All birds share the following
characteristics: (1) tetrapods
with feathers; (2) forelimbs modified to form
wings; (3) respiration through lungs; (4) endothermic; (5) internal
fertilization; (6) shelled amniotic eggs; and (7) acute vision (Campbell 1993).
Birds fill a wide range of ecological roles. Nectar-eating species such as
hummingbirds, herbivorous species such as ducks which feed on aquatic
vegetation, and granivorous
species such as doves and buntings all depend on
plant matter for a substantial portion of their diet. These species represent
primary consumers in the food web. Insectivorous species such as swallows and
warblers, and shorebird species such as plovers and sandpipers that feed on
crustaceans and molluscs represent the next higher trophic
level. Omnivorous
species such as grackles and crows eat a combination of plant and animal matter
which represent multiple trophic levels. Raptor species such as hawks, eagles,
and owls represent the highest bird trophic level. These birds feed on mammals,
reptiles, and amphibians
as well as other birds and are considered top
predators. Scavengers such as vultures and gulls play an important role in
removal of dead animal matter and nutrient recycling.
The ACE Basin study area has an extremely rich bird life. Over half of the
species of birds that occur in North America inhabit the 320,000 hectares
(790,000 acres) of the ACE Basin study area. Several bird surveys have been
conducted in the ACE Basin study area. The
South Carolina Department of
Natural Resources (SCDNR) Wildlife and Freshwater Fisheries
division has
conducted colonial waterbird surveys since 1969. This program uses both ground
and aerial surveys to determine the number of various birds species that nest
in colonies in the ACE Basin study area. Nesting colonies can be found in a
variety of habitats including upland forests, forested wetlands, beaches, and
bird keys (Colonial waterbird nest
locations
). The South Carolina Center for Birds of
Prey
has conducted the South Carolina Coastal Hawk Migration
Survey since 1995. This survey focuses on the hawk species that migrate into
coastal South Carolina during the winter. The ACE Basin survey site is located
three miles inland on Edisto Island in estuarine
marsh habitat surrounded by
forested wetlands, croplands, and upland forests. The United States Geological
Survey (USGS) has conducted the Breeding
Bird Survey (BBS)
in the
ACE Basin study area at a site near Walterboro since 1970. This is a
large-scale roadside survey of North American birds with the objective of
estimating population change for songbirds. The Walterboro route of this survey
travels through upland forests, forested wetlands, and old- field habitats (Breeding bird survey route
). The Audubon Society has coordinated the
Christmas Bird Count
(CBC)
in the
ACE Basin study area since 1990. Groups of volunteers work together to identify
and count all birds possible within a 24-km (15-mile) circle. The survey site
is centered at Brickyard Launching Bridge, Bennetts Point Road and includes
both freshwater and estuarine non-forested wetlands, forested wetlands, upland
forests and old- field habitats (Christmas bird survey
). The SCDNR has compiled an ACE Basin Bird Checklist
that lists all the birds inhabiting the area along with their residency and
abundance status.
There are about 8,600 species of birds in the world divided into 28 orders. Of these, approximately 280 species of birds in 17 orders occur in the ACE Basin study area. Many of these birds migrate in tremendous numbers to South Carolina from northern breeding grounds to spend their winters or to rest before continuing their migration to more southern areas. Because birds can fly, the barriers that restrict travel for many animals are easily avoided by birds. Therefore, birds are rarely restricted to one environment and are often found in a variety of habitats (Potter et al. 1980). However, birds frequently exhibit a preference of one habitat over others and this habitat preference affects distribution and abundance. The avifauna of different habitats in the ACE Basin study area will be discussed below. This discussion is based mainly on information obtained from Sandifer et al. (1980) and Potter et al. (1980).
- Marine Subtidal Waters
- Beaches
- Bird Keys
- Dunes
- Maritime Shrub
- Maritime Forest
Marine Subtidal Waters
An estimated thirty species of birds utilize the marine subtidal
habitat as
feeding grounds. Eight of these species are common year-round residents and
seven are common in winter (Bird
habitat
). Five species that
utilize this habitat (e.g. shearwaters, phalarope, gannet, and petrel) are
mainly pelagic
and are seen only rarely near shore. The concentration of fish
and invertebrates
by the Gulf Stream provide ample prey for these birds. The
majority of birds in this habitat are piscivorous
and catch fish by aerially
diving (e.g. terns, gannets); surface diving (e.g. loons, cormorants); or
surface skimming (skimmers, gull-billed terns). Two species (e.g. Wilson’s
petrel and northern phalarope) feed on nearshore zooplankton
while diving ducks
consume benthic
organisms (Sandifer et al. 1980). The brown pelican and the
double-crested cormorant are probably the most common permanent resident
species that feed in subtidal habitats of the ACE Basin study area.
