Distribution

Breeding Range

Figure 1 . Breeds in coastal and inland wetlands, but location of colony sites within range often changes from year to year. Overall range limits have also changed over time in response to the effects of hunting, habitat loss, and other environmental factors. Following range description includes breeding locations described since 1970.

Canada. Since 1984 small numbers have summered in mixed-heron colony on Bon Portage I., Nova Scotia. Breeding probable but unconfirmed (Erskine 1992).

Western United States. Breeds in se. and s.-central Oregon (Harney Basin and s. Lake Co.; Gilligan et al. 1994), California (mainly Siskiyou Co., Modoc Co., Humboldt Bay, San Francisco Bay area, s. Sacramento Valley, San Joaquin Valley, San Diego Co., Salton Sea, and Colorado River; Small 1994), s. Idaho (Snake River Valley; Stephens and Sturts 1997), sw. Wyoming (Oakleaf et al. 1992), northern half of Nevada and se. Nevada (T. Floyd pers. comm.), and n. Utah (Behle et al. 1985). Also breeds more locally in w. Montana (Bergeron et al. 1992), central and se. Wyoming (Oakleaf et al. 1992), central Colorado (San Luis Valley; River-side Reservoir, Weld Co.; Barr Lake, Adams Co.; Ryder 1998), s. Arizona (Colorado River and w. Gila River; Arizona Breeding Bird Atlas [BBA] 1993–1998 unpubl.), and s. New Mexico (central and s. Rio Grande River and s. Pecos River.; Hubbard 1978).

Eastern United States. Breeds locally in e. South Dakota (Brown, Kingsbury, Day, Charles Mix Cos.; S. Dakota Ornithol. Union 1991, Peterson 1995); from the Gulf coast north through e. Texas (mainly east of 98°W; Texas BBA 1987–1992 unpubl.), and e. Oklahoma, to s.-central Kansas (Arkansas River; Kansas BBA 1992–1997 unpubl.), and north through Louisiana and w. Mississippi to central Arkansas (Arkansas River; James and Neal 1986) and the Mississippi lowlands of se. Missouri (Robbins and Easterla 1992), w. Tennessee (Ford 1998), and w. Illinois (St. Clair and Madison Cos; Bohlen 1989). Breeding range extends east along Gulf coast to include s. Alabama, s. Georgia, and all of Florida, and extends north locally along the immediate Atlantic coast from Georgia to Wood I. in s. Maine (Ryder 1978, Adamus 1987). Also breeds locally from the lower e. Chesapeake Bay (Robbins and Blom 1996) north to the lower Susquehanna River of se. Pennsylvania (Schutsky 1992), north to upper Delaware Bay (Pea Patch I., DE; Hess et al. in press), in nw. Ohio on islands of w. Lake Erie (Peterjohn 1989), in ne. Wisconsin (Brown and Oconto Cos.; Robbins 1991), and in s. Ontario (Curry and Bryant 1987). First inland breeding in S. Carolina reported in 1987 (Belser and Post 1987).

West Indies. Breeds throughout the Bahamas, Greater Antilles, and in the Virgin and Cayman Is., Antigua, Guadeloupe, and Barbados (Raffaele et al. 1998).

Mexico and Central America. Breeds locally (in coastal lowlands) along the Atlantic and Pacific coasts from Sonora and Tamaulipas, Mexico, south to at least Honduras (Land 1970, Howell and Webb 1995). Has been found less frequently in interior valleys of Mexico (Durango and from Jalisco east to Guanajuato; Howell and Webb 1995) and on larger lakes in Guatemala (Land 1970, Ryder 1978). Also breeds locally in Mexico along the Pacific coast of Baja California Sur (Howell and Webb 1995). South of this area known breeding locations include lower Tempisque Basin (Palo Verde National Park) of nw. Costa Rica (Stiles and Skutch 1989), and 2 islands (Changamé and Taborcilla) in Bay of Panama, but likely breeds elsewhere in Costa Rica and Panama (Wetmore 1965, Ridgely and Gwynne 1989). No information on current status in Nicaragua.

South America. Breeds on Bonaire, Curaçao, Aruba, Trinidad and Tobago off coast of n. South America (ffrench et al. 1980, Voous 1983), and throughout Colombia, Venezuela, Suriname and Brazil, mainly in coastal lowlands, but also occurs inland where suitable wetlands exist (Meyer de Schauensee 1964, Meyer de Schauensee and Phelps 1978, Hilty and Brown 1986, Dubs 1992, Sick 1993, Haverschmidt and Mees 1994). Breeds south to Valdivia, Chile, and Córdoba and Buenos Aires, Argentina (Meyer de Schauensee 1970, Hancock and Kushlan 1984).

Above description generally representative of core breeding range, but species unpredictable and sometimes breeds erratically in other locations (e.g., 1977 in Minnesota; Janssen 1987).

