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 total perceived color 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 parasitic eggs than parasitic 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 within the avian perceivable color space remains unclear. Therefore, we built upon 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...