Foraging behavior of migrant warblers in mixed-species flocks in Venezuelan shade coffee: interspecific differences, tree species selection, and effects of drought

Shade coffee has been identified as an important habitat for Nearctic-Neotropical migrants during the non-breeding season, including species of conservation concern such as Cerulean Warblers (Setophaga cerulea). To better understand habitat features important for migrants in shade coffee, we studied the foraging behavior of migrants in mixed-species flocks at six shade-coffee farms in the Cordillera de Merida, Venezuela, in 2008–2009 and the El Niño drought year of 2009–2010. We examined interspecific differences in foraging behavior and tree species selection of three foliage-gleaning migrants, Blackburnian (Setophaga fusca), Cerulean, and Tennessee (Oreothlypis peregrina) warblers, and aerial-foraging American Redstarts (Setophaga ruticilla). For morphologically similar Blackburnian and Cerulean warblers, we also examined factors influencing foraging rates (attack and movement rates), capture of large prey, and maneuver/substrate type. We found that aerial-foraging American Redstarts foraged lower, used more aerial maneuvers, showed no tree species selection, and were less likely to forage in flocks than foliage-gleaners. Although foraging rates were similar for Blackburnian and Cerulean warblers, the three foliage-gleaners differed in foraging height and use of maneuvers. Cerulean Warblers foraged lower than the other two species, whereas Blackburnian Warblers used the greatest proportion of woody gleans. All three foliage-gleaners selected Inga spp. (a commonly planted shade tree in shade-coffee farms) for foraging, and Blackburnian and Cerulean warblers captured a greater proportion of large prey in Inga spp. than in other tree species. During the drought year, Blackburnian and Cerulean warblers captured half as many large prey and used a greater proportion of woody-gleans. We found that interactions among behavioral, floristic, and environmental drivers influenced the foraging behavior of migrants wintering in shade coffee. Our results support those of previous studies suggesting that migrants partition resources behaviorally during the non-breeding season, that foliagegleaners may benefit from the presence of shade trees, especially Inga spp., in agroforestry systems, and that drought may influence the foraging behavior of foliage-gleaning migrants, presumably due to reduced prey availability. El café bajo sombra ha sido identificado como un hábitat importante para especies migratorias neartico-neotropicales durante la época no reproductiva, incluye