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GULL-BILLED TERN Sterna Nilotica

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GULL-BILLED TERN Sterna Nilotica hllGRATOKY NONGAME IllHDS OF IMANAGEMENT CONCERN IN TIE NOXTIIEhST 1 GULL-BILLED TERN Sterna nilotica Jerry Via 710 South Gate Drive Blacksburg, Virginia 24601 David C. Duffy Box 1095 Shelter Island Heights, New York 11965 Gull-billed terns nest in colonies with other waterbirds along the Atlantic coast from New York south to the Gulf coast. In the Northeast region, the species is scarce from New York to Maryland, and the largest colonies in Virginia have declined considerably. Loss of upland foraging sites and island nesting sites has probably been the greatest threat to gull-billed tern populations, although predation and competition with gulls, and human disturbance at colonies have also contributed to population declines. Protection of nesting tern colonies by posting, gull control, and creation of artificial nesting islands are management techniques that could assist with recovery of populations. Annual nesting snweys should also be conducted to monitor population status. Research activities should address foraging requirements and movement of populations amon2 colonies as well as the ecoloev of the s~eciesduring mieration and in winter. DESCRIPTION Morphology and Plumages Taxonomy The gull-billed tern is a stout, white, blunt- billed tern that feeds in the marshes and The gull-billed tern, Sfemn nilotica, is adjacent coastal uplands of the southern and placed in the genus Srema in North America, Gulf coasts of the US. (Forbush 1925, Harrison whereas Europeans have retained it in the 1983, Cramp 1985). It is similar in size to other monotypic genus Geochelidon (American medium-sized terns, but lacks the long tail Ornithologists' Union 1983, Cramp 1985). The streamers of common terns (Sterna hirundo), gull-billed tern is a nearly cosmopolitan species Forster's terns (S. forsten), and roseate terns (S. with six subspecies, only one of which, S. nilotica dougallii) terns. The gull-billed tern is also aranea, occurs along the east coast of the U.S. longer-legged and broader-winged than these south to the West Indes and Mexico. A second species. It can be told from the sandwich tern subspecies, S. nilorica vanrossenti, occurs on the (S. sandvicensis) by its shorter beak and tail, Pacific coast. shallower wingbeat in flight, and more upright 2 CULCBtLLEI) TICIIN - Vlu posture while sitting. This tern's flight is usually more buoyant and gull-like than that of other terns (Vinicombe and Harris 1989). Nesting While in breeding plumqc, a black cap extends from the lores of the gull-billed tern, Wilson (1840) and Stone (1908) reported including the eyes, to the nape. The rest of the that the gull-billed tern was a marsh-nesting upper parts, wings, and tail are a pale pearl species along the coast of New Jersey. Bent gray. The side of the head, underparts, wing (1921) concluded that this species had been linings, tail, and undenving coverts are white. driven to nest on barrier beaches because of The primaries are grayish-black underneath and hunting at sites on inner dunes, saltmarshes, and frosty-gray above. The legs and feet are black. islands. Nesting sites are presently confined to The heavy black bill lacks the sharp tip of other sandy barrier islands, beaches, sandy shorcs of terns and is stouter and proportionately shorter. saline lagoons and marshes, and artificially- The sexes are similar in appearance. Winter produced dredge spoil islands (Clapp et al. plumage is similar to the breeding plumage 1983). Regional differences in nest sites occur, except that the black cap is nearly absent, with with the percentage of gull-billed terns nesting only some remnant spotting near the rear of the on spoil islands ranging from 28% in New crown. A blackish patch extends from the eye Jersey to 6040% in North Carolina and 70.84% to the auriculars, although the extent of this is in Texas (Clapp et al. 1983). quite variable (Harrison 1983, Cramp 1985). The gull-billed tern is a colonial nester and Recently fledged juveniles are similar to typically forms interspecies colonies with adults in winter plumage except that the head is common terns and black skimmers (Rynchops darker with more blackish spots and the gray niger). Least terns (Sterna anrillnn~m),royal back and upper wing are edged in tan, giving terns (S. mnrinta), sandwich terns, and caspian the back and wing a buff-colored appearance terns (S. caspia) may also be present. Most of when the bird is in flight. the gull-billed tern colonies studied in Virginia The downy young are variable in and North Carolina were small (mean = 45 appearance but generally cream, buff, or peach birds) (Erwin 1978). colored, with darker down on the dorsal Nests are generally located close to some surfaces. Young usually have nvo dorsal stripes landmark, such as a plant or piece of driftwood, on the crown, nape, and back and a distinctive and are usually a slight