Limited adult vocal learning maintains call dialects but permits pair-distinctive calls in red crossbills

Vocal imitation can inform receivers about the social background of signallers because shared signals reflect learning experiences. Open-ended imitation can generate shared signals that indicate current social affiliation, whereas limited vocal plasticity in adulthood can permit signals to reflect an individual's origin. I examined the relations between vocal learning, signal sharing and social dynamics in red crossbills, Loxia curvirostra. Two levels of shared variation exist within crossbills' contact calls: discrete call variants distinguish ecologically diverged crossbill forms, and within these variants, some bonded pairs produce calls with nearly identical structures. Assessment of vocal and social behaviour of experimentally housed birds revealed that some adult crossbills converged on calls shared with companions of their own form, but no birds learned new, categorically distinct call variants. Also, while the process of call convergence within the bounds of discrete variants was associated with affiliation, birds that produced different contact call variants interacted less frequently and less amicably. These results suggest that call learning is limited in adult crossbills, generating calls with two levels of shared variation and multiple social functions: discrete contact call variants reflect birds' ecological forms, while pair-distinctive calls within those discrete variants reflect current social affiliation. Thus, a single social call concurrently reflects bonding among conspecifics of common origin and promotes the social isolation of ecologically diverged forms of red crossbills.