Host Responses to Foreign Eggs across the Avian Visual Color Space

Despite extensive research on the sensory and cognitive processes of host rejection of avian brood parasites' eggs, the underlying perceptual and cognitive mechanisms are not sufficiently understood. Historically, most studies of host egg discrimination assumed that hosts rejected a parasite's egg from their nest based on the perceived color and pattern differences between the parasite's egg and their own. A recent study used a continuous range of parasitic egg colors and discovered that hosts were more likely to reject browner foreign eggs than foreign eggs that were more blue green, even when their absolute perceived color differences from the hosts' own egg colors were similar. However, the extent of these color biases across the avian perceivable color space remains unclear. Therefore, we built on this previous study by testing European blackbirds' (Turdus merula) responses to model eggs spanning an unprecedented volume of the avian color space. We found that host decisions depended on avian perceived hue, saturation, and luminance of the parasite's egg; hosts generally accepted eggs that were bluer or more blue green and more often rejected eggs that were less saturated or darker. We suggest that future studies investigate the underlying mechanisms of foreign egg discrimination in other host lineages to determine the prevalence and phylogenetic conservation of such perceptual biases among birds.