FORAGING SOCIABILITY OF NESTING WADING BIRDS (CICONIIFORMES) AT LAKE OKEECHOBEE, FLORIDA

摘要

From 1989-1992 at Lake Okeechobee, Florida, I followed 356 adult Great Egrets (Casmerodius albus), 236 Snowy Egrets (Egretta thula), 82 Tricolored Herons (E. tricolor), and 286 White Ibises (Eudocimus albus) from several nesting colonies to foraging sites during chick-rearing periods. White Ibises departed in groups more often than the other species (64% of the birds followed), tended to travel in larger groups, rarely landed in areas devoid of other birds, and tended to join larger, more concentrated feeding flocks than other species. A majority of all species joined other birds at foraging sites. However, Great Egrets and Tricolored Herons often landed in areas devoid of other birds (29% and 43% of the birds, respectively) and more often landed near rather than in the immediate vicinity of other birds. A tendency toward group departures has been interpreted as support for the "information-exchange" hypothesis about the benefits of colonial nesting relative to food-finding. However, many of the birds in this study that departed in groups dispersed before landing, which is contrary to the expectations of the hypothesis. Many individuals did, however, choose foraging sites already occupied by other birds, which is indicative of reliance on "local enhancement" for identifying productive foraging grounds. Differences in degree of sociability among the species may, however, primarily reflect differences in foraging strategy. White Ibises are more social probably because foraging in groups enhances the effectiveness of the grope-foraging technique they employ, whereas Tricolored Herons and Great Egrets often employ foraging techniques more suited to solitary effort and are not as likely to benefit from foraging in aggregations. The evidence also suggested that Snowy Egrets, in particular, act as attractors for other species that rely on local enhancement to identify profitable foraging opportunities. This association may result from a combination of factors; i.e., white plumage serves as an attractor, but flocks of Snowy Egrets may also be particularly indicative of high-quality habitat and/or concentrations of universally attractive prey species.