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 h osts, but still manage to monopolize a disproportionate share of the food d elivered by adult hosts. We hypothesized that: (1) the low levels of host-s pecific pathogens of such brood parasite nestlings provide them with an adv antage in competition for limited food; (2) a higher provisioning rate and the resulting better body condition of brood-parasite nestlings compared wi th that of host nestlings may increase the efficiency of their immune defen ce; (3) brood parasites may decrease the risk of parasitism and improve the immune response level of their offpring by selecting hosts with a more eff icient immune defence (low pathogens load) and by selecting high-quality ho sts, 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 gre at spotted cuckoo Clamator glandarius. Cuckoo nestlings are reared together with magpie nestlings, but the former usually manage to outcompete the lat ter. The results of our experiment were as follows. Magpie and great spotte d cuckoo nestlings had similar prevalence of a generalist, directly transmi tted haematophagous Diptera of the family Carnidae. Magpie nestlings had co nsiderably higher prevalence and intensity of a haematozoan parasite of the genus Leucocytozoon than nestlings of the great spotted cuckoo. Great spot ted cuckoo nestlings had greater immune responses, measured in terms of T-c ell response to an injection with phytohaemagglutinin, sedimentation rate o f erythrocytes and colour of the plasma than magpie nestlings. Heavier magp ie nestlings had greater immune responses than lighter conspecifics. We fou nd no support for the third hypothesis, however. Therefore, brood-parasite nestlings may outcompete host nestlings because of pathogen specificity, an d because of their efficient immune system mediated by their higher rate of food intake.