Double-crested cormorants dive from the surface to feed on small fish and can
often be seen swimming with just their head and neck above water. These birds
feed in subtidal waters but are most likely to nest in stunted cypress trees
along heavily forested lake shores (Potter et al. 1980).
Beaches
An estimated forty-four avian species can
be found on the intertidal
beaches of the ACE Basin study area. Twenty-three of these
are common year-round residents of the beaches. The avifauna
community of
beaches can be divided into three broad categories: (1) marine species such as
brown pelicans, royal terns, and
black skimmers which feed on fish and use the beaches only for resting or
breeding; (2) macrobenthic
predator species such as sanderlings, American
oystercatchers, plovers, and sandpipers which hunt in the sand for annelids,
crustaceans, and mollusks; and (3) beach scavengers such as gulls, crows,
boat-tailed grackles, and vultures which feed mostly on dead animal matter that
litters the beaches (Sandifer et al. 1980).
Although most species of birds found on beaches are not limited to this environment, there are a few species (e.g. sanderlings, red knots, piping plovers, and Wilson’s plover) which almost exclusively inhabit beaches. Sanderlings are small shorebirds that forage for mollusks, worms, and crustaceans in the surf zone. These birds are common year-round residents of the ACE Basin study area but breed in the Arctic (Sandifer et al. 1980). Brown pelicans are one of the most common marine seabirds seen on the beach. This species breeds on bird keys and feeds on fish by plunging from the air into the ocean. Both the black and turkey vulture can be seen on ACE Basin beaches. These birds hunt for carrion in a variety of habitats but nesting occurs most often in woodlands or swamps. Black vultures mostly hunt by sight over open habitat while turkey vultures are most often forest hunters and therefore depend on smell to locate prey. Both species are common year-round residents and are important ecologically because of their consumption of dead animal matter. The peregrine falcon is a winter predator in this habitat and is regularly seen in the ACE Basin study area during the fall migration in October. This species is endangered due to pesticide exposure, but numbers in the ACE Basin study area are steadily increasing as the bird recovers. Peregrine falcons are frequently seen perched low on remote barrier island beaches where they feed on shorebirds (Murphy, pers. comm.).
Bird Keys
Bird keys and banks are small isolated islands that usually occur in tidal
inlets and broad bays. They are very dynamic habitats because they are
susceptible to over wash by storm action and spring tides and because they tend
to migrate in response to inlet
morphology (Sandifer et al. 1980). Bird keys
are used by a number of colonial birds for breeding because they provide
protection from terrestrial predators such as raccoons. Historically, several
bird keys in the ACE Basin study area were utilized by colonial waterbirds.
However, recent anthropogenic
alterations in channel flows have destroyed these
keys (i.e. Egg Bank Island at the mouth of Harbor River). Currently, the only
bird key in the ACE Basin study area on which birds nest is Deveaux Bank at the
mouth of the North Edisto River.
Twelve species of birds are estimated to
currently nest on Deveaux Bank. Forty-two percent of all colonial waterbird
nests identified in the ACE Basin study area since 1969 were found to occur on
Deveaux Bank (Colonial waterbird
nesting sites
). Royal terns, brown pelicans, laughing gull, sandwich
terns, and black skimmers are the dominant nesting species on bird keys (Colonial waterbirds. Colonial birds that breed on bird keys exhibit distinct habitat
preferences. Brown pelicans prefer higher grounds where there is sufficient
vegetation for nest construction. The number of brown pelican nests on Deveaux
Bank has decreased since the 1970s (Pelican abundance
) probably because of erosion of Deveaux Bank
over the past 20 years. Prior to the 1970s, pelican populations declined
because of DDT but since the ban on DDT in 1972, pelican populations have been
on the rise and in 1985 they were delisted as an endangered species
on the
Atlantic coast. Royal tern nests are nothing more than depressions in the sand;
if the nest is destroyed, a new nest is built and new eggs are laid. Royal tern
colonies are the largest colonies on Deveaux Bank with thousands of nests often
present during one breeding season (Royal tern abundance
). Sandwich terns also occur in large numbers
and nest in mixed colonies with royal terns. Laughing gulls establish their
colonies on the fringes of brown pelican and royal tern colonies in order to
exploit the food source provided by pelican and tern eggs and young. Since
1975, laughing gull nests have been recorded in the ACE Basin study area only
at Deveaux Bank. Both black skimmers and gull billed terns nest in unvegetated
areas of Deveaux Bank above the mean high tide mark.
