
protected by a gutter. Although chicks may be injured by running into the
wire, net survival should increase. Chicks can also fall into drain pipes and air
conditioning vents unless these openings are fully screened.

Provide Protection for Chicks.-Wooden pallets can be placed on roofs
to provide shade for young chicks during the hot portion of the day, shelter
during severe weather, and protection from predators.


POPULATION ESTIMATION AND METHODS OF COLONY CENSUS

Censusing black skimmer colonies is necessary to monitor population and
nesting habitat trends. Colony size can be estimated in several ways. Total
numbers of adults can be estimated from the colony periphery by counting
birds loafing at the colony site and adding the number of birds flying overhead.
Nest numbers can be estimated by counting the numbers of unattended nests
and the numbers of adults sitting on nests. These counts can be made from
outside the colony using binoculars or a spotting scope. If the colony is very
large, a portion of the colony can be counted and extrapolated to the entire
colony. Estimates of roof colonies can be performed by counting the maximum
number of adults observed in flight over the building (O'Meara and Gore
1988), although this method may greatly underestimate colony size (Gore
1991). Aerial photography is also effective in assessing total colony size, as the
dark color of the skimmers contrasts with the light colored sand and roof gravel.

In order to assess nesting success, a survey within the colony is required.
Only an experienced observer possessing the appropriate permits and
permission from property owners should enter a colony. Serious studies of
nesting success should involve periodic visits to each colony. Surveys should be
performed either early in the morning or late in the afternoon in order to
minimize disturbance. Visits should be limited to <30 minutes to minimize the






16 FLORIDA GAME AND FRESH WATER FISH COMMISSION FINAL PERFORMANCE REPORT


amount of time each bird is kept off the nest. Nests on the ground can be marked
with wooden tongue depressors and roof nests can be marked with metal
washers or wooden blocks painted white and numbered. During each visit,
nests, eggs, and chicks should be counted, and each egg checked for cracks.

In intensive studies of reproductive success, visits should be made once a
week. For less comprehensive studies, visits and counts should be made :3
times during the nesting season in order to minimize disturbance to the colony.
Generally, counts should be performed in late May to early June (egg laying
and incubation period), mid- to late June (hatching), and July (fledging)
(O'Meara and Gore 1988), but actual dates should be based on observations of
nesting behavior at individual colonies.

Any information gathered from colonies regarding size, location, habitat,
chicks produced, etc., can be mailed to the Section Leader, Survey and
Population Monitoring, Nongame Wildlife Program, Florida Game and Fresh
Water Fish Commission, Rt. 7, Box 3055, Quincy, FL, 32351.


ACKNOWLEDGMENTS

We thank the regional nongame wildlife biologists of the Florida Game
and Fresh Water Fish Commission and the Apalachicola National Estuarine
Research Reserve staff for providing unpublished nesting data compiled as a
component of this project. Persons interested in these data should address
their inquiry to: Section Leader, Survey and Population Monitoring, Nongame
Wildlife Program, Game and Fresh Water Fish Commission, Rt. 7, Box 3055,
Quincy, FL, 32351.


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___






20 FLORIDA GAME AND FRESH WATER FISH COMMISSION FINAL PERFORMANCE REPORT




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__


~_I_






22 FLORIDA GAME AND FRESH WATER FISH COMMISSION FINAL PERFORMANCE REPORT




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24 FLORIDA GAME AND FRESH WATER FISH COMMISSION FINAL PERFORMANCE REPORT


APPENDIX A

Regional offices of the Florida Game and Fresh Water Fish Commission. To obtain Nesting Area signs
or other technical assistance in protecting black skimmer colonies, please contact the nongame biologist in
the nearest regional office.


Northwest Region:





Northeast Region:





Central Region:





South Region:





Everglades Region:


Nongame Biologist
Florida Game and Fresh Water Fish Commission
3911 Highway 2321
Panama City, FL 32409
904-265-3677

Nongame Biologist
Florida Game and Fresh Water Fish Commission
Route 7, Box 440
Lake City, FL 32055
904-758-0525

Nongame Biologist
Florida Game and Fresh Water Fish Commission
1239 SW 10th Street
Ocala, FL 32674
904-732-1225

Nongame Biologist
Florida Game and Fresh Water Fish Commission
3900 Drane Field Road
Lakeland, FL 33803
941-648-3205

Nongame Biologist
Florida Game and Fresh Water Fish Commission
551 North Military Trail
West Palm Beach, FL 33415
407-640-6108
















Date Due

Due Returned Due Returned
SEP 0