Food acquisition by common cuckoo chicks in rufous bush robin nests and the advantage of eviction behaviour

The common cuckoo, Cuculus canorus, is a brood parasite that monopolizes parental care of its host species: soon after hatching, the chicks remove the host offspring. Although cuckoo chicks trick their foster parents into providing enough food, it is unknown whether cuckoo begging behaviour represents an advantage over that of the host chicks in a hypothetical competitive scenario. We studied the feeding behaviour of rufous bush robins, Cercotrichas galactotes, when rearing their own and parasitic chicks in natural and in experimental nests where a cuckoo and a host brood were presented simultaneously to parents. In natural parasitized and nonparasitized nests, the feeding rate for cuckoo chicks did not differ from that of a single host chick of the same age, but cuckoos were fed with a different diet and with larger prey. Thus, cuckoo chicks received a similar amount of food to that received by a whole host brood. Cuckoo chicks in experimental nests did not receive a diet, prey size or feeding rate different to that received by a single host chick and thus received considerably less food than cuckoo chicks in unmanipulated nests. These results suggest that cuckoo chicks could not outcompete host chicks, at least when parasitizing this host species, and thus that their eviction behaviour is beneficial. We discuss various explanations for the inability of cuckoos to outcompete host chicks.