Dunes
The maritime dune habitat is a harsh environment with many stressful
physical attributes (e.g. blowing sand, high summer temperature, limited
freshwater and sparse vegetation). Sea oats are the dominant plants and the
abundance of seeds are food for the many granivorous
birds which inhabit the
dunes. Seeds make up the bulk of the diet for eleven of these species of birds
including doves, blackbirds, sparrows, and cardinals. The Savannah sparrow is a
common winter resident that feeds on dune plant matter. This species consumes
more insects than other sparrows but the majority of the diet consists of grass
and weed seeds. Insectivores (e.g. nighthawk, swallows, chimney swift, and
warblers) are the next largest trophic
group. The common nighthawk is a common
summer resident which nests directly on the sand dunes and in many other open
areas. These birds eat a variety of insects including flying ants, mosquitos,
beetles, and gnats. The fish crow is the dominant scavenger and the great
horned owl is the most abundant raptor. Large numbers of great horned owls have
been observed hunting over sandy beaches for rodents and waterfowl (Sprunt and
Chamberlain 1970).
Shorebirds such as terns, plovers, and sandpipers use the dunes for resting, feeding, and nesting. The least tern uses the dune habitat for breeding habitat with nesting occurring in the fore dune area. The least tern has declined in numbers over the years because of anthropogenic disturbance of beach habitats and is now listed as a state threatened species. This species has adapted to the changing environment by establishing nesting colonies on rooftops. Large shopping centers with gravel rooftops can support a surprising number of least tern nests. However, gravel rooftops have recently been replaced by rubber rooftops which are less expensive. Only K-mart still maintains the gravelled rooftops which can be used as nesting sites by the least tern (Murphy pers. comm.).
Maritime Shrub
Moving inland from the dune community, is the maritime shrub community which
offers fewer habitats to bird species than does the nearby maritime forest. The
lack of an understory
and the low plant density provide little food for
granivorous
and herbivorous species. Many species in this habitat are
omnivorous either year round (mockingbird and grackles) or seasonally (tree
swallows). Most species in the shrub community consume insects (e.g. kingbird,
yellow-throat, and sparrow hawk). The majority of species in this habitat are
residents of the adjacent dune or maritime forests and utilize the shrub
community only as feeding or nesting grounds. Two species, tree swallow and
yellow-rumped warbler, are closely associated with this habitat because during
cold weather when insects are scarce they consume wax myrtle berries. Also, the
ground dove nests in wax myrtle and is, therefore, dependant on this habitat.
Ground doves are granivores and consume a wide variety of seeds. This species
is currently a state-threatened species
along with the least tern, glossy ibis,
and Wilson’s plover. A top predator in this habitat is the sharp-shinned
hawk which is a fairly common winter resident. This raptor inhabits woodlands
and ventures into the shrub community to hunt for small birds, insects, and
small mammals.
Maritime Forests
Maritime forests provide more diverse
habitats than the beach, dunes, or shrub habitats, therefore, they contain a
more diverse avian community. The dense vegetation and low canopy height of
undisturbed maritime forests, however, act to decrease
habitat types compared to inland forests. Therefore, avian species diversity is
usually lower than that in inland forests (Sandifer et al 1980). Of the 280
birds found in the ACE Basin study area, almost one- third (87) can be found in
the maritime forests . Many of the birds in
the maritime forests are passerine birds including flycatchers, swallows,
crows, nuthatches, wrens, kinglets, thrushes, vireos, warblers, sparrows,
blackbirds, grackles, and finches. Insects make up all or part of the diet of
most passerine birds with grains and fruits also important. Warblers, swallows,
vireos, and flycatchers feed almost exclusively on insects, while sparrows,
buntings, and finches feed mainly on vegetarian matter such as fruits, seeds,
and grains (Sprunt and Chamberland 1970). The yellow-rumped warbler, or myrtle
warbler, is the most common winter warbler in the ACE Basin study area. This
species consumes many small insects along with the fruits of various plants
(e.g. waxmyrtle and bayberry). The painted bunting is probably the most
visually spectacular bird found in this habitat. This species is a common
summer resident that builds its nest in bushes, trees, or Spanish moss and
raises up to three broods a year. Turkey and black vultures are both common
year-round scavengers in maritime forests. Top year-round predators in the
maritime forests include three species of owls (screech owl,
barred owl, and great horned owl)
and two species of hawks (red-tailed and red-shouldered). The
red-shouldered hawk occupies the ecotone
between wetlands and forests and
frequently feeds in wetland forests (Murphy pers. comm.). This species nests high in large trees near dense
woodlands and feeds mostly on frogs and snakes.
