| Abstract: | Passerine hosts of brood parasitic birds usually vary in their ability to discriminate and reject alien eggs. Two main hypotheses have been suggested to explain the persistence of acceptor individuals in species that are exploited by brood parasites. The evolutionary lag hypothesis postulates that some hosts have not yet evolved the ability to discriminate against alien eggs. Once the ability to recognize the parasitic egg has appeared by mutation, it spreads because of the selective advantage of rejection. Parasites are then selected to produce more mimetic eggs, in order to escape host discrimination, which eventually ends up in an arms race between the parasite and the host. The evolutionary equilibrium hypothesis is based on the assumption that rejection behaviour is costly in the absence of parasitism, because of recognition errors. Acceptor hosts can persist when parasitism rate fluctuates or is consistently low. Indirect evidence for costs of rejection in the absence of parasitism has been provided by Cruz and Wiley, who reported low rejection rate for a population of village weavers (Ploceus cucullatus) introduced from Africa to Hispaniola (West Indies) more than a century ago. In Africa the species is parasitized by Chrysococcyx cuckoos and shows high levels of egg discrimination. Since no brood parasite was present in Hispaniola, Cruz and Wiley suggested that rejection was selected against in the absence of parasitism due to recognition costs. Introduction of village weavers in Hispaniola, therefore, provided a unique opportunity to test the decline of an adaptation. During the past century the shiny cowbird (Molothrus bonariensis) has expanded its range-invading most of the West Indies from South America. It was first observed in Hispaniola in 1972, and it started to exploit village weavers as a host. Given that shiny cowbirds substantially reduce the reproductive success of weavers, we should expect higher rejection rates nowadays compared to those reported by Cruz and Wiley 16 years ago. In agreement with this prediction, we found a high rejection rate of cowbird model eggs (89.3%, 95% CI = 81.1 to 97.5%), moderate levels of rejection of non-mimetic weaver model eggs (67.5%, 95% CI = 52.5 to 82.5%) and rather low levels of rejection of mimetic weaver model eggs (25%, 95% CI = 4 to 46%). The rejection rate of artificial cowbird eggs has therefore increased from 13.8% (95% CI = 5 to 22.6%) to 89.3% in 16 years. To check whether this rapid increase in host resistance is compatible with a genetic microevolutionary change, we built a population dynamics model where, as an upper bound, resistance is inherited by the progeny with a probability of one. This simple model shows that observed changes of rejection rate are compatible with a genetic microevolutionary shift only under the most favourable scenario for rejecters to spread. Relaxing one or several of these assumptions (e.g. high parasitism rate, absence of rejection costs) considerably lengthens the period needed for rejecters to spread. We suggest that both genetic and learning processes might be involved in the observed changes. |
