Prey selection by Swainson's Warblers on the breeding grounds

Abstract

Swainson's Warbler (Limnothlypis swainsonii) breeds in bottomland hardwood forests across the southeastern United States, where it is believed to be one of the rarest breeding songbirds. Although information on its nest-site habitat is considerable, little is known about its foraging habitat except that the species is insectivorous, with a large bill used to flip fallen leaves on the forest floor. We captured Swainson's Warblers and flushed their crops to determine their diet and sampled leaf-litter arthropods and vegetation at each location of capture. We compared the proportion of arthropod orders in the crop samples to the proportion of arthropods collected in the leaf litter to determine the warbler's prey in proportion to its availability. Although Acari (mites and ticks) and Chilopoda (centipedes) were the most abundant arthropods in the leaf-litter samples (51% and 18%, respectively), these orders rarely occurred in the warblers' crops. Conversely, Araneae (spiders) and Coleoptera (beetles) were uncommon in leaf-litter samples (2% and 5%, respectively) but were the most abundant arthropod orders in the warblers' crops. Binary logistic regression with presence or absence of Araneae as the response variable and habitat measures as the predictor variables revealed that the probability of spiders occurring in the leaf litter increased as leaf-litter depth increased. To promote foraging habitat for Swainson's Warbler, deep leaf litter should be maintained by maintaining patches of closed-canopy forests and restoring natural regimes of flooding. © The Cooper Ornithological Society 2010.