Socially-facilitated habitat selection by two migratory forest songbirds.

Abstract

One proximate cue important for habitat selection by territorial birds is the location and behavior of conspecifics. For migratory forest songbirds returning to their breeding grounds in early spring, conspecific song may facilitate territory search and settlement. Song is a behavioral mechanism that excludes conspecifics from the territory of the singer, but song may also act to recruit conspecifics. I explored socially-facilitated habitat selection by two migratory passerines nesting in Michigan's Upper Peninsula. I first examined territory establishment by American redstarts <italic> Setophaga ruticilla</italic> in response to conspecific song playbacks on plots with low or high redstart density, based on pre-experiment surveys. Redstarts exhibited a significant numerical response to playbacks and settlement was strongest on low density plots, where space for additional territories was greater. Adult males new to my plots were the primary respondents, whereas yearling males failed to respond to playbacks. Yearlings preferentially settled on high density plots, however, and arrived substantially later than adult males. These results demonstrate that conspecific attraction is an important mechanism for breeding habitat selection, as moderated by redstart age, reproductive experience, and arrival timing. I next assessed whether reproduction is negatively affected by higher redstart densities, either natural or experimentally increased through playbacks. Higher densities had no significant negative impacts on nest survival or site fidelity. I also found no consistent reproductive advantages from socially-facilitated habitat selection, though neighbor density and playback carryover were positively related to nest success in 2006, the year following treatment. High local density was also linked to early spring arrival, demonstrating that the plot-level response I detected operates as attraction to individual birds. The behavioral cues provided by early-arriving males may help explain breeding distributions across the landscape. Finally, I tested whether conspecific song can be used to shift breeding populations of black-throated blue warblers <italic>Dendroica caerulescens </italic>, a species of conservation concern. This proof-of-concept experiment showed that playbacks significantly increased territorial occupancy and shifted territories closer to the speakers. Because loss of breeding habitat is implicated in the population declines of many songbirds, song playbacks should be further investigated as a tool for mitigating the effects of disturbance.