Whitlock, Peter L. – Spatial visual acuity in American Kestrels
Spatial visual acuities were estimated in five American kestrels (Falco sparverius) at several bird-stimulus distances: 0.2 m, 0.9 m, 1.0m, and 2.0 m. The kestrels discriminated between a uniformly gray field and black-and-white square-wave gratings of varying spatial frequencies in a forced-choice procedure. Positive food reinforcements were given for correct discrimination responses, and incorrect responses generated a 15-sec delay before the next discrimination could be made. Gray and square-wave grating stimuli were randomly exchanged between two positions from one trial to the next over a minimum of 35 trials per test session. Spatial visual acuity was estimated as the spatial frequency at which the kestrel reliably discriminated 75% of trials. At 0.2 m the spatial visual acuities of three birds (n = 3) ranged from 3.2 – 5.0 cycles/degree (c/d). Spatial visual acuities obtained at 0.9 and 1.0 m ranged from 9.0 – 20.3 c/d (n = 3) and at 2.0 m form 12.1 – 15.2 c/d (n = 2). These acuity estimates are lower than expected; however, they appear reliable given the consistency of estimates between individuals within each bird-stimulus distance, the bright stimulus illumination used, and the careful control of bird-stimulus distance. Thus, the spatial visual acuity of American kestrels in this study appears to be lower than that commonly assumed for diurnal raptors and lower than that previously reported for American kestrels. Because the bird-stimulus distance may have been incompletely controlled in earlier studies, the previously reported acuity estimates may be inflated. Anatomical studies suggest diurnal raptors have two primary visual fields, a frontal binocular field and a lateral monocular field, and that the spatial visual acuities may differ between these fields. A significant difference was found between the acuities obtained at 0.2 m and each of two longer bird-stimulus distance categories (1 m and 2 m); therefore, the results reported here are consistent with the hypothesis that the spatial visual acuities differ between fields.