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R. Michael Erwin
USGS Patuxent Wildlife Research Center
Department of Environmental Sciences
University of Virginia
Charlottesville, Virginia 22903

Background and History

For two years, Patuxent biologists, headed by Dr. Michael Erwin of the Branch of Migratory Bird Research, investigated the
nesting ecology and dispersal of juvenile wading birds (Snowy Egrets and Black-crowned Night-Herons) in coastal Virginia.
The study was part of an assessment of whether aspects of wading bird reproduction could be used as a bioindicator of
estuarine wetland conditions.(Click here for a picture of a Snowy Egret chick)

At two Chincoteague colonies in 1992 and 1993, biologists monitored sample nests for hatching, survival, and growth of
young. When the young were large, small transmitters (with mortality switches) were attached to leg bands. This permitted
late-nestling survival to be followed (which has seldom ever been monitored). It also allowed young that dispersed from the
colony to be followed for up to 90 days in some cases. Thus, post-fledging survival and habitat use could be determined. (Click here for a picture of a Snowy Egret chick with a transmitter attached to its leg)

The results indicated that Snowy Egrets were more sensitive than Night-Herons in growth rates between years. In general,
1993 was a better "food year" for both species than was 1992. Survival and growth were strongly correlated for both species
and survival during the nesting period was very high, at least for the oldest (A) chicks. Survival after fledging was variable and
low, ranging from about 0.3 to about 0.6 after 45 days post-fledging. We suspected owl predation as the major source of

Dispersal was farther for Snowies than Night-Herons, and distances moved were much greater for Snowies in 1993 vs. 1992,
but no such differences were found for the Night-Herons. Many Snowies moved north into New Jersey and Delaware from
Chincoteague, but few herons did. Habitat use indicated that herons tended to use manmade wetlands proportionately more
than did Snowy Egrets which were usually found in tidal marshes. Some birds are very site-specific after dispersal, with a few
remaining in the same wetland complex for more than 6 weeks! Results have been compiled and submitted to The Auk and
The Wilson Bulletin.