Rethinking visual supernormal stimuli in cuckoos: visual modeling of host and parasite signals

Some parasitic cuckoo chicks display a vivid-colored gape to their host parents when begging for food. Their mouth color was once regarded as a supernormal stimulus, yet owing to a lack of experimental support, the idea has fallen out of favor. However, previous experiments were conducted without considering the vision of avian receivers. We compared the color and visibility of begging signals between chicks of a brood parasite, the Horsfield's hawk-cuckoo (Cuculus fugax), and that of its host, the red-flanked bluetail (Tarsiger cyanurus), considering bird vision. We investigated the mouth palate of host and parasite chicks, and a gape-colored skin patch on the wing of parasite chicks, which has previously been demonstrated to induce host parental feeding. We found that, in terms of stimulation of the birds' photoreceptors and visual discrimination thresholds, visibility of parasite signals, particularly of the wing-patch, was quantitatively greater than that of the host chick signal. Meanwhile, host and parasite signals were qualitatively different in the hue, which was driven mostly by greater ultraviolet reflectance of the parasite signals. Evidence from previous studies indicates that the visual attributes of the parasite signals may induce parental provisioning, suggesting that signal exaggeration of the parasite has evolved to stimulate hosts effectively in the dark nest environment. Overall, our results suggest that the color of hawk--cuckoo chicks' signaling traits can work as a supernormal stimulus, although host parental responses to exaggerated stimuli need to be tested experimentally. Copyright 2011, Oxford University Press.