Visual abilities in two raptors with different ecology

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 spectraldomain 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.

Eye Size, Fovea, and Foraging Ecology in Accipitriform Raptors

“…During the study period, the birds were housed in their aviaries and moved to a dark room for the measurements. Additionally, we included data from Ruggeri et al [2010] and Potier et al [2016b] in our analyses. In total, foveal SD-OCT images of 16 species were used in this study.…”

“…This glare reduces contrast in the retinal image [Koch, 1989;Aslam et al, 2007] and thus counteracts the potentially high spatial resolution of large eyes more than of small eyes with low resolution [Land and Nilsson, 2012;Martin, 2007Martin, , 2014. Accessory structures such as the suborbital ridges, mainly found in largeeyed species, might help to reduce the "disability glare" by shading the eye from direct sunlight [Martin and Katzir, 2000]; however, they also reduce visual field size above the head [Martin and Katzir, 2000;Potier et al, 2016b]. This reduction in the visual field may be especially costly for carrion eaters, as they need to scan the horizon for conspecifics while foraging [Houston, 1974].…”

“…Raptors from the orders Accipitriformes and Falconiformes have relatively larger eyes than other birds [Howland, et al, 2004], with eye masses 1.4 times greater than an average bird of the same body mass [Brooke et al, 1999], and the most acute vision of all birds known to date [Fischer, 1969;Hirsch, 1982;Reymond, 1985Reymond, , 1987McIsaac, 2001;Potier et al, 2016aPotier et al, , 2016b. However, the need for acute vision might differ between raptors depending on their foraging ecology; some of them are predators hunting moving prey of various sizes on the ground or in the air, while others are mainly or exclusively carrion eaters, usually searching for large carcasses or aggregations of other scavenging species [Del Hoyo and Elliot, 1994].…”

Comparison of functional and anatomical estimations of visual acuity in two species of coral reef fish

“…If we applied the threshold criterion of 65% used by Champ et al (2013) to our study, behavioural acuity would be increased to 1.44 and 1.76 cycles deg −1 from 1.29 and 1.69 cycles deg −1 for P. amboinensis and P. fuscus, respectively. We followed recently published studies in which a threshold of 72.5% was used (Lind and Kelber, 2011;Potier et al, 2016). It is important to remember that by increasing the number of sessions, any accuracy level different from chance can become statistically significant, and that therefore the threshold also depends on the number of trials and sessions carried out.…”

“…It is important to remember that by increasing the number of sessions, any accuracy level different from chance can become statistically significant, and that therefore the threshold also depends on the number of trials and sessions carried out. We used 40 trials per spatial frequency, which was identical to Lind and Kelber (2011) but higher than Potier et al (2016).…”