The estuarine
subtidal
habitat is an open
water system used mainly by birds for resting and feeding. All of the birds
found in this habitat are water birds which feed on fish, benthos, carrion, or
insects. Three species of gulls (laughing, herring, and ring-billed) are
considered common year-round residents of the ACE Basin study area. Herring and
ring-billed gulls are abundant in winter but rare in other seasons and laughing
gulls are abundant in summer but rare in winter. All three species eat a
variety of food items and are important in the consumption of dead animal
matter. Terns, cormorants, and brown pelicans inhabit open water areas to feed
on fish. The double-crested cormorant is a common year-round resident that
doesn’t breed in South Carolina. Other water birds such as the lesser
scaup, ring-necked duck and ruddy duck feed on aquatic vegetation. The lesser
scaup is a common winter resident that breeds in Canada. It eats the seeds of
water lilies, pondweed, water milfoil, and widgeon grass along with mollusks,
crustaceans, and insects. The osprey is the only bird of prey to utilize this
habitat where it feeds on its primary prey, fish. Ospreys can be seen
year-round but are rare in December and January.
The intertidal salt marshes provide habitat for a variety of avian species. Eighty-seven species of birds utilize salt marshes for feeding or breeding. Thirty-two of these are common year-round residents while an additional thirteen are common winter residents. Wading birds such as herons and egrets use this habitat for feeding on their primary prey which includes mummichogs, mullet, menhaden, and penaeid shrimp. Other birds such as rails, swallows, wrens, and blackbirds use the smooth cordgrass as feeding and nesting grounds. The clapper rail is a strict inhabitant of ACE Basin salt marshes. This species feeds, roosts, nests, and raises its young on the Spartina marsh (Sandifer et al. 1980). The clapper rail feeds on crabs, minnows, shrimp, and marsh insects and, in turn, is an important food item for a variety of mammalian and avian predators.
Two sparrows, the
sharp-tailed and the seaside, also rely heavily on salt marshes. The sharp-
tailed sparrow is a common winter resident while the seaside is a year-round
resident that nests on the marsh surface where Juncus and
Spartina are the dominant plants (Bent 1968). Both species consume
mostly animal matter including insects, crustaceans, and marine worms. The
insectivorous long-billed marsh wren is another dominant species of the salt
marsh. This species is found only in estuarine and freshwater wetlands and its
eggs and young are heavily preyed upon by salt marsh mammals such as raccoons,
marsh rice rats, and minks. Four raptors
are found in estuarine emergent
wetlands. Of these, the northern harrier or marsh hawk is probably the most
important. This raptor is a common winter resident that is most abundant during
migrations. Marsh hawks prey upon rodents and small birds that inhabit the
marsh.
The abundance of fish and invertebrates in intertidal flats provides excellent feeding opportunities for many avian species. Fifty-five species of birds are estimated to occur in this habitat and over half of these are common year-round residents in the ACE Basin study area. Almost all of the species found here are wading birds or shorebirds including herons, egrets, ibises, gulls, plovers, sandpipers, and terns. The boat-tailed grackle and fish crow are the only non-aquatic species found regularly on intertidal flats. Both species feed on small fish and invertebrates.
Eight species of herons and egrets utilize the intertidal flats as feeding grounds with the great egret, snowy egret, and tricolored (Louisiana) heron being the most abundant. Many of the shorebirds feed extensively in this habitat but breed in others (e.g. beaches or bird keys). Migrations into and out of the intertidal flats can greatly affect abundances of some shorebirds in this habitat. Abundances of herring gulls, ring-billed ducks and American oystercatchers increase in the winter as northern birds migrate south. Other species such as the semipalmated plover, ruddy turnstone, and least sandpiper decline in abundance during the summer as they leave to breed in other habitats.
The American oystercatcher is possibly the most notable bird in this
habitat. Although it breeds on beaches, this bird feeds exclusively on the
flats. Oystercatchers feeds mainly on mollusks including oysters and clams.
Upon finding a gaping oyster the bird plunges its beak between the shells to
cut the adductor muscle. This causes the shell to fall open and the oyster is
easily obtained. Oystercatchers were hunted to near extinction in the early
1900's (Sandifer et al. 1980) but today the species is considered a common
year-round resident. Although this bird is now common, the number of nests
observed in the ACE Basin study area is low (Oystercatcher abundance
) because of anthropogenic
disturbance of its nesting
habitats.
