Acoustic adaptations for parent-offspring recognition in swallows.

We have used field and laboratory studies to investigate acoustic adaptations for parent-offspring recognition in two closely related pairs of swallows: (a) bank swallow (Riparia riparia) and northern rough-winged swallow (Stelgidopteryx serripennis), and (b) cliff swallow (Hirundo pyrrhonota) and barn swallow (Hirundo rustica). Cross-fostering and playback experiments show that bank swallow and cliff swallow parents recognize their offspring by voice while rough-winged swallow and barn swallow parents do not. We argue that this species difference is due to an evolutionary history of strong selection for recognition in bank swallows and cliff swallows, which live in large, dense colonies, and of weak or no selection for recognition in rough-winged swallows and barn swallows, which live solitarily or in small groups. We consider two possible acoustic adaptations which may underlie the observed species difference. First, the "signature" calls of cliff swallow and bank swallow chicks appear to be more individually distinctive than the homologous calls of rough-winged swallows and barn swallows. This conclusion is supported by a sonographic analysis of among- and within-individual call variation: The information content of bank swallow and cliff swallow calls is considerably greater than that of rough-winged swallow or barn swallow calls. We also discuss our more recent work on the hypothesis that the colonial swallow species are better able to discriminate these sorts of auditory stimuli. We conclude with the caution that auditory specializations may be unnecessary given the signature call adaptation and the general capabilities of the avian ear.