Intraspecific brood parasitism in the moorhen: parentage and parasite-host relationships determined by DNA fingerprinting

Parasitic female moorhens (Gallinula chloropus) lay from one to six eggs in the nests of conspecific neighbours. DNA fingerprinting was used to show that parasitic eggs could be correctly identified when they appeared in addition to or outside the host’s laying sequence. Moorhen hosts accept all parasitic eggs laid after the 2nd day of their laying period. To understand why moorhen hosts tolerate parasitic eggs, we tested two hypotheses. (1) The quasi-parasitism hypothesis: females lay their eggs in the evening when the host males are normally in attendance at the nest, so host males may allow parasitic females to lay in their nests in exchange for fertilizing their eggs. However, DNA fingerprinting showed that all the parasitic eggs were sired by the parasites’ mates. Parasitic moorhens frequently continue laying a clutch in their own nest, without a break in the laying sequence after a parasitic laying bout. The eggs laid by brood parasites in their own nests were also sired by their own mates. Therefore this hypothesis was rejected. (2) The kin selection hypothesis: if one or both members of the host pair are close relatives of the parasite, the costs of rearing parasitic chicks will be to some degree offset by inclusive fitness benefits. We examined the genetic relationships between parasites and their hosts using DNA fingerprinting and genealogical data. Natal philopatry by both sexes was relatively common in this population, and the probability that a neighbour of either sex was a first-order relative (parent-offspring) was calculated as 0.18. Although first-order relatives were not preferentially chosen as hosts over individuals that were not first-order relatives, even through random host selection there is almost a one-in-five chance that brood parasites in this population are closely related to their hosts. This may facilitate host tolerance of parasitic eggs. Other hypotheses are also discussed. Received: 3 February 1995/Accepted after revision: 27 August 1995