The eye of a passeriform bird, the European starling (Sturnus vulgaris): eye movement amplitude, visual fields and schematic optics

Abstract

The European starling,Sturnus vulgaris, is a diurnal, ground feeding bird of the order Passeriformes. Aspects of its visual fields, eye movements and the optical structure of its eye are described. 1. The magnitude of eye movements as a function of elevation in the median sagittal plane of the head was measured (Fig. 1). Maximum eye movement amplitude (32°) occurs in a plane which passes 50°–60° below the closed bill while the minimum amplitude (11°) occurs in a plane approximately orthogonal to this. 2. The ways in which eye movements can alter the binocular and cyclopean retinal visual fields are described (Figs. 3 and 5). When the eyes are fully converged maximum binocular overlap equals 43° and this occurs approximately 40° below the bill. The bill intrudes into the visual field and when the bill is fully open, maximum binocular field width occurs between the mandibles. The eyes can also be swung backwards and upwards so that the margins of the monocular fields just coincide behind the head to give extensive visual coverage of the celestial hemisphere. When the eyes are in this position the frontal binocular field is almost abolished. 3. The monocular retinal field is asymmetric about the optic axis and this can be attributed to asymmetry in the gross structure of the eye rather than asymmetry in the optical system (Figs. 4 and 9). From purely optical considerations the starling could have a maximum binocular field width twice that actually found. 4. A schematic model of the starling eye optical system (Fig. 4 and Table 2) shows that optically this ‘flat-diurnal’ eye is a small scaled version of the larger ‘tubular-nocturnal’ eye of an owl (Figs. 6 and 7). In addition the calculated optical fields of these two eyes are very similar (Fig. 8), and these eyes differ in their maximum retinal image brightness by only 0.33 log10 (Table 4). However, although the rat and starling eyes are of similar total dioptric power they are found to be of quite different optical design (Fig. 7). It is concluded that no particular ‘diurnal’ feature can be discerned in the optical structure of the starling eye. 5. The starling feeds mainly on the ground where it is known to employ vision to locate prey at short range. However, when foraging it is vulnerable to aerial predators. The visual problems which this presents are considered. How the bird's visual fields and eye movements may cope with these problems are discussed. There is evidence that the starling eye may embody a ‘ramp’ retina as a static accommodatory device which could facilitate the simultaneous detection of both close prey and distant predators (Fig. 9 and Table 5).