Mechanisms of egg recognition in brown-headed cowbird hosts: the role of ultraviolet reflectance

One of the most effective adaptations to counter avian brood parasitism is rejection of the parasitic egg, yet relatively few hosts reject eggs of the brown-headed cowbird, Molothrus ater. Studies have demonstrated that ultraviolet (UV; 300-400 nm) reflectance of eggs plays a role in egg rejection by hosts of parasitic cuckoos Cuculus spp. Only two studies have experimentally tested whether a cowbird host utilizes UV light when making egg rejection decisions, and those studies found no evidence that UV light was a significant factor. We experimentally blocked the UV reflectance of one host egg in the clutches of three rejecter species: brown thrasher, Toxostoma rufum, American robin, Turdus migratorius, and grey catbird, Dumetella carolinensis, to determine whether they utilize UV reflectance when rejecting eggs. We also measured the UV reflectance of each species' eggs. All host species rejected more of their own UV-blocked eggs than they did control eggs, but brown thrashers were significantly more likely to reject their own UV-blocked eggs than were American robins and grey catbirds. Brown thrasher eggs also reflected significantly more UV light than both American robin and grey catbird eggs. Our results coupled with those from similar studies suggest that hosts with brighter UV-reflecting eggs should be more likely to reject UV-blocked eggs than hosts with duller UV-reflecting eggs. This is the first study to demonstrate that UV reflectance is a parameter used by hosts of the brown-headed cowbird when rejecting eggs and further increases our understanding of the mechanisms of egg recognition in brood-parasitic hosts.