Winter Range

Key areas are generally on the Atlantic Coast, Bahamas, Cuba, Greater Antilles, Gulf Coast into Central America and Pacific coast to Central America (Mikuska et al. 1998).

United States. Winters in Oregon (Coos Bay; Gilligan et al. 1994), California (from the lower Sacramento Valley south throughout California breeding range, and along all but the northernmost coast), Arizona (mainly lower Colorado River), along the immediate Atlantic coast from s. New Jersey (sparingly) south to Georgia, throughout Florida and southernmost Georgia, and in the coastal plain from Alabama west to Texas, with wintering extending farther inland along some major rivers in s. Texas (Christmas Bird Count data). Individuals reported wintering as far north as Mt. Desert Island, ME, Cape Cod and Nantucket, MA; and on Block Island, RI (Heilbrun 1976), although regular wintering on Atlantic Coast probably does not occur north of s. New Jersey. Also rare in interior southwestern states.

West Indies and Bermuda. Transients winter in Bermuda and throughout the West Indies south to s. Lesser Antilles wherever suitable habitat exists (Evans 1990, Raffaele et al. 1998). Described as regular vagrant in Bermuda, mainly during spring and summer (Amos 1991).

Mexico and Central America. Transients winter throughout Mexico (including all of Baja) and Central America south to Panama, although primarily in coastal lowlands in Costa Rica and Panama (Ridgely and Gwynne 1989, Stiles and Skutch 1989, Howell and Webb 1995).

South America. Transients from U.S. recovered as far south as Guyana, Venezuela, and Colombia (Ryder 1978), indicating that northern transients probably regularly winter in this area, but full extent of species’ South American winter distribution unknown.

Vagrants documented in Iceland and the Azores (Alström and Colston 1991). Repeatedly seen on Tristan da Cunha near African coast (Hancock and Kushlan 1984). Reported as rare straggler to Hawaiian Is. and American Samoa (Scott et al. 1983).

Breeding distribution has undergone radical changes since late nineteenth century. Numbers plummeted in response to extreme predation pressure on breeding adults exerted by the millinery trade between 1880 and 1910 (Ogden 1978). Before plume hunting, species was “common” on Atlantic Coast as far north as Long Island, NY, but was subsequently extirpated locally south to N. Carolina (Ogden 1978) and nearly became extinct in Florida (Frohring et al. 1988). With the enactment of the McLean Bill in 1913, which forbade transportation of plumage of any bird into the U.S. (Dawson 1940), and the Migratory Bird Treaty Act in 1916, populations showed a consistent recovery toward former levels of abundance. They began occupying breeding sites on both coasts that subsequently extended north of previous range (Cooke 1913). First post-1930 breeding records for New Jersey, New York, Massachusetts and Maine occurred in 1939, 1949, 1955, and 1961, respectively (Ogden 1978). Along Gulf Coast, Louisiana functioned as a primary source of individuals during expansion since 1950, whereas Texas has maintained relatively stable numbers (Ogden 1978, Spendelow and Patton 1988). Elsewhere breeding populations became re-established during 1940s at Malheur Lake, OR (Gilligan et al. 1994). First breeding records noted in 1947 at Topock Marsh along the lower Colorado River, AZ (Rosenberg et al. 1991); 1937 in Colorado (Ryder 1998); 1970s in S. Dakota (S. Dakota Ornithol. Union 1991); 1975 in Wisconsin (Robbins 1991); 1983 in Ohio (Peterjohn 1989); and 1975 in Pennsylvania (Schutsky 1992). Remarkable population expansion was largely into estuarine habitats, and inland along large river drainages (Peterjohn and Rice 1991). Population appears stable in Maryland (Robbins and Bloom 1996) and perhaps increasing in Massachusetts (Veit and Petersen 1993). However, the species has declined or disappeared as a breeder in many parts of the Northeast and Midwest. In 1978, 1,228 pairs nested in 21 colonies on Long Island, NY. By 1985, only 650 pairs nested in 15 colonies (Andrle and Carroll 1988). A similar decline in New Jersey from 3,178 individuals in 27 colonies to 1,343 in 26 colonies has also occurred (Walsh et al. 1999). Nest-site competition with Cattle Egrets (Bubulcus ibis) in certain areas is one potential cause (Burger 1978b). Competition for nest sites with Black-crowned Night-Herons (Nycticorax nycticorax) may be the cause of a 99% decline in Snowy Egrets at a colony in Avalon, NJ from 1995 to 1996 (TLM). The night-heron population has risen 78% during the same time span. Since the 1970s, abundance has declined by 25 to 30% in the ne. U.S. (KCP). Increased predation at semi-isolated nesting sites has largely redistributed birds to true islands, which may be limiting (KCP).

Although some specimens reportedly discovered from the Pleistocene and prehistoric sites (Brodkorb 1963), Olson (1985) claims fossil record historically misinterpreted and very incomplete.