One possible explanation for the evolution of white plumage in herons is an adaptive advantage for foraging. Under this hypothesis, white plumage is cryptic to aquatic prey; thus white-plumaged birds may be more prone to forage in deep water habitats, where they would be cryptic, using passive foraging tactics. Dark-plumaged birds foraging in shallow water habitats, where they are presumably more visible, use active tactics. These hypotheses were tested by investigating choice of water depth and choice of foraging tactics by conducting paired foraging observations between color morphs within the plumage dimorphic Reddish Egret (Egretta rufescens). Water was divided into four depths classes: deep (>15 cm), intermediate (5-15 cm), shallow (1-5 cm), and mudflat (<1 cm). There was no difference in time spent in different water depth classes between the two color morphs of the Reddish Egret. However, there was a significant interaction between color morph and depth of water for time spent actively foraging. The white morph of the Reddish Egret was more active in intermediate depths of water than the dark morph while the dark morph was more active in shallow depths of water. These results support the hypothesis that Reddish Egrets may alter foraging tactics based on their degree of crypsis to prey.
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Vol. 28 • No. 4