Population Dynamics of Avian Brood Parasitism

The population dynamics of avian brood parasitism have received little attention in spite of mounting evidence that many host species are adversely affected by brood parasites. We develop models for the dynamics of brood parasitism and apply these models to host-parasite associations for which demographic data are available. For generalized brood parasites, which use many host species, we assume that the population dynamics of the parasite are unaffected by any one host species and that the hosts accept the eggs of parasites. The demographic consequences of parasitism for a population of a given host species can be estimated if one knows the probability of parasitism, juvenile and adult mortality rates, and the average number of offspring successfully fledged by parasitized and unparasitized females (eqs. 3, 4). Because these demographic data are difficult to obtain in the field, they are available for few species. Applications of this model using available data suggest that females of many parasitized species must renest several times within a season in order to replace themselves. Furthermore, several species are in danger of extirpation as a result of brood parasitism by cowbirds (Molothrus spp.), which are increasing in population and have expanded their ranges and come into contact with many species that have not evolved a resistance to brood parasitism. One-host-one-brood-parasite associations represent a generalization of insect host-parasitoid associations, which have been extensively studied. Using reasonable estimates of the demographic parameters for both avian parasite and host, we show that these associations have a stable equilibrium point (eqs. 6, 7), in contrast with the oscillatory behavior exhibited by the simplest (Nicholson-Bailey) host-parasitoid models. In general, the longer the host lives, the more likely the system is to be stable rather than oscillatory. A review of the scant demographic data for one-host-one-parasite associations suggests that most specialized brood parasites are too rare to have an impact on the overall host population, although some local populations may be affected. These models can be modified to include the effects of host resistance (individuals of some host species reject the eggs of parasites), the effects of spatial heterogeneity (whereby some host populations are more vulnerable to brood parasites than others), and renesting following parasitism. More complete demographic data are needed if we are to understand the population dynamics and conservation requirements of host species subject to brood parasitism.