![]() ![]() (b) Placement of eyes in the skull is depicted by a diagram of a horizontal section through the head and eyes of a broad-winged hawk ( Buteo platypterus). Although the optical and analyzing systems are linked together their essential characteristics can evolve independently. The optical system determines the brightness, size, and optical quality of the image, while variation in the distribution of photoreceptors and ganglion cells across the retina determines the limits on the information extracted from different regions of the image. (a) Diagrammatic cross-section through an eye of a white-tailed eagle ( Haliaeetus albicilla) highlighting the optical system, which projects an image of the world onto the retina, which is the first stage of the image analysis system. The implication of this is that variation in the vision of raptors is likely to occur within and between orders, families, genera, and even individuals: so broad generalizations may have restricted application.įigure 1. The sensory systems of birds exhibit a high degree of variation that appears subtly tuned to the perceptual challenges posed by the conduct of specific tasks, especially foraging, and hence sensory capacities are seen as a vital part of each species’ ecology (Martin, 2017). For example, palm-nut vultures Gypohierax angolensis lack large talons, and almost 90% of their diet is composed of palm-nut fruits (Thiollay, 1994). Furthermore, not all birds included in these taxa have the typical raptor features of hooked bills and curved talons, or even a diet based upon other animals. Falconiformes are, in fact, considered more closely related to parrots (Psittaciformes) and songbirds (Passeriformes), than they are to eagles and vultures (Hackett et al., 2008 Jarvis et al., 2014 Prum et al., 2015). Thus birds referred to as raptors include taxa that have been independently evolving lineages for as long as most other extant avian orders. The evolutionary origin of these orders is placed at about 60 Mya during the major period of avian radiation when present-day bird forms first became established (Jarvis et al., 2014). The term “raptor” has been applied to birds, which have separate evolutionary origins and are placed in three avian orders (Jarvis et al., 2014): Falconiformes (falcons, caracaras, etc.), Accipitriformes (eagles, vultures, hawks, etc.), and Strigiformes (owls). “Raptor” is derived from the Latin word “rapere,” which means “plunderer.” When applied to birds it is used synonymously with “bird of prey.” These labels have historically been applied to a diversity of species that were originally grouped together principally by a reference to their dietary preference for other animals and possession of hooked bills and sharply hooked claws. These and other visual properties underpin prey detection and pursuit and show how these birds’ vision is adapted to make them successful diurnal predators. As a result of the low density of rods, and the narrow and densely packed single cones in the central fovea, the visual performance of diurnal raptors drops dramatically as light levels decrease. The eyes of diurnal raptors, unlike those of most other birds, are not very sensitive to ultraviolet light, which is strongly absorbed by their cornea and lens. Moreover, pursuit-hunting and scavenging raptors also differ in configuration of visual fields, with a more extensive field of view in scavengers. Scavenging carrion eaters do not possess a temporal fovea that may indicate different needs in foraging behavior. ![]() Pursuit-hunting raptors have a second, shallower, temporal fovea that allows for sharp vision in the frontal field of view. The deep central fovea allows for the highest acuity in the lateral visual field that is probably used for detecting prey from a large distance. The highest density of single cones occurs in one or two specialized retinal regions: the foveae, where, at least in some species, rods and double cones are absent. The raptor retina has rods, double cones, and four spectral types of single cones. Some large raptors, such as wedge-tailed eagles and the Old World vultures, have visual acuities twice as high as humans and six times as high as ostriches-the animals with the largest terrestrial eyes. The high visual acuity in some raptor species is possible due to their large eyes, both in relative and absolute terms, and a high density of cone photoreceptors. Diurnal raptors (birds of the orders Accipitriformes and Falconiformes), renowned for their extraordinarily sharp eyesight, have fascinated humans for centuries. ![]()
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