Social experience modifies behavioural responsiveness to a preferred vocal signal in red crossbills, Loxia curvirostra

Although plasticity in signal production is well recognized as a means for animals to modify their communication systems, modifying responsiveness to signals could be equally important. We examined plasticity of responsiveness to social signals that mediate group formation and cohesion in red crossbills. The red crossbill is composed of several ecologically diverged forms, defined by variation in morphology, ecological specialization and vocal behaviour; each form is associated with a distinct variant of the species 'contact call.' To understand how plasticity in response to signal variants may influence social dynamics, we examined the response of red crossbills to distinct call variants before and after experimental pairing with a companion of a different form. In a playback study conducted soon after birds were captured we found that adult crossbills responded more to calls that were similar to their own calls. However, social experience with a companion that produced a different contact call variant eroded this response selectivity; at the end of the study birds experimentally housed with a companion that produced a different call variant responded equally to all species-typical calls, but control birds continued to respond selectively to calls that were similar to their own. The observed form of response plasticity may permit birds to gain information from a wider range of conspecifics but is unlikely to facilitate the social intermixing of crossbills of different forms; birds that respond to all crossbill call variants probably would not gain acceptance into flocks of other forms.