- Impoundments
- Non-Forested Wetlands
- Palustrine Forests
Impoundments
Impoundments are estuarine
or freshwater wetlands which have been diked to
create managed bodies of water. Most impoundments
in the ACE Basin study area
are managed for waterfowl and are characterized by brackish
or freshwater
vegetation. The waterfowl in impoundments consists of geese (Canada geese);
puddle ducks (e.g. mallards, teals, gadwalls, wigeons, and shovelers); and
diving ducks (e.g. ring-necked ducks, buffleheads, mergansers, and ruddy
ducks). Puddle ducks are the most abundant group of waterfowl. These species
preferentially feed on wild rice, spikerush, pondweeds, smartweeds, bulrushes,
and wigeon grasses and, therefore, many impoundments are managed for these
plant species (see Harvest
management of waterfowl
). The Christmas Bird Count (CBC) found that the
green-winged teal
and the American
wigeon (i.e. baldpate) were among the five most abundant species of birds found
within the survey area (Christmas
bird count
). About 75% of the survey area is non-forested wetlands
that contain numerous impoundments.
Rails, coots, and gallinules are also commonly found in impoundments. The
king rail inhabits freshwater impoundments while the clapper rail is restricted
to estuarine impoundments. Virginia rails, sora rails, American coots and the
common gallinules (i.e. common moorhen) inhabit both estuarine and freshwater
impoundments where they feed on snails, insects, fish, and aquatic plants.
Herons, egrets, and ibises also utilize impoundments for feeding. The CBC found
the white ibis to be the most abundant wading bird in the survey area with the
great egret, glossy ibis, and great blue heron also abundant (Christmas bird count
).
Other shorebirds that are common in impoundments include yellowlegs, plovers, dowitchers, sandpipers and avocets. The American avocet is a rare winter inhabitant of the ACE Basin study area. This species feeds by sweeping its long bill through shallow water and consuming the aquatic insects and marsh plant seeds that it stirs up (National Geographic Society 1987; Potter et al. 1980). The bald eagle and osprey, although uncommon, both use this habitat as hunting grounds.
Non-Forested Wetlands
The avifauna
of palustrine
non-forested wetlands contains many of the same
species that occur in impoundments
and estuarine
emergent wetlands. The
transitional zone from brackish
to freshwater contains a high diversity of
vegetation for food and habitat. Ninety-two of the 177 avian species in the ACE
Basin study area are estimated to occur within non-forested wetlands.
Twenty-one of these species are considered common year-round residents while an
additional 25 are common winter residents (Bird habitat
). The CBC found that the
common grackle and the red-winged
blackbird
were the most abundant species within the survey area. These birds are both omnivores, but there is a
seasonal shift from a diet of mostly insects in the spring and summer to mostly
seeds and grains in fall and winter (Bent 1965).
Non-forested wetlands provide nesting grounds for a variety of birds including gallinules, wrens, swallows, red-winged blackbirds, and king rails. The tree swallow is probably the most abundant swallow during the winter while the purple martin is the most abundant summer resident. The purple martin nests in hollow trees and bird houses and forages for insects over open areas such as ponds, rivers, and marshes (Bent 1963a). Other birds such as herons, egrets, and ibises use freshwater wetlands as feeding grounds. Great egrets are a common sight in ACE Basin wetlands. This wading bird nests in mixed species colonies in tall trees near or over water.
Waterfowl are abundant in this habitat because the freshwater vegetation is often preferred over salt marsh vegetation for food. Most waterfowl species such as mallards, teals, gadwalls, and pintails are winter residents and, in general, are present in the ACE Basin study area from September to May. The ring-necked duck is especially abundant here during the winter because of its preference for the seeds of freshwater plants such as waterlilies and watershields.
Several insectivores including the common yellowthroat,
marsh wren, barn swallow, and
purple martin are common in palustrine wetlands while granivorous
species
include the swamp, song, and Savannah sparrows. Numerous raptor species are
found in non-forested wetlands. Ten of the thirteen hawk species identified by
the South Carolina Coastal Hawk Migration Survey inhabit non-forested wetlands
(Hawk migration
). Both the red-tailed and red-shouldered hawks are common
year-round residents and marsh hawks and sparrow hawks are the most common
winter residents.
Bald eagles are the top
predators here, although these birds nest in trees and use the wetlands only
for hunting. In 1977, there were only 13 occupied bald eagle breeding areas
known to remain in all of South Carolina. Six of the 13 were in the ACE Basin.
