Sternula antillarum

Formerly (AOU 1983, 1998) included in the genus Sterna but separated on the basis of genetic data that correspond to plumage patterns (Bridge et al. 2005) (AOU 2006). The American Ornithologists’ Union (AOU) recognizes three subspecies: S. a. browni from California, the Eastern, S. a. antillarum, and the Interior, S. a. athalassos (AOU 1957).

Often has been considered conspecific with Old World S. albifrons (AOU 1983). Appears to constitute a superspecies with S. albifrons, S. superciliaris, S. lorata, and S. saundersi (AOU 1998). Massey (1976) evaluated morphological, vocal, and behavioral characteristics and concluded that nominal subspecies antillarum and browni are indistinguishable. Thompson et al. (1992) examined morphological and electrophoretic variation and found little evidence of differentiation among populations of the nominal subspecies antillarum, athalassos, and browni; they recommended that the subspecific taxonomy of the S. antillarum complex be reassessed. Johnson et al. (1998) used a quantitative colorimetry analysis to study variation among antillarum, athalassos, and browni and found differences significant enough to warrant the validity of the taxa and their importance as entities for conservation.

Widely distributed, but numbers are reduced and/or declining in some areas. Nesting colonies are sensitive to disturbance.

BREEDING: Pacific coast, central California to southern Baja California and Chiapas (Garcia and Ceballos 1995); since 1970, most nesting has occurred from Santa Barbara to San Diego County, California. Interior U.S.: locally along the Colorado, Red, Arkansas, Missouri, Ohio, and Mississippi river systems; formerly more widespread and common; has been eliminated from much of former habitat; now breeds locally in this region, north to Montana and North Dakota, east to southwestern Indiana, central Kentucky, and western Tennessee, west to eastern Colorado. Atlantic-Gulf coast: Maine south to Florida and west to Tamaulipas, coast of Yucatan Peninsula, and in West Indies (Bahamas [Sprunt 1984], Greater and Lesser Antilles [van Halewyn and Norton 1984]); islands off coast of Belize, Honduras, and Venezuela; and Bermuda (Thompson 1995, AOU 1998). About 2/3 of world population breeds in the southeastern U.S.; largest colony is at Gulfport, Mississippi (Clapp and Buckley 1984). NON-BREEDING: regularly along Pacific coast from southern Mexico to Peru and eastern coasts of Mexico, Central America, and South America to Brazil and northern Argentina (Thompson et al. 1997, AOU 1998). May remain in wintering areas during first year (Thompson et al. 1995). Casual in Hawaii (Whitman 1988).

Widely distributed, but difficult to estimate because nesting habitat is ephemeral and nesting sites may change location from year to year.

Populations were decreased greatly by formerly extensive plume hunting. Current major problems are human use and development of nesting habitat and predation on adults, eggs, and young by birds and mammals (see Burger and Gochfeld 1990). Some habitat is lost due to encroachment of vegetation, but this may be offset by habitat created when storm overwash removes vegetation from portions of barrier beaches or creates overwash fans of sandy/gravelly substrate. Storms during spring tides sometimes wash out nests. Replacement of gravel-covered roofs by plastic-covered roofs, together with increasing human use of beaches, may seriously limit the availability of productive nesting sites in the southeastern U.S. (Gore and Kinnison 1991). Birds and mammals prey on eggs and young, and ants may prey on young in pipped eggs and on newly hatched young (Moseley 1976). Ghost crabs (OCYPODE QUADRATA) may be important predators of eggs and chicks in South Carolina (Blus and Prouty 1979). Other predators in various regions include coyote (CANIS LATRANS), red fox (VULPES VULPES), mink, weasels, raccoon (PROCYON LOTOR), striped skunk (MEPHITIS MEPHITIS), opossum (DIDELPHIS VIRGINIANA), domestic cats and dogs, feral hogs (SUS SCROFA), Norway rat, black rat, various gulls, night-herons, American kestrel (FALCO SPARVERIUS), northern harrier (CIRCUS CYANEUS) red-shouldered hawk (BUTEO LINEATUS), fish crow (CORVUS OSSIFRAGUS), and grackles (Whitman 1988, Burger and Gochfeld 1990, Thompson et al. 1997). In New Jersey, Burger (1984) found that mainland colonies with over 80 birds suffer more from predators than colonies with fewer than 80 birds, perhaps because the former are large enough to make their presence known to predators. Larger colonies may incur greater losses to predation than do smaller colonies because the former are more stable and hence known to predators (Burger 1984). On two South Carolina barrier islands, harsh weather was the primary cause of unsuccessful nesting (Cowgill 1989). Bird banding, photography, and other activities that cause young to scatter or keep adults away from nests for substantial periods of time may result in increased mortality (Zickefoose 1985). Wayward young may be attacked by nonparental adults. Exposed eggs or young may succumb to overheating and be subject to increased predation. Potential threats include chemical spills and pesticide or heavy metal pollution. Decline of interior nesting populations has been coincident with human modification of river flow (e.g., reduction of spring floods by dams) and bank stabilization and channelization, resulting in reduced availability of bare island/sandbar nesting habitat; loss of aquatic habitat diversity and resulting changes in fish species composition and abundance also may have contributed to the reduced tern population (Figg 1993). Desalination projects may reduce shallow water fish populations and thus decrease tern food resources (Schulenberg and Ptacek 1984). In arid regions, irrigation may be a threat by lowering water levels/flows and reducing river areas when terns are breeding (Schulenberg and Ptacek 1984). Grazing cattle may trampled eggs (Schulenberg and Ptacek 1984). Heavy rains and other severe weather have resulted in significant nesting losses along the Cimarron River in Kansas and on salt plains in Oklahoma (Grover and Knopf 1982). Colonies reduced by severe weather may incur increased predation (Schulenberg and Ptacek 1984). Interior populations nest mainly on riverine sandbars or salt flats that become exposed during periods of low water (Hardy 1957). As a result of vegetational succession and/or erosion, preferred nesting habitat typically is ephemeral. Hardy (1957) implied that breeding in riverine situations depends on the presence of sandbars, favorable water levels during nesting season, and sufficient food. Nests are usually located at higher elevations and away from the water. Water levels determine the size of sand bars and the extent of nesting areas (USFWS 1990). Dams above colonies generally lower habitat quality by eliminating the spring floods that are necessary for alluvium deposition and the scouring of vegetation. Ducey (1982) reported successful breeding at two privately-owned sand and gravel companies along the Platte River in Nebraska. As old breeding sites became unsuitable due to vegetation encroachment, the terns simply moved to more recently created sand deposits. See also Ziewitz et al. (1992) for information on nesting habitat in the Platte River in Nebraska. Populations in Kansas have nested on oil well sites (Schulenberg and Ptacek 1984).