Barn owls (Tyto alba) use accommodation as a distance cue

Summary

We have investigated the role of ocular accommodation in the distance estimation during pecking in the barn owl (Tyto alba). Owls were trained to peck at pieces of food presented on a small platform (Fig. 2) while one eye was occluded to eliminate binocular distance cues (Fig. 1). The other eye was defocussed by spectacle lenses ( −4, −2, +2 diopters) or had normal vision. Pecking behavior was analyzed by video taping the owl from the side and drawing the owls' head position off-line frame-by-frame. Additionally, during some trials the owls' accommodation behavior was recorded continuously by infrared photoretinoscopy with a second camera. This camera was aligned with the line of sight of the owl so that the refractive state could be measured just prior to pecking. We made the following observations:

• 1. The lenses resulted in errors in distance estimation which were highly predictable from the power and sign of the lenses (Fig. 3).

• 2. The lenses also changed the position of the owls' head during the period of fixation (Fig. 4). This finding together with the observation that the owls closed their eyelids during pecking suggested that ‘pecking length’ is a pre-programmed parameter in the owls' pecking behavior (Figs. 5, 6).

• 3. The photorefractions showed that the owls' accommodative effort during fixation was highly correlated with the power of the lenses. During fixation, negative lenses caused the owl to accommodate more than positive lenses (Fig. 8).

• 4. The results demonstrate that the accommodative effort necessary to fixate the target is one of the parameters by which the owls can estimate distance (Fig. 9), at least under monocular viewing conditions.

• 5. The total range of accommodation in the barn owls measured in our experiments did not exceed 6 diopters. This may be one of the reasons why the owls did not get their targets into best optical focus during pecking. The results are discussed with respect to the depth of field in owls and accommodation behavior in other birds.