Group foraging is common among wading birds, and the reasons why individuals forage in groups are of theoretical and practical interest. Although aggregations of foraging wading birds usually form within patches of high-quality habitat, individual wading birds can sometimes increase success by foraging near others. We investigated the hypothesis that individuals derive a benefit from foraging in groups separate from benefits due to habitat quality. We measured the capture rates of birds foraging solitarily and in groups at foraging sites and paired unused sites and used generalized linear mixed models and information-theoretic model selection to access the evidence that individuals increase their foraging success when foraging in groups. The capture rate of Great Egrets (Ardea alba) in groups was higher, supporting the hypothesis that individuals benefit from the presence of other foragers. This rate was highest in intermediate-sized groups but tapered off in larger groups, suggesting that interference competition limits success. Tricolored Herons (Egretta tricolor) showed a similar but less statistically precise pattern. Contrary to expectation, the capture rate of Snowy Egrets (E. thula) foraging alone was higher than that of those in groups. Although fish abundance was greater at used than at unused sites, and at sites with groups than at sites with individuals, we did not detect an effect of prey density on capture rate, probably because prey density was fairly high at all sites studied. Our study adds to the evidence for a direct benefit to individuals due to the presence of other foragers in aggregations.
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Vol. 114 • No. 4