Interactions between a brood parasite and its host in relation to parasitism and immune defence

Nestlings of many brood parasites are reared together with those of their hosts, but still manage to monopolize a disproportionate share of the food delivered by adult hosts. We hypothesized that: (1) the low levels of host-specific pathogens of such brood parasite nestlings provide them with an advantage in competition for limited food; (2) a higher provisioning rate and the resulting better body condition of brood-parasite nestlings compared with that of host nestlings may increase the efficiency of their immune defence; (3) brood parasites may decrease the risk of parasitism and improve the immune response level of their offpring by selecting hosts with a more efficient immune defence (low pathogens load) and by selecting high-quality hosts, respectively. This effect should provide brood-parasite nestlings with an advantage in their competition with host nestlings. This hypothesis was tested in the magpie Pica pica, which is the main European host of the great spotted cuckoo Clamator glandarius. Cuckoo nestlings are reared together with magpie nestlings, but the former usually manage to outcompete the latter. The results of our experiment were as follows. Magpie and great spotted cuckoo nestlings had similar prevalence of a generalist, directly transmitted haematophagous Diptera of the family Carnidae. Magpie nestlings had considerably higher prevalence and intensity of a haematozoan parasite of the genus Leucocytozoon than nestlings of the great spotted cuckoo. Great spotted cuckoo nestlings had greater immune responses, measured in terms of T-cell response to an injection with phytohaemagglutinin, sedimentation rate of erythrocytes and colour of the plasma than magpie nestlings. Heavier magpie nestlings had greater immune responses than lighter conspecifics. We found no support for the third hypothesis, however. Therefore, brood-parasite nestlings may outcompete host nestlings because of pathogen specificity, and because of their efficient immune system mediated by their higher rate of food intake.