Survival and anti-parasite defense in a host metapopulation under heavy brood parasitism: a source-sink dynamic model

The obligate brood parasite common cuckoo Cuculus canorus, widespread in Eurasia, occasionally reaches a high parasitism rate (over 20%), which usually exists only for a short period of time and in cases of new parasitism. Recent results from Hungary proved that a remarkably high parasitism rate (50-66%) can also be maintained constantly for several decades. In this paradoxical situation the reproductive success of the strongly exploited host population is lower than would be necessary for self-reproduction. We developed a model for a hypothetical host-brood parasite system that demonstrated that immigration of naive individuals from a highly reproductive (source) host population might explain the survival of the highly parasitized (sink) population. Our results also showed the possibility of maintaining the high parasitism rate and the imperfection of the host's counter-adaptation against the brood parasite over a longer period. Gene flow was necessary to maintain both the acceptor genes and the non-mimetic cuckoo eggs in heavy parasitism. When the immigration rate was low (1-2%), an early expansion of the mimetic cuckoos was followed by a spread of anti-parasite defense, and consequently, the parasitism rate stabilized at a lower, but still relatively high level of about 45-60%.