Nestling recognition in red-rumped and barn swallows

Nestling recognition and rejection is a rare defense strategy against avian brood parasitism that was previously thought to be too costly to evolve. Recently, several examples of nestling recognition and rejection by fosterers have been reported. Here, we tested for chick recognition ability in red-rumped swallows (Hirundo daurica), which build closed nests and do not discriminate against foreign eggs, by using cross-fostering experiments while controlling for responses to intraspecific and interspecific parasitism in closely related swallow species. We show that red-rumped swallows do not discriminate against conspecific or barn swallow (H. rustica) nestlings, but eject or reject cuckoo nestlings cross-fostered in red-rumped swallow nests by starving the introduced chicks to death. Assuming that the behavior of red-rumped swallows evolved in response to cuckoo parasitism, this study represents novel empirical evidence showing that chick discrimination evolved when brood parasites have completely evaded host defenses at the egg stage. The evolution of such recognition ability may be explained as a defense against avian brood parasitism by small-sized cuckoos or a pre-adaptation to specific communication between host parents and offspring. Our results are consistent with the "rarer enemy" hypothesis predicting that nestling discrimination should evolve only in hosts that accept all naturally laid parasite eggs and thus do not decrease effective parasitism rate at the nestling stage. In contrast, the findings were inconsistent with a number of alternative hypotheses.