White tail spots and tail-flicking behavior enhance foraging performance in the Hooded Warbler

Abstract

Many species of insectivorous birds have contrasting plumage patches that are often displayed during foraging. Although such displays are widely hypothesized to flush potential prey and enhance foraging performance, experimental evidence that they function in this way was previously available only from redstarts (Myioborus spp.). I provide additional evidence from an experimental study of the Hooded Warbler (Setophaga citrina) in northwestern Pennsylvania, USA. Hooded Warblers regularly flick their tails while foraging, revealing large white spots on the outer tail feathers. I tested the function of the white tail spots with plumage manipulation experiments. At nests containing 5-day-old nestlings, the attending adults were captured and assigned to one of two treatment groups. For experimental birds, I temporarily darkened the white tail spots with a brown marking pen; for sham-darkened controls, I applied a comparable amount of ink to the dark inner rectrices. Experimental birds with darkened tails showed significantly reduced foraging performance compared with controls, and the decline in foraging performance was driven almost entirely by a decreased frequency of aerial prey attack. High-definition video recording at nests showed that plumage manipulation also altered the types of prey females delivered to nestlings; females with darkened tails delivered significantly fewer winged insects, and proportionally more insect larvae, than controls. However, the types of prey delivered by males were unaffected by plumage manipulation. Tail-flicking behavior codevelops with independent foraging in juveniles and is a significant positive predictor of juvenile foraging performance, even when the effects of juvenile age and activity are controlled statistically. Collectively, these results provide strong support for the hypothesis that white tail spots and tail-flicking behavior of Hooded Warblers function to flush visually oriented winged prey and enhance foraging performance. They also raise questions about sexual dimorphism in the tail pattern and its relationship to possible sex differences in foraging strategies.