No egg or nestling-retrieval behavior in a cavity-nesting cuckoo host

The arms race between avian brood parasites and their hosts provides a classic model for studying coevolution. In one of the most widespread obligate brood parasites, the Common Cuckoo (Cuculus canorus), chicks typically evict all host progeny (eggs and nestlings) from the nest cup, resulting in complete reproductive failure for the host. Host parents of Common Cuckoos could thus potentially benefit from retrieving evicted eggs and nestlings into the nest cup. However, whether hosts of the Common Cuckoo exhibit such retrieval behavior has been scarcely studied. In this study, we experimentally investigated the occurrence of retrieval in a nestbox-breeding population of Daurian Redstarts (Phoenicurus auroreus), a common cavity-nesting host of the Common Cuckoo. To test the redstarts' response to an egg or a nestling outside the nest cup, we experimentally placed either a conspecific egg, a model cuckoo egg, or a redstart nestling near the rim of the nest cup. We found that redstarts never showed retrieval behavior of either eggs or nestlings. All hosts ignored the experimental nestling and conspecific egg, but most ejected the model cuckoo egg from the nestbox. Our results suggest that selection for retrieval behavior in this cavity-nesting host may be weak or even negative. We discuss several ecological and evolutionary factors that may explain the absence of retrieval in this system.