The largest of their group, Caspian Terns are the size of smaller gulls with stout bodies, broad wings and thick, red bills. These structural features enable them to fly slowly or rapidly as well as hover as they search for fish in shallow waters near their breeding colonies or wintering areas. On spotting a suitable fish, a Caspian Terms hover and then “plunge-dive” into the water, often submerging completely (Cuthbert and Wires 1999).
DISTRIBUTION. During the 1987-1992 field work seasons of the TBBA project, volunteers found 113 confirmed, 8 probable and 26 possible breeding sites for Cassin’s Terns on or near the coast in the Coastal Prairies, Coastal Sand Plain and South Texas Brush Country regions (see the region map in Lockwood and Freeman ).
North American Breeding Bird Survey (BBS) data show relative abundances of 1-3 Caspian Terns per route along the central Texas coast, San Francisco Bay area of California, northeast California and southern Oregon, around Moses Lake in eastern Washington and Great Salt Lake in Utah. Lower abundances were found in Florida and along the Gulf Coast, along the Pacific Coast from San Francisco to Vancouver Island and in Idaho, Nevada and Wyoming (Sauer et al. 2008). Breeding has also been recorded from southern Alaska across Canada to the Atlantic Coast, along the Atlantic Coast, at scattered inland location in the eastern and central United States and on the coast of northwest Mexico. In winter these terns are found in the southern United States, Mexico Central America, the West Indies and northern South America. Other populations occur throughout the Old World (Howell and Webb 1995, Am. Ornithol. Union 1998, Cuthbert and Wires 1999).
SEASONAL OCCURRENCE. Caspian Terns are common residents along the Texas coast and rare to uncommon migrants and winter visitors inland in the eastern half of the state. Northbound migrants depart the coast between late February and late May. Resident birds breed from mid-March to mid-July, based on egg collection dates from March 25 to June 18 and young seen as late as July 7. Fall migrants return to the coast from mid-September through October (Oberholser 1974, Lockwood and Freeman 2004).
BREEDING HABITAT. Caspian Terns breed in Texas near sea level on barren spoil; islands and shell beaches (Oberholser 1974). Most of them breed in colonies, sometimes associated with colonies of other terns and gulls, although the Caspian Tern nests are segregated. Across the North American range of this species, islands are popular, although these terns adapts to a wide variety of sites, generally with little or no vegetation (Harrison 1979 ,Cuthbert and Wires 1999).
Caspian Tern nests are hollows in the ground, excavated by both parents using their feet to dig and breasts to shape the hole. The hole is sometimes lined with bits of dry vegetation, pebbles or shell fragments. In it the female usually lays 2-3 dull, somewhat rough, buff to medium brown eggs at 2-3 day intervals. Markings are varied, ranging from small dark brown or black spots and speckles to large spots or irregular blotches. The eggs, pictured by Harrison (1979), are similar to those of Royal Terns (S. maxima) and gulls. Incubation starts when the first egg is laid and continues for 26-27 days. The young birds are semi-precocial, remaining in or near the nest for about a week. Parents start feeding their chicks fish on day one, small at first. For the first week, depending on the weather, the chicks are brooded almost continuously. Young remain dependent on their parents for at least several months, since diving and fishing abilities develop slowly (Harrison 1979, Cuthbert and Wires 1999).
STATUS. Caspian Terns are common residents along the Gulf Coast of Texas (Lockwood and Freeman 2004). The breeding and summer symbols along the coast on Oberholser’s (1974) map match the TBBA map. Only 4 BBS routes in Texas sample these terns at relative abundances <3 terns per route, inadequate for a biologically meaningful trend. Data from 66 routes in North America suggests a small annual population increase (Sauer et al. 2008). Dredging on the Intercoastal Waterway should continue to provide nest sites for Caspian Terns along the Texas coast.
Text by Robert C. Tweit (2008)
American Ornithologists’ Union. 1998. Checklist of North American birds, 7th ed. Am, Ornithol. Union, Washington, DC.
Cuthbert, F. J. and L. R. Wires. 1999. Caspian Tern (Sterna caspia), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology. Retrieved from: : http://bna.birds.cornell.edu/bna/species/403
Harrison, H. H. 1979. A field guide to western birds’ nests. Houghton Mifflin, Boston, MA.
Howell, S. N. G. and S. Webb. 1995. A guide to the birds of Mexico and northern Central America. Oxford University Press, New York.
Lockwood, M. W. and B. Freeman. 2004. The TOS handbook of Texas birds. Texas A&M University Press, College Station.
Oberholser, H. C. 1974. The bird life of Texas. University of Texas Press, Austin.
Sauer, J. R., J. E. Hines, and J. Fallon. 2008. The North American breeding bird survey, results and analysis 1966-2007. Version 5.15. 2008. USGS Patuxent Wildlife Research Center, Laurel MD < http://www.mbr-pwrc.usgs.gov/bbs>