Deceptive plumage signals in birds: manipulation of predators or prey?

Several species of raptors have two ocelli (eye-like patches) in the back of their head, giving them the appearance of a false face, although this trait has rarely been reported. According to our observations, these markings may be wide- spread in the family Falconidae, some Accipiter hawks, and some owls (e.g. Glaucidium, and some Athene, Aegolius, and Surnia). In this study, we outline general classes of hypotheses that may account for ocelli on the heads of rap- tors. The most frequently evoked general hypothesis is that ocelli offer protection against attackers. For example, a predator may abort the effort, be deflected by the ocelli, or be warned that the bearer will retaliate if attacked. We propose two alternatives. Most raptors with ocelli typically include a large proportion of passerine birds in their diets. The false faces may have evolved to aid in the hunting of small birds by actually provoking, or manipulating the nature of, a mobbing response. The benefit of doing so may either be immediate because there are numerous accounts of mobbers being killed, or the benefit may be postponed if the predator is using mobbing as a means of eval- uating hunting prospects in a given area. An analysis of the pygmy-owls of the genus Glaucidium indicates that spe- cies displaying ocelli in the nape tend to have a high proportion of small birds in their diets and live in open habitats, whereas the opposite is true for species without ocelli. Pygmy-owls with ocelli are also considerably smaller and, col- lectively, these findings are most consistent with false faces being a conspicuous visual signal to deceive mobbing birds so they can be preyed upon.