Barn owls have symmetrical accommodation in both eyes, but independent pupillary responses to light

We have studied accommodation behaviour in the barn owl (Tyto alba). By defocussing one eye with various spectacle lenses and recording the refractive state in both eyes continuously during pecking, we found that the owls' accommodation was symmetrical in both eyes, with no regard to the power of the lens used. Even with no visual input to one eye, the amount of accommodation was always identical in both eyes. On the other hand, pupillary responses to light were independent. This finding differs from an earlier observation in the chicken, where both accommodation and pupillary responses were found to be independent. The result is discussed with regard to current knowledge on the central pathways for control of accommodation and pupillary responses in birds.