Some observations on the visual optics of kingfishers (Aves, Coraciformes, Alcedinidae)

Summary

We studied visual optics using ophthalmoscopy in six species of coraciform birds, five species from the family Alcedinidae (kingfishers) and one from the family Meropidae (bee-eaters). All six species had large angular separations between the two foveae of one eye (angle α); angle α was greater than 40° in all cases, the largest separation so far reported for any group of vertebrates. In all kingfishers, but not in the bee-eater, the plane containing the projections of both foveae in one eye (angle ϑ) was rotated from the horizontal plane so that the projection of the monocular fovea was lower than the projection of the binocular fovea.

Retinal ganglion cell isodensity maps were obtained for the sacred kingfisher (Halcyon sancta) and laughing kookaburra (Dacelo gigas). These maps were constructed in the usual way for the peripheral regions of the retina. For the high-density, multi-layered, central region of the retinal ganglion cell layer, we used a combined retinal wholemount-cross-sectional technique. It was discovered that the ‘horizontal streak’, formed by the elliptically shaped contours of retinal ganglion cell isodensity, deviated inferiorly at its nasal extent from the line of the foveae. This deviation had the same sign, and slightly greater magnitude, as the rotation (angle ϑ) observed ophthalmoscopically when the eyes were in their primary position.

Our new observations provide new insights into the functional significance of the bifoveate visual organization. In particular, the relationship between angle α and angle ϑ suggests one strategy which could be used by kingfishers to maximize their visual capabilities when hunting from a perch above the substrate.