Foraging sociability of nesting wading birds (Ciconiiformes) at Lake Okeechobee, Florida

-FrOtn 1989-1992 at Lake Okeechobee, Florida, I followed 356 adult Great Egrets (Casmerodius albus), 236 Snowy Egrets (Egretta thula), 82 Tricolored Herons (E. tn’color), 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. Received 29 Aug. 1994, uccepted I Feb. 1995. Researchers often have debated the issue of whether or not species benefit from colonial nesting because individual birds can exploit conspecifics or similar species for information about profitable foraging locations (Ward and Zahavi 1973, Krebs 1974, Ogden 1978, Custer and Osbom 1978, Pratt 1980, Erwin 1983, Waltz 1983). Ward and Zahavi (1973) initiated the concept of information-sharing at colonies; i.e., birds departing without prior knowledge of profitable foraging grounds may reduce their search time and gain advantage by following other birds to foraging grounds. Observation of birds traveling in groups from colonies was considered corroborating evidence. However, Erwin (1983) noted that ’ Dept. of Wildlife Ecology and Conservation, Univ. of Florida, Gainesville, Florida 32611. (Present address: Forest and Rangeland Ecosystem Science Center, National Biological Service, 3200 S.W. Jefferson Way, Corvallis, Oregon 97331.)