Snowy Egret Habitat Model
go to: USFWS Gulf of Maine Watershed Habitat Analysis
go to: Species Table

Draft Date:
November 2002

Species:
Snowy egret, Egretta thula

Use of Study Area Resources:
Reproduction. Snowy egrets breed along the Eastern seaboard, primarily from Massachusetts south, most of Florida, the Gulf of Mexico coast and  Caribbean, and most of Central and South America. They breed inland in the southern mid-west to the Rockies. Snowy egrets undertake inland post-nesting dispersal (Palmer 1961).  On the Atlantic coast birds may travel north into Canada, returning when weather becomes cold and prey scarce (late September and October).  This species expanded its range into New England prior to the 1970's, but breeding colonies still tend to be smaller in the Northeast than many of those in the Southeast (Spendelow and Patton 1988, Andrews 1990, Veit and Petersen 1993).

Habitat Requirements:
Cover. Snowy egrets use freshwater sites, and even dry fields (Palmer 1961), but most frequently are found in brackish and sheltered saltwater areas (Spendelow and Patton 1988); in New England they breed only in coastal locations (Andrews 1990). They often nest in mixed colonies with other waterbirds (Parsons and Master 2000).

Nesting predominantly coastal, on islands or in salt marsh; inland they nest in swamps, ponds, marshes or lakes. Nesting typically is in mixed-species colonies (Parsons and Master 2000). Nest substrates include buttonbush, willow, Phragmites, bulrushes or cane (Palmer 1961), or nests may be built on the ground among low vegetation (Spendelow and Patton. 1988). Over time, vegetation may be damaged by nest building activities and excrement, with the result that the birds move or colonies are abandoned until the site regenerates.

Foraging. Snowy egrets feed on small fishes, crustaceans (particularly shrimp, fiddler crabs, crayfish), frogs, lizards, snakes, worms, snails, insects, and evern mice (Palmer 1961). In Lake Okeechobee, Florida, Smith (1997) found them to feed on smaller freshwater fishes than other waders.  Maccarone and Parsons (1994) noted that snowy egrets faced greater competition with other waders in a coastal freshwater pond than in nearby tidal waters, but that the latter were not available much of the time, because of excessive water depths during higher tidal stages. Snowy egrets have a number of foraging behaviors, and feed most efficiently in aggregations with conspecifics or other water birds (Parsons and Master 2000).

During the breeding season the nesting birds require adequate forage within a suitable flight distance (< 10 km, Parsons and Master 2000). Average foraging travel distances were below 5 km for snowy egrets in North Carolina (Custer and Osborn 1978 in Erwin et al. 1987), and 3 km, with a maximum of 30 km at Lake Okeechobee (Smith 1995).

Habitat Mapping:
Data Sources: Data on nesting colonies were obtained from the Maine Department of Inland Fisheries and Wildlife (MDIF&W) Seabird Nesting Island databases, from Andrews 1990, and from Massachusetts Department of Fisheries, Wildlife and Environmental Law Enforcement (Brad Blodgett).  Pierson et al. (1996) gave narrative descriptions of  foraging sites frequented by snowy egrets in Maine

All upland areas of active (most recent survey, at least since 1984) colonies were regarded as optimal nesting habitat, and so scored 1.0. Sites used since 1970,  but not in most recent surveys, were scored 0.8.

Foraging habitats were mapped by selecting fresh and marine wetlands (see table, below) within range of nesting colonies or of other sites having regular documented use.  This range was estimated as the extent of USDA Forest Service Ecological subunits (Keys et al. 1995) intersecting either a 30 km buffer of nesting colonies, or other documented use sites.  Foraging areas within 10 km of active nesting colonies were scored with the values in the table; those more distant from nesting colonies (but within the subunit polygon) were scored at half those values. Foraging scores range from 0.9, as optimal, to 0.5, as suitable, allowing foraging and nesting areas to be distinguished the single (combined) output grid.

