Early learning of discrete call variants in red crossbills: implications for reliable signaling.

The identification of appropriate companions and mates is essential to both speciation and the maintenance of species through prezygotic isolation. In many birds, social assortment is mediated by vocalizations learned through imitation. When imitative vocal learning occurs throughout life, emergent shared signals reflect current social associations. However, when vocal and genetic variation arises among populations, shared learned signal variants have a potential to reflect cultural or genetic origin and to limit social and reproductive intermixing, provided that signal learning occurs prior to dispersal. The red crossbill (Loxia curvirostra) is a bird species in which discrete contact call variants are associated with morphological variation, raising the possibility that learned calls play a role in limiting intermixing. I examined the process of early call learning to determine if contact call variants have a potential to limit intermixing in crossbills. I conducted a captive playback study to nestlings to evaluate potential learning predispositions. I also cross-fostered nestlings to adoptive adult pairs of either their own or a different call variant than their biological parents to assess the degree of vocal learning plasticity. Results show that young crossbills imitate the call structures of adoptive parents, generating shared family-specific calls, which could facilitate family cohesion. Learning processes that generate family-specific calls could also ensure that discrete call variants are transmitted across generations, making call variants reliable signals of crossbills' morphological and genetic backgrounds.