Egg recognition and interspecific brood parasitism rates in red bishops

The observed rates of interspecific brood parasitism by diederik cuckoos, Chrysococcyx caprius, on red bishops, Euplectes orix, are reported for four colonies. Experimental manipulations were used to examine how tolerant red bishops were towards foreign eggs. Red bishops did not discriminate mimetic cuckoo-sized models on the basis of egg size, an increase in clutch size or the stage at which the parasite egg appeared in the nest. Only eggs that were heavily maculated or had a darker ground colour and were quite unlike the host's light-blue eggs, were significantly rejected. Low rates of rejection (11.1%) of cuckoo-egg models may be a consequence of evolutionary lag in a coevolutionary arms race between host and parasite. Since the cuckoo egg is a good mimic the cost to a red bishop of making a mistake in the closed nest and rejecting an 'own' egg may constrain rates of egg rejection. The parasite may be unable to exploit this constraint because corporate vigilance and defence of large colonies probably prevents any significant parasitism from taking place in a population; rates of interspecific parasitism were negatively correlated with colony size. The potential exists for an evolutionary equilibrium between host and parasite in which the parasite can exploit breeders only in small, low density and poorly defended colonies. (C) 1996 The Association for the Study of Animal Behaviour