Interspecific egg rejection as ecological collateral damage from selection driven by conspecific brood parasitism

Distinguishing between interspecific and intraspecific coevolution as the selective driver of traits can be difficult in some taxa. A previous study of an avian obligate brood parasite, the black-headed duck, Heteronetta atricapilla, suggested that egg rejection by its two main hosts (two species of coot) is an incidental by-product of selection from conspecific brood parasitism within the hosts, not selection imposed by the interspecific parasite. However, although both species of coot can recognize and reject eggs of conspecific brood parasites, which closely resemble their own, they paradoxically also accept a moderate fraction of duck eggs (40-60%), which differ strikingly in shape and colour from their own eggs. Here we test the key assumption of the incidental by-product hypothesis that natural selection for egg recognition solely from conspecific brood parasitism can result in intermediate levels of rejection of nonmimetic eggs. We repeated the same egg rejection experiments conducted previously with the two Argentine hosts in a third closely related species that experiences only conspecific brood parasitism, the American coot, Fulica americana. These experiments yielded the same intermediate rejection rates for nonmimetic duck eggs. Our results confirm that selection from conspecific brood parasitism can lead to counterintuitive intermediate rejection rates of nonmimetic interspecific eggs and further support the suggestion that selection from antagonism within species can incidentally affect interactions between species.