The visual impediment of cranial ornamentation in male Chrysolophus pheasants

Sexually selected traits such as feather ornamentation of male birds can act as an impediment to movement and predator detection. Here, we report a previously undocumented example of an impediment derived from a sexually selected trait: the cranial feather ornamentation in male Chrysolophus pheasants restricting their visual field. Visual fields define the space around an animal from which visual information can be retrieved. Out of the 300 bird species studied to date, there have been no significant differences reported in the visual fields between sexes. Our findings reveal that the cranial feathers of male golden (C. pictus) and Lady Amherst's (C. amherstiae) pheasants significantly restrict their visual field relative to females and may impede their ability to gather information from the world about them. This effect is most extreme in the vertical extent, where a 30° and 40° difference is evident between the sexes of golden and Lady Amherst's pheasants, respectively. The two Chrysolophus pheasant species are the first species studied to show a difference in visual fields between sexes; this difference was absent in two closely related species also measured in this study, silver pheasants (Lophura nycthemera) and green pheasants (Phasianus versicolor).