Abstract
Differences in visual capabilities are known to reflect differences in foraging behaviour even among closely related species. Among birds, the foraging of diurnal raptors is assumed to be guided mainly by vision but their foraging tactics include both scavenging upon immobile prey and the aerial pursuit of highly mobile prey. We studied how visual capabilities differ between two diurnal raptor species of similar size: Harris's hawks, Parabuteo unicinctus, which take mobile prey, and black kites, Milvus migrans, which are primarily carrion eaters. We measured visual acuity, foveal characteristics and visual fields in both species. Visual acuity was determined using a behavioural training technique; foveal characteristics were determined using ultra-high resolution spectral-domain optical coherence tomography (OCT); and visual field parameters were determined using an ophthalmoscopic reflex technique. We found that these two raptors differ in their visual capacities. Harris's hawks have a visual acuity slightly higher than that of black kites. Among the five Harris's hawks tested, individuals with higher estimated visual acuity made more horizontal head movements before making a decision. This may reflect an increase in the use of monocular vision. Harris's hawks have two foveas (one central and one temporal), while black kites have only one central fovea and a temporal area. Black kites have a wider visual field than Harris's hawks. This may facilitate the detection of conspecifics when they are scavenging. These differences in the visual capabilities of these two raptors may reflect differences in the perceptual demands of their foraging behaviours.
Highlights
The ability of animals to detect food items and predators depends upon their sensory capabilities
While some species search for food when flying at high altitude, others search from a perch or by walking on the ground (Del Hoyo and Elliot, 1994), and we suggest that raptor species with different foraging ecology might differ in their visual fields, eye and head movements (O’Rourke et al, 2010a,b) and perhaps in their visual acuity
The visual acuity determined in the behavioural test ranged from 27.4 to 43.7 cycles deg−1 (N=5, mean 37.3±2.9 cycles deg−1) for the Harris’s hawks and from 25.9 to 32.9 cycles deg−1 (N=2, mean 29.3±3.4 cycles deg−1) for the black kites (Fig. 2, Table 2)
Summary
The ability of animals to detect food items and predators depends upon their sensory capabilities. Birds are known to differ highly in their visual capabilities (Hart, 2001; Kiltie, 2000; Martin, 2007) and these differences must result in differences in the ability of species to retrieve information from their environments. Raptors differ greatly in their foraging ecology and may differ in their visual abilities. While some species search for food when flying at high altitude, others search from a perch or by walking on the ground (Del Hoyo and Elliot, 1994), and we suggest that raptor species with different foraging ecology might differ in their visual fields, eye and head movements (O’Rourke et al, 2010a,b) and perhaps in their visual acuity. Significant differences in the vision of closely related birds that differ in their foraging behaviour have been described in other species (Guillemain et al, 2002; Martin and Piersma, 2009; Martin and Portugal, 2011)
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