We tested the prediction that the calls of sibling cliff swallow (Hirundo pyrrhonota) chicks are more similar than those of sibling barn swallow (Hirundo rustica) chicks. This prediction was derived from the hypothesis that the call of the colonial cliff swallow, but not the call of the noncolonial barn swallow, has been selected for signature function (i.e., for individual distinctiveness). In Study 1 we examined the calls of 22 cliff swallow sibling pairs and 23 barn swallow sibling pairs. The intraclass correlations for 4 of the 5 cliff swallow variables were significantly different from zero, and each of the 4 was approximately 0.5. Only one of the 4 barn swallow call variables was significantly different from zero. In a discriminant-function analysis of these data, cliff swallow chick calls were correctly identified as to sibship in 82 % of the cases, barn swallow chick calls in only 46 % of the cases. In Study 2 we cross-fostered eggs between cliff swallow nests to create foster sibships (all chicks in a nest were unrelated). We found no similarities among foster sib calls, and thus no evidence for call imitation of the calls of sibs or parents, suggesting that genetic differences are the main source of variance in cliff swallow chick calls.