Intensive nest predation by crabs produces source-sink dynamics in hosts and parasites

Nest predation is a common cause of reproductive failure. Here, we show for the first time unusually high rates of nest predation on eggs and chicks in nests of the Oriental Reed Warbler Acrocephalus orientalis and the brood parasitic Common Cuckoo Cuculus canorus caused by mudflat crabs Chiromantes dehaani. Crabs and the Common Cuckoo greatly reduce reproductive success of hosts at a site in coastal habitat, but only Common Cuckoos did so at an inland habitat. Thus, the parasitic Common Cuckoo suffered an indirect high predation cost by using bird species affected by crabs as hosts. The difference in the relationship between mudflat crabs, cuckoos and hosts between these two habitats can be explained by the source-sink dynamics of an ecological trap. The coastal habitats constitute sink populations for both host and brood parasite, while inland habitats with cuckoos only constitute sink populations for hosts. Because the population densities of both hosts and parasites are similar in the two kinds of habitats, these similarities might be maintained by immigration from source populations such as those without mudflat crabs and Common Cuckoos, respectively.