Binocular vision and nocturnal activity in Oilbirds (Steatornis caripensis) and Pauraques (Nyctidromus albicollis): Caprimulgiformes

Visual field characteristics were measured using an ophthalmoscopic reflex technique in Oilbirds (Steatornis caripensis) and Pauraques (Nyctidromus albicollis). Despite these birds' apparent frontal-eyed appearance, their functional binocular fields are relatively small with a maximum width of 38 degrees and vertical extent of 100 degrees in Oilbirds, and 25 degrees x 110 degrees in Pauraques. Maximum width occurred 10 degrees above the horizontal in both species. In Pauraques, the blind area behind the head equals 89 degrees and the monocular retinal field equals 148 degrees. The apparent (optical) binocular field is 65 degrees wide, compared with the functional (retinal) field approx. 25 degrees. The binocular fields in these highly nocturnal birds are similar to those of strongly diurnal species. This supports the hypothesis that binocularity in birds is concerned with the contra lateral projection of each eye rather than achieving disparate images of the same object from two eyes. These contra lateral projections result in a section of the field in each eye that provides a symmetrically expanding optic flow field, rather than a basis for the extraction of higher order information from disparate images.