depression containing dark smudge behind the eye. The bill is the eggs with a rim of dried straw andlor shell typically light pink at hatching and darkens with fragments (Harrison 1975, Sears 1978). Some age. The feet are light pink and darken to an nests are more elaborate piles of accumulated orange-brown with age (Harrison 1983, Cramp shell fragments, which may serve to provide 1985). protection from drifting sands (Sears 1978). The appearance of the nest lining and perimeter Vocalizations varies greatly between nests and from day to day in the same nest, depending on the individual, The typical call of the gull-billed tern is a the time available for placing the lining, and nasal "tee-hee-hee" or "kat-y-did" (Bent 1921). weather conditions (Sears 1976, 1978). Terns attacking terrestrial predators will Distances between nests ranges from 2-114 frequently utter a harsh "grack" call during m, with a mean of 21 m (Sears 19- cited by defensive dives (Sears 1981). Other calls are Cramp 1985), although the inter-nest distance described by Bent (1921), Sears (1981), and may vary as a function of colony size (M$ller Cramp (1985). 1982). Vegetation in nesting areas is sparse, e. approximately 15% of the ground was covered in a North Carolina colony (Soots and Parnell 1975). Clutch size is one to three eggs (Bent 1921, Sears 1975, Mgller 1951) and occasionally four (Bent 1921, Forbush 1925, Pemberton 1927, Foraging normally takes place within 10 Harrison 1984). Clutch size in one study was km of the colony (Sears 1976). Unlike other found to be significantly greater in larger species of terns, gull-billed terns rarely feed over colonies (Mgller 1951). Renesting attempts open water (Ashmole 1971); instead they forage usually result in smaller clutches (Sears 1978). by flying low over marshes, open fields, shrubs, The background color of the eggs may vary and agricultural fields, with a gull-like flight. from buff to olive and the mottling is somewhat Upon locating food, they stall briefly, then drop finer grained than the pattern of the common to the ground, vegetation, or substrate to tern egg. The eggs have a characteristic "frosty" capture their prey (Cramp 1985). Their feeding appearance which also distinguishes gull-billed behavior is similar to that of the laughing gull, tern eggs from the those of the common tern Lams atricilla, with which they are frequently (Bent 1921, Harrison 1975). The eggs of the observed feeding over agricultural fields gull-billed tern are cryptically colored, and the (Meanley 1981). immediate area of the nest is not white-washed. Gull-billed terns sometimes forage over Like many terns, the gull-billed tern has a quiet estuaries and ponds, where they pick fish highly ritualized courtship behavior which and invertebrates from the surface. Unlike other includes aerial flights, a variety of terrestrial species of terns, they rarely dive. They displays, and courtship feeding (Bent 1921, Lind sometimes feed with black skimmers on small 1963, Sears 1976, 1981). fish concentrated in drying marsh pools (J. Via Gull-billed terns display a variety of pers. obs.). Gull-billed terns rarely feed in behaviors which reduce the risk of nest conspecific groups because many of their predation. For instance, they walk a short invertebrate prey tend to be dispersed rather distance from the nest to defecate (Sears 1978). than aggregated like the fish schools exploited They also frequently remove eggshells from the by other species of terns (Erwin 1978). nest after hatching (Cullen 1960), but this Major dietary items include arthropods, response is not as strong as it with other tern locusts, grasshoppers, dragonflies, insects, species, probably because the cryptically-colored spiders, and marine life such as fiddler crabs, young leave the nest several days after hatching crustacea, crabs, and sand bugs (Wilson 1840, (Sears 1978). Bent 1921, Sprunt 1954, Rohwer and The eggs are incubated by both the male Woolfenden 1968, Cramp 1985, Quinn and and the female for 22-24 days (Harrison 1975), Wiggins 1990). Vertebrate fauna consumed by and only one brood is raised per season (Bent the gull-billed tern include fsh, frogs, toads, 1921, Forbush 1925). The major factors that lizards, small mammals (Bent 1921, Dement'ev determine the time at which the young leave the et al. 1951, Bannerman 1962), green anoles nest are the age of the chicks, the proximity of (Anolis carolinensis) (Rohwer and Woolfenden vegetation, and disturbance (Sears 1978). The 1968), and mice (Hobbs 1976). young fledge at 4-5 weeks of age (Harrison Gull-billed terns are occasional 1975). opportunistic predators on the downy chicks of Nesting success appears to be low, with other beach-nesting birds including gull-billed many colonies producing no young at all (Blus (Zubakin 1975) and least terns (Densmore and Stafford 1980). The main causes of 1990), and possibly piping plovers, Charadrius breeding failure are flooding of low-lying nlelodus (R.
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