Feeding Associations between Snowy Egrets and Red-Breasted Mergansers

Abstract

-During the winter months Snowy Egrets (Egretta thula) often form feeding associations with Redbreasted Mergansers (Mergus serrator) at Bolsa Chica Ecological Reserve, Orange County, California. We quantified and compared foraging success of egrets feeding in associations with success of egrets feeding independently. During winter 1996-1997, egrets feeding with mergansers increased their rate of prey acquisition by increasing both their striking rate (strikes/min) and their striking efficiency (captures/strike). In contrast, during winter 1997-98 we found no differences in the rate of prey acquisition between egrets feeding independently and those feeding in association with mergansers. Although we did not quantify the size of prey taken, egrets feeding in associations in 1997-1998 may have captured larger prey than those that were feeding alone. Egrets may benefit from feeding in associations with mergansers by increasing their prey capture rate and/or by capturing larger prey. Received 15 September 2000, accepted 3 October 2000.