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Nature Of Migration In The Species
Vacate breeding areas of U.S., n. Mexico, and Caribbean islands in late summer or early fall, most moving well southward to Central and South America; stragglers may occur on Pacific and Gulf coasts during winter. Degree of migration in terns from much of Mexico and Caribbean uncertain. Terns from n. U.S. migrate greatest distance to South America. Winter areas for U.S. breeding population largely unknown, except California subspecies known to winter in s. Mexico. Banded juveniles from Atlantic Coast known to move to northern coast of South America (Thompson 1982).
Apparently migrates in small, loose groups, feeding en route in shallow water near land and resting on sandbars, beaches, pilings, and docks. During migration and postbreeding wandering, Least Terns have appeared in aquatic habitats through most of U.S. (Am. Ornithol. Union 1983) and into s. Ontario (Beardslee and Mitchell 1965). Unknown if breeders on Caribbean islands move to offshore waters or go farther south (Voous 1983).
Timing And Routes Of Migration
Varies with site and season. Spring migration into U.S. likely quite rapid; fall migration appears protracted, as adults with fledged young may linger in coastal breeding range for 6–8 wk before departing. Spring arrival times progress northward (for coastal and interior populations) from late Mar to early Apr on Gulf Coast (Thompson 1982, Toups and Jackson 1987), late Mar–mid-Apr in Georgia and S. Carolina (Tomkins 1959, Sprunt and Chamberlain 1970), mid- to late Apr in California (Massey 1974), mid-Apr in Kentucky (Monroe 1994), late Apr–early May in New York and Massachusetts (Wolk 1974, Veit and Peterson 1993), and mid- to late May in Illinois and Iowa (Stiles 1939, Bohlen 1989). Fall migration starts with initial movements from breeding colonies as early as Jun, depending on nesting chronology; distinct migration flocking apparent by late Jul and early Aug at coastal colonies; northern interior populations slightly later in Aug. Migration departure may be delayed by some birds to late Sep in coastal Massachusetts (Veit and Peterson 1993), mid-Oct in Kentucky (Monroe 1994) and California (late-nesters; JLA), and late Oct–mid-Nov in the Carolinas (Sprunt and Chamberlain 1970) and Caribbean (Voous 1983).
Primarily follows major rivers and marine coasts, where available; migrates across open water for more direct movements. Terns breeding at Gulf Coast sites appear to migrate southward around Gulf of Mexico, whereas Atlantic Coast terns tend to migrate across the ocean through Caribbean islands (Thompson 1982). California population moves southward to west coast of Mexico but unknown if route is around Baja California or landward to Gulf of California. Interior population appears to follow major river basins to confluence with Mississippi River and then south to Gulf of Mexico; route thereafter in fall unknown. Some terns from interior apparently migrate cross-country in late summer, as indicated by terns seen (1986–1993) in central Texas >150 km from known nesting in major river drainages (BCT). Spring migration likely follows similar major routes along marine coasts and rivers, but such movements not extensively documented or monitored.
Several weeks before fall migration, adults and young wander along marine coastlines or along interior rivers and reservoirs, congregating at prime fishing sites. Great Salt Plains Lake, Alfalfa Co., OK, is an important postnesting staging area (Talent and Hill 1985); staging sites along interior rivers are more unpredictable. Adults and juveniles tend to congregate at various traditional riverine and Gulf Coast sites in mid- to late Aug before disappearing for winter (Thompson 1982, Lingle 1993b, EMK). Little known of Least Tern migration in Mexico. Adults in Georgia still feeding fledged young when individuals depart in Sep (Tomkins 1959), suggesting that family units migrate together, as observed in California and Massachusetts (JLA).
Control And Physiology
No specific information.
Thompson, Bruce C., Jerome A. Jackson, Joannna Burger, Laura A. Hill, Eileen M. Kirsch and Jonathan L. Atwood. 1997. Least Tern (Sternula antillarum), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online: http://bna.birds.cornell.edu/bna/species/290