We quantified flocking behavior and examined the impact of social context (solitary, single-species flocks, and mixed-species flocks) on the foraging behavior of Prothonotary Warblers (Protonotaria citrea) wintering in a Costa Rican mangrove forest and surrounding habitats. Based on observations collected over two winters during 70 visits to four sites, 87% (483) of the 555 Prothonotary Warblers encountered moved in flocks and over 48% (271) of these individuals were in single-species flocks. Although the propensity to join flocks was 6% higher for Prothonotary Warblers in the second winter of the study, neither the average size of single-species flocks nor the average number of individuals or species in mixed-species flocks differed between years. Twenty-seven different species were identified in mixed-species flocks that had at least one Prothonotary Warbler, but Nearctic migrants dominated these flocks. Analyses of focal observations on 57 females and 93 males indicated that Prothonotary Warbler foraging behavior was largely independent of flock type and size. Foraging maneuver, substrate, and location did not differ significantly for individuals of either sex foraging alone, in single-species, or mixed-species flocks. The species is almost strictly insectivorous, gleaning made up 70% of 150 prey capture attempts observed and about half of all attempts (76 of 150) were directed towards leaf surfaces. Foraging generally occurred in the outer third of the tree, on branches less than 1 cm in diameter, in the bottom half of the canopy. Agonistic interactions among flock members that involved Prothonotary Warblers were uncommon and neither flock type nor size were useful predictors for rates of foraging, movement, preening, or vigilance.
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