NWI Designations
(wetlands only)
Cover Types Cover Suitability
(0 - 1 scale)
Upland deciduous forest
Upland coniferous forest
Upland mixed forest
Grassland
Upland scrub/shrub
Cultivated
Developed
Bare ground
PEM, L2EM Lake/pond, emergent vegetation 0.5
PFOcon Palustrine forest, conifer
PFOdec Palustrine forest, deciduous
PSSdec Palustrine scrub shrub, deciduous
PSScon Palustrine scrub shrub, conifer
PAB, L2AB Lake/pond, aquatic vegetation 0.5
L1UB, PUB Lake/pond, unconsolidated bottom
L2US Lake, unconsolidated shore 0.5
L2RS Lake, rocky shore 0.5
R1UB Riverine subtidal unconsolidated
Rper Riverine perennial 0.5
E1AB Estuarine subtidal vegetated
E1UB Estuarine subtidal unconsolidated bottom
E2AB Estuarine intertidal algae 0.9
E2EM Estuarine intertidal emergent 0.9
E2RS, R1RS Estuarine, tidal river rocky shore 0.9
E2SS Estuarine intertidal shrub 0.7
E2US/R1US Estuarine, Riverine intertidal unconsolidated shore 0.9
M1AB Marine subtidal vegetated
M1UB Marine subtidal unconsolidated bottom
M2AB Marine intertidal algae 0.7
M2RS Marine intertidal rocky shore 0.7
M2US Marine intertidal unconsolidated shore 0.7

NOTES

Sources:

Andrews, R. 1990. Coastal Waterbird Colonies Maine to Virginia 1984-85. U.S. Fish and Wildlife Service. 429 pp.

Erwin, M.E., J.E. Spendelow, P.H. Geissler and B.K. WIlliams. 1987. Relationships between nesting populations of wading birds and habitat features along the Atlantic coast. In W.R. Whitman and W.H. Meredith (eds): Waterfowl and wetlands symposium; proceedings of a symposium on waterfowl and wetland management in the coastal zone of the Atlantic flyway. Delaware Depart. of Nat. Res. and Environm. Control, Dover DE. 552 pp.

Keys, J.E., Jr., J.C. Carpenter, S. Hooks, F. Koenig, W.H. McNab, W. Russell and W. Smith. 1995. Ecological units of the eastern United States - first approximation (map and booklet of map unit tables), USDA Forest Service. Atlanta, GA.

Maccarone, A.D. and K.C. Parsons. 1994. Factors affecting the use of a freshwater and an estuarine foraging site by egrets and ibises during the breeding season in New York City. Colonial Waterbirds 17(1):60-68.

Palmer, R.S. 1961. Handbook of North American Birds; Vol. 1. Yale University Press, New Haven CT. 567 pp.

Parsons, K.C. and T.L. Master. 2000. Snowy egret (Egretta thula). In A. Poole and F. Gill, (eds.) The Birds of North America, 489. Acad. Nat. Sci. Phil. and Amer. Ornith. Union, Washington D.C.

Pierson, E.C., J E. Pierson and P.D. Vickery. 1996. A Birders Guide to Maine. Down East Books, Camden, ME.

Smith, J.P. 1995. Foraging flights and habitat use of nesting wading birds (Ciconiiformes) at Lake Okeechobee, Florida. Colonial Waterbirds 18(2): 139-158.

Smith, J.P. 1997. Nesting season food habits of 4 species of herons and egrets at Lake Okeechobee, Florida. Colonial Waterbirds 20(2): 198-220.

Spendelow, J.A. and S.R. Patton. 1988. National Atlas of Coastal Waterbird Colonies in the Coterminus United States: 1976-82. U.S. Fish and Wildlife Service Biol. Report 88(5). 326 pp.

Tyler, H.R. 1977. Wading birds in Maine and their relevance to the Critical Areas Program. Planning Rept. No. 26. 51 pp.

Veit, R.R. and W.R. Petersen. 1993. Birds of Massachusetts. Massachusetts Audubon Society. 514 p.