A Role for the Avian Pectén Oculi in Orientation to the Sun?

Abstract

In addition to its well-accepted nutritive role, I propose a function for the pectén that is a synthesis of the views of Griffin (that it might play a role in migration), Menner (that it casts a shadow), and Barlow (that it blocks intraocular light rays scattered from the sun’s image). New measurements, supported by observations on a scale model, show that the pectén is precisely placed to produce a sharply defined shadow on the field of light scattered within the eye when the sun is imaged on one edge of the retina. The precision of the shadow-casting function becomes evident when one considers the sun’s image as the source of light, rather than direct rays from the sun itself, as Menner had done. The sharp definition of the shadow, its extended linear contour, its proximity to the image of the horizon, and its position within the area of highest retinal resolution would all facilitate precise observation of the sun.