Migration

Partial, medium-distance migrant. Along Gulf and Pacific coasts, some year round residents; most immatures remain on wintering grounds throughout year; extent of migration not known (R. Clay pers. comm., C. Collins pers. comm.). Birds banded in the Great Lakes were recovered during the winter months (Nov–Feb) on average 2,040 km away from the banding area (n = 46 band recoveries; Ludwig 1942). Along the Pacific coast, distances flown to wintering areas, as determined from band recovery data, ranged from 1,640 to 2,550 km (n = 118 band recoveries from birds banded in San Diego, CA, San Francisco, CA, and Grays Harbor, WA; Mewaldt 1983). However, may travel much farther distances; nearest potential wintering areas for Caspian Terns breeding on Neragon I., Bering Sea, are in Japan (4,300 km away) and Mexico (5,000 km away; McCaffery et al. 1997). Fall migration slower and less direct than spring migration, and average distance flown in fall is greater than in spring; fall migration is characterized by adults with juveniles wandering and lingering at abundant food sources (Ludwig 1942, Kilpi and Saurola 1984). In spring, adults travel faster and roost less, because unaccompanied by young. Same flyways generally used between breeding and wintering sites and by all age groups and both sexes (Ludwig 1965, Cramp 1985, L’Arrivee and Blokpoel 1988). In North America, migrates primarily along coasts and large rivers and bays. Not recorded over open ocean (Garrett and Dunn 1981).

Timing

Varies with location. Along Pacific Coast, generally arrives at breeding areas beginning in late Mar; bulk of migration complete by late Apr (Kirven 1969, Penland 1976, Gill and Mewaldt 1983, D. Roby pers. comm.). Migrants recorded late Mar–mid-May in lower Colorado River valley of s. Arizona (Rosenberg et al. 1991); arrive late Mar, peaking in Apr, along coast of Oregon, with inland migration occurring later (Gilligan et al. 1994). Earliest arrival dates for British Columbia are 19 Apr (coast) and 7 May (interior; Campbell et al. 1990). In Great Lakes and other northern latitudes, spring arrival typically corresponds with ice breakup. Begins arriving in Great Lakes in mid-Apr; most migration complete by 1 Jun (Cuthbert 1981). In Manitoba and ne. Alberta, nearly all birds are still incubating eggs in mid- to late Jun (Vermeer 1970, Weseloh and Cocks 1979), so likely does not arrive on breeding grounds before mid- to late May. In Northwest Territories, northernmost limit of Nearctic breeding range, arrives after 15 May (Sirois et al. 1995). Along Atlantic Coast, rare spring migrant in Cape May, NJ, primarily in May (Sibley 1993), and uncommon spring migrant in Massachusetts from late Apr to mid-May (Veit and Petersen 1993).

Extreme fall dates for British Columbia are 11 Nov (coast) and 10 Nov (interior; Campbell et al. 1990). Along Oregon coast, fall migrants arrive late Jun (presumably coming from breeding locations in Washington State); peak mid-Jul–early Sep; stragglers into Nov (Gilligan et al. 1994). In lower Colorado River valley, fall migrants recorded primarily during 2 periods: Jun–early Jul, and late Aug through Sep (Rosenberg et al. 1991). Migration from Great Lakes begins Jul–Aug, and continues through Oct. Adults with juveniles are fairly common to locally abundant in Ohio along Lake Erie from Aug to mid-Sep; some remain until mid-Nov (Peterjohn 1989). For Great Lakes birds, average arrival dates of 17 Aug in Virginia, 11 Sep in N. Carolina, 12 Sep in Louisiana, Nov–Dec in Missis-sippi, Georgia, and Florida (Ludwig 1965). Migrants generally recorded in Missouri early Sep–mid-Oct, peaking in mid-Sep (Robbins and Easterla 1992); in Massachusetts, mid-Sep–mid-Oct (Veit and Petersen 1993); and in Cape May, NJ, late Jul–late Oct, peaking mid-Sep–early Oct (Sibley 1993). Some juveniles, however, fledge and arrive at wintering sites rapidly—e.g., in Colombia (3 Sep) and in Dominican Republic (by Aug; L’Arrivee and Blokpoel 1988). On Atlantic Coast, departs N. Carolina sites mid- to late Aug (Parnell and Soots 1979). Departs Northwest Territories before 15 Sep (Sirois et al. 1995). In Panama, has been recorded Nov–Mar (Ridgely and Gwynne 1989), but timing of migration within winter range is difficult to determine because some individuals remain in most parts of winter range throughout year from West Indies north to Florida (Stevenson and Anderson 1994, Raffaele et al. 1998), and from Costa Rica north through Middle America to s. California (Stiles and Skutch 1989, Howell and Webb 1995, Small 1994).

Routes

Great Lakes birds winter mostly across Gulf and Atlantic Coast states, Caribbean, and n. South America; migrate to and from these areas along U.S. Atlantic Coast and Mississippi Flyway to Gulf of Mexico and Caribbean (Ludwig 1965, L’Arrivee and Blokpoel 1988). Also along Lakes Erie and Ontario to watersheds of New York and Pennsylvania and on to Atlantic Coast (Ludwig 1965, L’Arrivee and Blokpoel 1988). Pacific Coast birds winter mostly along west coast of Mexico and in Guatemala; migrate south along coast to wintering areas (Gill and Mewaldt 1979, 1983). Migrants from inland nesting sites in Oregon follow Columbia River to ocean (Gilligan et al. 1994). An individual fledged in Idaho and recovered in Sinaloa, Mexico, suggests that west-central interior birds winter in same area as Pacific Coast birds (Gill and Mewaldt 1983). Migration routes and wintering areas for birds breeding in other geographic regions are not well known.

About 1 mo after fledging, juveniles and adults gather at traditional feeding sites in coastal bays, lakes, and inlets close to breeding areas and migrate south in stages. Extent and conditions under which birds flock are not well understood; birds travel individually, in families (often single parent with 1–2 young), and in groups (Gilligan et al. 1994). In Georgia, rarely observed in flocks (Burleigh 1958), but along Lake Erie flocks of 30–120 individuals typically are seen during spring migration; slightly larger flocks in fall (Peterjohn 1989). Elsewhere (Florida, w. Missouri), flocks of hundreds, rarely thousands, seen in migration (Robbins and Easterla 1992, Stevenson and Anderson 1994).

Information on other migratory behavior is not available for North American population, but a detailed study in Poland reported migratory behavior and patterns of birds that breed in Baltic (Jozefik 1969). Migration flight usually observed twice a day: late morning and late afternoon–early evening; nocturnal migration not uncommon. Migration flight is at higher altitude than food-searching flight—80–100 m versus 10–30 m, respectively—and characterized by straight-line, rather than circuitous flight; average speed 40–50 km/d for autumn transcontinental migrants from s. Poland. While migrating, maintained family connections using vocalizations. Navigation depended largely on lines with fixed direction, such as rivers and coastlines. Environmental factors at time of migration—e.g., cloud cover, wind direction and speed, river depth and width, reduced space orientation—affected individual route choice. Effect of wind most significant, often determining route and diminishing ability of individuals to fly in group.

No information.