This remnant population allowed for the recovery of the eagle population to
occur far more rapidly then in adjacent states where no nesting eagles remained
(Bald eagle nests
). The ACE Basin provides high quality
nesting habitat with abundant prey, large trees for nesting and protection from
disturbance at nest and foraging sites.
Typically, an eagle’s diet is composed of 80% fish, 10% birds, 5% mammals and 5% carrion. The presence of abundant bird prey such as coots, moorhens and ducks, however, results in a greater percentage consumption of birds in the diet. This may have mitigated some of the effects of pesticides, with fish prey more heavily contaminated then birds. The extensive acreage of managed marsh impoundments that occur in the ACE Basin study area and elsewhere in the state, may explain why South Carolina maintained a remnant eagle nesting population.
During the 1998 nesting season, 30 of the 129 occupied breeding areas in
South Carolina were in the ACE Basin (Bald eagle nests
). While
the percentage of nests in the ACE Basin has declined as eagle nesting has
repopulated statewide, it still remains one of the high density centers for
nesting. Nesting occurs during the winter with the peak of egg laying the last
week in December. Winter nesting is adaptive as water clarity is maximum,
wintering bird prey are available and shad and herring runs coincide with the
maximum energy demands of the chicks in the nest.
Bald eagles can be seen during any month of the year, but are most abundant during the winter. They reach their lowest density during July and August because many birds move north after the nesting season.
Palustrine Forests
Forested wetlands in the ACE Basin study area provide avifauna
a wide
variety of habitat types. The occurrence of wet and dry tree species and both
grassland as well as closed canopy sites contribute to a high diversity of
birds (Sandifer et al. 1980). An estimated 132 species of birds can be found in
forested wetlands; the highest diversity of any environment in the ACE Basin
study area. Of these 132 species, 34 are common year-round residents and 23 are
common winter residents. Common year-round residents include omnivores such as
the American crow, common grackle, and
red-winged blackbird;
herbivores such as wood ducks and mourning doves; insectivores such as
Carolina wrens, white-eyed vireos, and common yellowthroats; scavengers such as
black and turkey vultures; and raptors
such as bald eagles, barred owls,
red-shouldered hawks, and red- tailed hawks. Common winter residents include
American goldfinches, robins, cedar waxwings, dark-eyed juncos, eastern
phoebes, hooded mergansers, lesser scaups, song sparrows, and ruby- crowned
kinglets.
Palustrine forests are important nesting grounds for wading birds
such as herons, ibises, and egrets. White ibises which are the second most
abundant colonial nesting bird in the ACE Basin study area, nest almost
exclusively in wooded swamp habitats (Wading bird nesting habitat
). Cattle egrets and great egrets also nest in wooded swamps with
71% and 41%, respectively, of the nests occurring in this habitat. In the ACE
Basin study area, wood storks
also
nest exclusively in wooded swamps. These birds are federally endangered and,
until 1981, no eggs or young had been reported in South Carolina. Nesting
populations of wood storks have steadily increased since 1981 and in 1997, 653
wood stork nests were identified in the ACE Basin study area (Wood stork abundance
).
Ten species of birds are closely associated with forested wetlands. Most of
these species are warblers which feed on the large number of insects that occur
in this habitat. Seven of these species, the blue-winged warbler, golden-winged
warbler, Tennessee warbler, Swainson’s warbler, black-throated warbler,
gray-cheeked thrush, Louisiana waterthrush, and worm-eating warbler, are rare
or uncommon but can be found in forested wetlands of the ACE Basin study area
(Bird habitat
). The Breeding Bird
Survey (BBS) which includes forested wetland habitats identified
Swainson’s warbler in 1977, 1993, and 1995. The BBS also found the
Kentucky warbler, which is considered a fairly common summer resident of the
ACE Basin study area, to be present every year from 1991 to 1996.
The riverine system as discussed here is limited to the open water areas of rivers and does not include adjacent wetland areas. The avifauna of the riverine systems of the ACE Basin study area is made up of species that occur in other habitats and use the rivers for feeding or resting (Sandifer et al. 1980). Species found here forage in the rivers for aquatic plants or animals. Grebes and wading birds hunt for fish either by diving (grebes) or by fishing from shore (wading birds). Shorebirds such as sandpipers and plovers also fish from shore in the rivers for crustaceans, mollusks, fish, and aquatic insects while gulls and terns forage on the rivers for similar prey.
About eleven species of ducks use the river to forage for aquatic vegetation
such as pondweeds, wigeon grass, wild rice, eelgrass, and marsh grass. The wood
duck is a dominant year-round resident in riverine systems. This species nests
on or near water in the natural cavities of dead or live trees (Potter et al.
