Visual fields, foraging and collision vulnerability in gulls (Laridae)

Abstract

Wide variation in visual field configuration has been recorded among avian species and it is hypothesized that this variation is driven primarily by foraging ecology and predator detection. It has also been shown that visual field configurations can render some species more vulnerable to collisions with human artefacts that extend into open airspace, such as power lines and wind turbines. Visual fields have three main components: the monocular fields describe the extent of the world seen by each eye, the binocular field describes the region where the monocular fields overlap, and the blind area describes the region in which no vision is provided. Among birds, the topography of the binocular field, and the extent and position of the blind area, show considerable interspecific variation. Although Laridae (gulls, terns, skimmers) are a large and cosmopolitan taxon, visual field characteristics of only one species, Black Skimmer Rynchops niger, have been determined. However, skimmers are distinct from other Laridae species because they use a specialized foraging technique based upon tactile cues. We determined visual fields in three species of gulls (European Herring Gulls Larus argentatus, Lesser Black-backed Gulls Larus fuscus, Black-legged Kittiwakes Rissa tridactyla), and found that they show the key characteristics associated with visually guided foraging. However, the binocular field does not extend through the full height of the frontal field. This results in a blind sector, which can project in the direction of flight when gulls pitch their heads sufficiently far forwards to visually search the surface below. This could render gulls vulnerable to collisions with anthropogenic structures (power lines, wind turbines) that extend into the open airspace. Photographs show that gulls in level flight do pitch their heads forward sufficiently to render them almost blind in the direction of travel, and further work on the head positions adopted by gulls in flight are recommended. The visual field of skimmers differs markedly from those of gulls. Their binocular field topography is interpreted as functioning in the control of bill position when skimming (flying just above the water surface with the elongated, blade-like, rhamphotheca of the mandible extending through the water surface). Skimmers also have a blind area, which projects forwards in the direction of travel when skimming. This can be associated with the vulnerability of skimmers to collisions with objects that extend just above the water surface.