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Nature Of Migration In The Species
Partially migratory. Populations breeding in the North American interior and along the n. Atlantic coast completely migratory. Extent of migration in other populations unknown. Migratory individuals winter from central and s. U.S. south to at least n. South America. Many individuals (mostly immatures) remain in wintering areas (particularly in Middle America) throughout the year (Stiles and Skutch 1989, Howell and Webb 1995). Migratory individuals often appear outside of regular breeding range (north to s. Canada) during spring, summer, and early fall (see Fig. 1).
Timing And Routes Of Migration
Northward migration generally begins in early Mar. At Cape May, NJ, returns late Feb–mid-Mar (Sibley 1993), arriving in breeding colonies by mid-Apr (Burger 1978b). Both sexes arrive at same time; reside approximately 2 wk in area before breeding activity at colony commences. In Massachusetts, begins arriving late Mar (earli-est 12 Mar 1955 at Chatham, MA), with most arriving early–mid-Apr (Veit and Petersen 1993, KCP). In Ohio, usually arrive 25 Apr–7 May (Peterjohn 1989).
Arrives at breeding colonies in Arkansas third to fourth week Mar with migration continuing through mid-May (James and Neal 1986). Arrives in Missouri and Illinois in early Apr, reaching peak abundance in late Apr and May (Bohlen 1989, Robbins and Easterla 1992). Arrives in Oklahoma in mid-Apr (earliest 29 Mar; Baumgartner and Baumgartner 1992) and in S. Dakota in late Apr (earliest 7 Apr; S. Dakota Ornithol. Union 1991). In San Luis Valley, CO, arrive first half Apr (Ryder 1998). In Oregon, arrives late Apr–early May (Gilligan et al. 1994).
During spring, summer, and early fall, many individuals (particularly birds dispersing after breeding) typically appear in areas outside of breeding range, although these movements not as pronounced as in other herons (Ryder 1978). During this period, individuals have been recorded throughout North America north to southernmost portions of all Canadian provinces, and in n. Ontario (Am. Ornithol. Union 1998), but numbers appearing in any given location may vary greatly from year to year, and species may be absent for long periods. Wandering birds have been recorded 17 Apr–19 Nov in extreme sw. British Columbia (Campbell et al. 1990), and 10 Apr–12 Oct in Minnesota (Janssen 1987).
In Oregon, most breeding Snowy Egrets depart in Sep, but have been recorded as late as 25 Oct, with migrants seen through early Nov (Gilligan et al. 1994). In Ohio, breeding population departs by late Sep, with migrants recorded through 10–15 Oct (Peterjohn 1989). In other areas where distinctions between movements of breeders and other migrants have not been made, the species gener-ally recorded through third week Sep in Colorado (Andrews and Righter 1992), mid-Sep in S. Dakota (latest date 25 Oct; S. Dakota Ornithol. Union 1991), mid-Sep in Wisconsin (latest date 3 Nov; Robbins 1991), and most have left Massachusetts by Nov (Veit and Petersen 1993). In New Jersey, noticeable southbound flights observed late Sep–late Oct, peaking in mid-Oct (Walsh et al. 1999). Few observed in Arkansas after Sep (latest date second week Oct; James and Neal 1986). In southern portions of range where species is resident, exact periods of migration difficult to determine. Migrants and wintering individuals recorded in Mexico by Aug (Howell and Webb 1995). Migrants also recorded on Bermuda 22 Aug–24 Nov, peaking through-out Oct (Amos 1991).
Along the Atlantic coast, migrates through Florida, the Caribbean, and n. South America. Majority of eastern birds winter along the Gulf Coast and on e. Caribbean islands (Ryder 1978). Caribbean recovery sites of hatch-year birds banded as nestlings in e. North America included Haiti (14 recoveries as of 1990), Cuba (12), and the Bahamas (11; Murphy 1992). An individual color banded in Virginia was seen on 22 Jan 1970 in Bonaire in the West Indies (P. and F. Buckley pers. comm.). Birds generally remain in Caribbean from Oct to May (Raffaele et al. 1998). Some Florida birds may have migrated to Panama or remained as residents (Loftin 1967). Individuals banded in Louisiana and Mississippi subsequently recovered in Panama (Wetmore 1965). A nestling banded in New Jersey on 10 Aug 1968 was recovered in Trinidad on 18 Nov 1968 (ffrench et al. 1980).
Some western birds withdraw to central Cali-fornia and n. Mexico, especially Baja California (Dawson 1940, Hancock and Kushlan 1984, del Hoyo et al. 1992; see Distribution: the Americas, above). Individuals banded in Colorado have been recovered on both coasts and the interior plateau of Mexico (Andrews and Righter 1992). Oklahoma individuals recovered in Mexico, Nicaragua, and Costa Rica (Baumgartner and Baumgartner 1992). Limited migration of w. U.S. populations to central California resembles Black-crowned Night-Heron migration in that region (Davis 1993)—and Black-crowneds often found with Snowy Egrets at colony sites. Adults from central California largely resident, although hatch-year birds may migrate to Mexico (Ohlendorf et al. 1988). Nestlings banded in Nevada and Idaho recovered in w. Mexico and Guatemala (Henny et al. 1985). Most Central American populations described as a mixture of resident and migratory birds during winter (Wetmore 1965, Land 1970, Ridgely and Gwynne 1989, Thurber et al. 1987, Stiles and Skutch 1989, Howell and Webb 1995).
Mexican and Central American birds appear sedentary. Migration of South American breeding populations not well known but suspected. Southward postbreeding dispersal sketchily reported (Hancock and Kushlan 1984).
Very little information. Cold weather reported to prompt fall migration. Nocturnal, offshore sightings (112 km east of mid-Atlantic coastline) suggest oceanic, southward migratory route used (Brady 1990–1991, 1992–1993). Oklahoma bird recovered 400 km west of Costa Rica in Pacific Ocean (Baumgartner and Baumgartner 1992).
Control And Physiology