1980) and young hatch in April or May. Both year-round residents and wintering
residents of wood ducks can be found in the ACE Basin study area. Insectivores
such as swifts and swallows hunt over the rivers for aquatic insects. The
chimney swift is a voracious insectivore which feeds over rivers. This small,
dull-colored bird often gathers in large numbers and spends most, if not all,
of its day on the wing catching beetles, flies, and ants (Bent 1964; Sprunt and
Chamberlain 1970). 
The osprey is the only bird of prey to utilize this habitat extensively. Ospreys can be seen during all months of the year, but are in low numbers during December and January. The osprey not only hunts in the riverine waters for fish but it also commonly nests on dead snags, channel markers, and power line poles in rivers. The osprey is almost exclusively a fish eating species and, like the bald eagle, suffered a dramatic population decline in numbers as a result of pesticide contamination. By the mid 1970’s, it is estimated that the statewide population had declined to 300 nesting pairs. Currently this population has recovered to more than 1,000 nesting pairs. Populations within the ACE Basin continue to increase with nesting concentrated on the Combahee River. Unlike the Charleston Harbor population of ospreys, the birds nesting in the ACE Basin only occasionally build nests on manmade objects. The ACE Basin study area serves as a control that can be used to evaluate the effects of development on the osprey population (Murphy pers. comm.).
Old field habitats consist of croplands, fields, and pastures along with their adjacent edge communities in which secondary succession is just beginning. Diversity and densities of birds tend to be low in newly abandoned farmlands and to increase as succession proceeds. The edge community supports a high diversity and density of avifauna. The combination of open grasslands, transitional shrubs, and trees provides ideal habitat for many species by providing access to feeding grounds, nearby escape cover, and prime nesting habitat.
Seventy-four birds are estimated to occur
in old-field habitats of the ACE Basin study area. Of these, 20 are common
year-round residents, 14 are common winter residents, and seven are common
summer residents. Many of the birds (e.g. cardinals, mockingbirds, mourning
doves, buntings, and sparrows) that occur here fulfill part or all of their
dietary needs from the seeds, grains, and fruits that are plentiful in these
fields. Others (e.g. Carolina wren, common yellowthroat, brown thrasher, and
eastern meadowlark) consume the insects that are feeding in this habitat.
Because many old-field birds consume vast numbers of insects, they are
extremely beneficial to farmers in insect control. Omnivores common in this
habitat include American crow, red-winged blackbirds, and common grackle. The
Breeding Bird Survey and the Christmas Bird Count, which both traverse areas
containing old field habitats, found that these species were quite abundant.
Eight birds of prey are found in this habitat. Seven of the eleven species
of migratory hawks identified on Edisto Island by the South Carolina Coastal
Hawk Migration Survey inhabit old-field communities (Hawk migration
). The red-tailed hawk is probably the dominant raptor
in this habitat. This large broad-winged hawk feeds primarily by the perch and
wait method of hunting. Food consists primarily of small mammals. Because of
the extensive time this species spends perching, it is conspicuous on the
landscape and is frequently blamed for the depredations of other species of
predators. This is the largest and most common of the broad-winged (Buteo)
hawks found in the ACE Basin study area (Murphy pers. comm.).
The avian fauna in pine forests of the ACE Basin study area is less diverse than many other habitats. Coastal plain pine forests don’t develop dense understories because of frequent fires as well as poor soil conditions. The lack of a dense understory combined with the low habitat variability in monotypic pine canopy results in a low species diversity. Fifty-two species of birds are estimated to occur in pine forests and almost half are considered common year-round residents. Insect-eaters, generalists, and seed-eaters are represented by warblers, bobwhites, and the brown-headed nuthatch, respectively.
Four species of warblers can be found in pine forests and the pine warbler is most commonly associated with this habitat. Pine warblers build their nests in pine trees and forage for grasshoppers, locusts, moths, beetles, flies, and other insects. The brown-headed nuthatch is a seed-eating species that is most common in pine forests. This species builds its nests in cavities of decaying trees and lines the nest with the sheaths of pine seeds. The nuthatch consumes mostly pine seeds along with some insects (Sprunt and Chamberlain 1970). The bobwhite quail is an omnivore that is abundant in pine forests. Bobwhites are a popular game bird in the ACE Basin study area and are hunted extensively (See related subsection: Hunting: Bobwhite Quail.) This species builds its nest in areas where vegetation is dense and provides abundant cover. Bobwhites eat a myriad of foods including seeds, insects, fruits, leaves, spiders, crustaceans, and tubers (Bent 1963b).
