Nest desertion and cowbird parasitism: evidence for evolved responses and evolutionary lag

Nest desertion with subsequent renesting is a frequently cited response to parasitism by the brown-headed cowbird, Molothrus ater, yet the role of des ertion as an antiparasite defence is widely debated. To determine whether d esertion represents an evolutionary response to brown-headed cowbird parasi tism, we searched the primary literature, yielding data on the desertion fr equencies of 60 host populations from 35 species. Species were categorized according to three habitat types (forest, intermediate and nonforest). Beca use cowbirds prefer open habitat and rarely penetrate deeply into forests, nonforest species have long been exposed to widespread cowbird parasitism, whereas forest species have not. However, due to increased forest fragmenta tion, forest species are being increasingly exposed to extensive parasitism . The frequency of desertion of parasitized nests was significantly higher in nonforest than forest species, suggesting that the latter experience evo lutionary lag. We also considered whether desertion is affected by predatio n frequency, degree of Current or recent sympatry with cowbirds, parasitism frequency, length of host laying season, phylogenetic relationships, and p otential cost of cowbird parasitism. None of these variables created biases that could account for the observed difference in desertion frequencies of nonforest and forest species. However, species that incur large costs when parasitized had higher desertion rates among nonforest species but not amo ng forest species. These results indicate that increased nest desertion is an evolved response to cowbird parasitism, as one would otherwise expect no relationship between desertion frequency and: the costs and length of expo sure to cowbird parasitism. Although nearly all hosts have eggs easily dist inguished from cowbird eggs, few or none desert in response to cowbird eggs . Instead, desertion may be a response to adult cowbirds. The scarcity of s pecies that desert in response to cowbird eggs suggests that egg recognitio n is more difficult to evolve than heightened desertion tendencies and that egg recognition quickly leads to ejection behaviour once it does develop. (C) 2000 The Association for the Study of Animal Behaviour.