Nest Defense by Willow Flycatchers to Brood-Parasitic Intruders

Abstract

Nest defense behaviors are defined as counteradaptations to brood parasitism if the defensive behaviors are (1) beneficial and (2) specific to brood parasites. Specificity implies recognition of parasites and host responses tailored to brood parasite intrusions. We investigated the adaptive value and specificity of Willow Flycatcher (Empidonax traillii) responses to simulated and live intrusions of Brown-headed Cowbirds (Molothrus ater). We hypothesized that flycatchers which were quieter near their nests were less likely to be parasitized. Parasitism was associated with noisier flycatcher pairs, suggesting that inconspicuous behavior was adaptive for this heavily parasitized population. The flycatchers' responses to cowbird intrusions differed depending on the context of intrusions. Flycatchers chased female cowbirds near the nest (<2 m) but not cowbirds further from the nest nor other species which approached to within 2 m. Preincubation behavior (sitting in the nest) was observed in 42% of flycatcher nests during live and simulated cowbird intrusions. We tested whether flycatchers responded adaptively (quietly) to playbacks of female cowbird vocalizations distant (5 m) from the nest. In our first experiment, flycatchers were quieter in response to calls of a brood-parasite than they were to a playback of a nonparasitic species (Rufous-sided Towhee). In a second experiment, parasitized and unparasitized flycatchers did not differ in their response to female cowbird calls. Taken together, our results suggest that this population of Willow Flycatchers differentially recognized female cowbirds and differentially responded to female cowbirds, depending on the cowbird's distance from the nest. This response did not appear to differ with the experience of parasitism.