Woodpeckers are also abundant in this habitat, with the red-bellied being
most abundant. The Breeding Bird Survey found the red-bellied woodpecker (Woodpecker abundance
) was the most abundant woodpecker species in
upland areas. Seven birds of prey can be found in pine forests. The
screech-owl, which often builds its nest in woodpecker holes, is a dominant owl
species in this habitat (Sandifer et al 1980). Both red-shouldered and
red-tailed hawks were found to occur in the upland areas surrounding
Walterboro.
Pine-Hardwood Upland
Forests
Pine-hardwood forests in the ACE Basin study area have more bird species
than the other upland communities. These forests have extensive subcanopy and
understory
growth that is not found in either pine forests or in old-field
communities (see the Plants: Upland Community
). The
addition of the subcanopy and understory vegetation greatly augments the
habitat types available and, therefore, more birds can be found here (Sandifer
et al 1980). As with many other forested habitats, insectivores are common in
mixed upland forests. Along with warblers and woodpeckers, the Carolina wren,
tufted titmouse and white-eyed vireo are abundant (Insectivores
). The Carolina wren is the state bird of
South Carolina and is found in a variety of habitats. This species will nest
almost anywhere a suitable nook or cavity is available including natural
cavities, birdhouses, and mailboxes. Carolina wrens feed on a variety of
insects and are, therefore, beneficial to farmers. Common granivores in this
habitat include the mourning dove
and the American goldfinch. Both of these species subsist almost entirely on
seeds. The mourning dove is an important game species and is a year-round
resident of the ACE Basin study area. The American goldfinch is a winter resident that consumes the seeds
of sweetgum trees as a major part of its diet (Martin et al. 1951). The same
species of raptors
occur in mixed forests as in the pine forests. Dominant
scavengers are the black and turkey
vultures
while the great-horned owl is the top
predator (Sandifer et al 1980).
Indiscriminate hunting for plumage or sport
around the 1900's had devastating effects on many species of birds. The great
egret, the snowy egret, and least tern were hunted almost to extinction for
their plumage while birds such as the wild turkey and
wood duck
were over-exploited by
sport hunting. Conservation efforts that began in the 1800's have resulted in
the protection of many of these exploited birds. The
Endangered Species Act of 1973
was established to protect species that are in danger of becoming extinct. All
migratory birds are now protected by federal law as are nonmigratory hawks and
owls. These birds can not be killed legally except during regulated hunting
season on specific species (Potter et al. 1980).
Unregulated pesticide use has also resulted in population declines of bird species. DDT, which was used in the United States prior to 1972, was particularly harmful because it was applied for a wide variety of purposes and its toxicity persists long after application. Ospreys, wading birds, and bald eagles are among the species of birds that were affected by pesticides such as DDT. These birds are generally higher level predators that are feeding on organisms that have bioaccumulated harmful pesticides. As these birds bioaccumulate pesticides, both lethal and sublethal effects can result. Birds can die from lethal effects or from weakness associated with the pesticides which causes increased susceptibility to disease or predators. One of the most common sublethal effects associated with pesticide use is thinning of eggshells. Thin eggshells are more likely to break, thereby killing the young. Obviously, when the young die, the population declines greatly. Through more strict regulations on pesticide use, bald eagles, pelicans, and ospreys, as well as many other bird species are slowly recovering.
The most serious conservation issue for birds of the ACE Basin study area is habitat destruction. Although much of the ACE Basin study area is protected from development, there are still concerns. The conversion of upland communities into pine plantations and croplands has limited habitat diversity resulting in a subsequent decline in species diversity. Development of the beach communities has decreased the nesting habitat available to shorebirds. Redivergence of river channels has destroyed several bird keys that are important to colonial waterbirds. Habitat destruction in other regions can significantly affect species in the ACE Basin study area. As discussed above, many birds are migratory and use the ACE Basin study area only for a part of the year. For example, songbirds such as the summer tanager migrate from their summer breeding grounds (which includes the ACE Basin study area) to Latin America for the winter. As nesting habitat (i.e. forests) in the ACE Basin study area and winter habitats in Latin America are converted to other land uses songbird populations decline. Recent declines in songbird population have been noted both nationally and in the ACE Basin.
To ensure the health of the avifauna, conservation groups must continue to strive to protect species from harm due to over-hunting, environmental pollution, and habitat loss. Careful control of hunting and pesticide use can protect species to a degree, but habitats must be preserved. Because every species has different needs, the effect of development on each species found in a habitat must be examined to ensure the continued existence of the population.
Author
L. Zimmerman, SCDNR Marine Resources Research Institute
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