Intraspecific brood parasitism: a strategy for floating females in the European starling

In many bird species, there is a floating population of females that are ex cluded from breeding because of competition for limited breeding resources. Female floaters may enhance their reproductive success by engaging in intr aspecific brood parasitism. We studied female floaters in a population of E uropean starlings, Sturnus vulgaris, in order to determine their identity a nd potential parasitic behaviour. Females were caught after being attracted to nestboxes with artificial nests during 1993-1995. None of the females w as known to have a nest of her own at capture but 47% of the females either laid an egg in the nest or carried a fully developed egg within the reprod uctive tract, indicating that they were intraspecific brood parasites. The floating females were significantly younger and smaller than breeding femal es. Of 13 females equipped with radiotransmitters and followed daily, all b ut one started a breeding attempt of their own after 3-8 days and the major ity settled as secondary females or mated with males where the original fem ale had disappeared.:This suggests that females that are unable to compete successfully for nest sites or males early in the breeding season may use i ntraspecific brood parasitism to enhance reproductive success during the pe riod that they are constrained from breeding. The importance of settling ra pidly because of a seasonal decline in reproductive success may also promot e the evolution of intraspecific brood parasitism in the starling. The rela tive reproductive success of combining egg dumping with breeding compared w ith traditional breeding will depend on the costs of delaying breeding as w ell as the probability of finding a mate later in the breeding season. (C) 1999 The Association for the Study of Animal Behaviour.