A behavioral study of refraction, corneal curvature, and accommodation in raptor eyes

Abstract

Since there is much speculation in the literature regarding the accommodative abilities of raptors, we undertook a behavioral study of accommodation in the five families of raptors. The resting refractive state and amplitude of accommodation were measured using infrared video photorefraction in a variety of wild-caught and captive-bred raptors. The resting corneal curvature and the extent of changes in corneal curvature during accommodation (corneal accommodation) were measured using video keratometry. External ocular and head dimensions were measured with calipers to look for correlates with accommodative amplitude. In general, all eyes examined were of high optical quality and relatively free of aberrations. No significant refractive errors were recorded in any of the birds examined (< 1.0 diopters (D)). In general, significant amplitudes of accommodation were measured in the hawks (up to 25.0 D), but little accommodation was seen in the owls. Corneal accommodation ranging from 2.8 to 6.2 D in magnitude was recorded in a number of the hawks. These differences in the accommodative behaviors of the owls and hawks are discussed with respect to the limitations of the behavioral techniques used, differences in the degree of cooperation of the different species, and real differences in the accommodative abilities of the owls and hawks in relation to their accommodative needs, such as when feeding.