A RISE IN NEST PREDATION ENHANCES THE FREQUENCY OF INTRASPECIFIC BROOD PARASITISM IN A MOORHEN POPULATION

1. A population of moorhens (Gallinula chloropus, Linnaeus) at Peakirk , Cambridgeshire, UK, was studied during three breeding seasons betwee n 1991 and 1993. In the first 2 years, when rodent predators were cont rolled, the rate of nest predation was relatively low (37% and 36% of nests depredated, respectively), and the rate of brood parasitism was also low (10% and 13% of nests, respectively). However, in 1993, when rodents were not controlled, the rate of nest predation increased to 6 5%, and correspondingly, the rate of brood parasitism nearly doubled t o 21% of nests. 2. The total number of eggs laid and the number of lay ing females did not differ greatly between the three years, but the pr oportion of eggs laid parasitically increased from 4% in 1991 and 1992 , to 9% in 1993. 3. The increase in the rate of parasitism in 1993 was due primarily to a large number of females engaging in parasitic layi ng bouts immediately prior to nesting some time after clutch loss, and to a lesser extent, to females laying parasitically immediately after partial clutch loss. 4. Female moorhens tended to show local synchron y in clutch initiation. In 1993, females in neighbouring territories w ere significantly more likely to begin laying within a few days of one another than females in non-neighbouring territories. 5. Hosts were a lmost exclusively the immediate neighbours of the brood parasites. The high rate of nest predation observed in 1993 directly influenced the rate of brood parasitism because it increased the level of synchrony i n laying between neighbours, giving them more opportunities to lay par asitically. 6. Together with other studies of brood parasitism, these results indicate that both low nest success rates and high nest availa bility can produce high rates of brood parasitism: the first, out of c onstraint when conditions are unfavourable, and the second as a bonus when conditions are particularly favourable.