Nest defence and egg rejection in the house sparrow (Passer domesticus) as protection against conspecific brood parasitism

Abstract

Nest site availability is a factor associated with conspecific brood parasitism (CBP) in birds. The house sparrow ( Passer domesticus ) population on Dassen Island, South Africa, suffers from limited nesting space availability. We estimated the incidence of CBP in this population and determined experimentally whether sparrows retain the capacity to recognise and reject parasitic eggs. Other strategies that have evolved to thwart brood parasitism include nest guarding and nest defence. The presence of an intruding female near a nest may result in antagonistic behaviour as well as increasing the rejection of parasitic eggs (i.e., additive stimulus). This study tests if the presence of an intruding conspecific female generates an additive stimulus or whether nest guarding/defence and egg rejection are antagonistic responses, which has been studied principally in cases of interspecific brood parasitism (IBP). Despite space limitations, CBP was not detected in this population, and one third of foreign eggs were rejected. However, nest guarding/defence was not related to egg recognition and rejection. Egg rejection in this population does not appear to depend on the probability of nests being parasitised or on an inability to assume the costs associated with nest guarding/defence and with egg rejection.