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
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 Wilsons 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.).
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.
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.).
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 doesnt 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.
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 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 eagles 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 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, Swainsons 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 Swainsons 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 1970s, 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 dont 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.
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.
L. Zimmerman, SCDNR Marine Resources Research Institute
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