Systematics

Sometimes placed in monotypic genus Leucophoyx, but no author has followed this classification recently.

Variation in size slight, otherwise no differences in coloration described. Although clinal variation in size suggested, no pattern clearly known, and careful study based on birds of known breeding locality needed. Birds from the Rocky Mtns. westward said to average larger than birds from e. North America (male tarsus averages 107.3 mm vs. 97.1 mm), but there is great individual variation within populations in any given region (Bailey 1928). Baja California breeders may average largest overall in North America, with a more massive bill (length, depth, and breadth at base), while populations in the interior West are somewhat smaller but still larger than those in the East (Rea 1983). Length of tarsus apparently not correlated with bill or wing length; for example, a long-legged bird may have relatively short wings and culmen (Bailey 1928). Breeding populations of North America, Central America, and n. South America (Venezuela, Colombia, Ecuador) suggested to average larger overall in bill and tarsus than those of s. South America (Oberholser 1974), but further study needed.

Two currently recognized subspecies (Payne 1979); these are poorly differentiated, however, and based entirely on slight differences in size (Bailey 1928, Palmer 1962).

E. t. thula (Molina, 1782): Breeds throughout e. North America south through Central America, the Greater Antilles, and all of South America. Smaller on average than brewsteri, with shorter tarsus. Breeding populations from North America south to n. South America (Venezuela, Colombia, Ecuador) possibly separable as E. t. candidissima; only those breeding in s. South America regarded as nominate thula (see Oberholser 1974 and Rea 1983).

E. t. brewsteri Thayer and Bangs, 1909: Breeds w. North America (west of Rocky Mtns.). Larger, especially birds from Baja California. Rea (1983) restricts brewsteri to Baja, noting that birds there have more massive bill, and merges all other North American populations under nominate thula (or candidissima if that taxon recognized). Oberholser (1974) suggested that birds from the Great Basin (Utah) south through central California, Arizona, and, possibly, as far south as nw. Mexico recognizable as separate subspecies (E. t. arileuca), being intermediate in size between brewsteri and nominate thula . Measurements by Bailey (1928), however, suggest that these populations more appropriately grouped with brewsteri (also see Browning 1974).

Snowy Egret thought closely related to Little Egret, both of which, along with Western Reef-Heron and Mascarene Reef-Heron (E. dimorpha), classified as a superspecies by Payne (1979). Some authors have regarded Little and Snowy egrets as conspecific based on coloration of lores and reproductive behavior (Curry-Lindahl 1971); others have emphasized the importance of differences in plumage, size, and foraging behavior (Bock 1956, Kushlan 1976a, Murphy 1992). Measurements of appendicular musculature very similar to Little Blue Heron, Reddish Egret, and Tricolored Heron (E. tricolor; Vanden Berge 1970); skeletal characteristics essentially identical to Little Egret (Payne and Risley 1976).

Snowy Egret belongs to a group of herons known collectively as the day-herons (tribe Ardeini; Am. Ornithol. Union 1998), which, along with the night-herons (Nycticoracini), form a sister group to the bitterns (Botaurini) based on DNA-DNA hybridization data, vocalizations, and preliminary cladistic analysis of mitochondrial DNA (cytochrome b) sequences (McCracken and Sheldon 1998).

Hybridization

With Little Egret (Perkins 1995); colonization of the New World by Little Egret may be facilitated by socialization with Snowy Egret, as was the case with Cattle Egret (Murphy 1992). Also known to have produced hybrids with Tricolored Heron (Meeks et al. 1996), Little Blue Heron, and Cattle Egret (Sprunt 1954, Parkes 1978).