Simon Potier

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: Harriss 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. Harriss hawks have a visual acuity slightly higher than that of black kites. Among the five Harriss 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. Harriss 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 Harriss 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. Summary: Differences in visual acuity, foveal specialization and visual field between Harriss hawks, Parabuteo unicinctus, and black kites, Milvus migrans, may reflect differences in the perceptual demands of their foraging behaviours.

Individual repeatability of foraging behaviour in a marine predator, the great cormorant, Phalacrocorax carbo

Intrapopulation variability, especially individual foraging specialization, has been investigated in many species. Nevertheless, the repeatability of foraging behaviour remains poorly understood. In particular, whether individuals differ in their respective degrees of repeatability still remains to be determined. Here, we estimated foraging behaviour repeatability in the great cormorant, at both the population and the individual levels, and assessed the effect of repeatability on individual foraging performance. At the population level, we found that some foraging variables were more repeatable (e.g. departure angle and trip duration) than others (e.g. time spent underwater per trip). At the individual level, we found differences in the degree of repeatability for each foraging variable, highlighting the presence of both highly flexible and highly consistent individuals in the population. The effect of repeatability on individual performance depended on the considered timescale: individual-level repeatability of time spent underwater per dive was negatively related to foraging efficiency while individual-level repeatability of time spent underwater per trip was positively related to foraging efficiency. Overall, our study demonstrates the importance of studying repeatability at the individual level and shows how both flexibility and consistency in animal behaviour shape their ability to extract energy from the environment.

Visual acuity in an opportunistic raptor, the chimango caracara (Milvago chimango)

Raptors are always considered to have an extraordinary resolving power of their eyes (high visual acuity). Nevertheless, raptors differ in their diet and foraging tactics, which could lead to large differences in visual acuity. The visual acuity of an opportunist bird of prey, the Chimango caracara (Mivalgo chimango) was estimated by operant conditioning. Three birds were trained to discriminate two stimuli, a positive grey uniform pattern and a negative grating pattern stimulus. The visual acuity range from 15.08 to 39.83 cycles/degrees. When compared to other birds, they have a higher visual acuity than non-raptorial birds, but they have the lowest visual acuity found in bird of prey so far. We discuss this result in the context of the ecology of the bird, with special focus on it is foraging tactic.

Eye Size, Fovea, and Foraging Ecology in Accipitriform Raptors

Birds with larger eyes are predicted to have higher spatial resolution because of their larger retinal image. Raptors are well known for their acute vision, mediated by their deep central fovea. Because foraging strategies may demand specific visual adaptations, eye size and fovea may differ between species with different foraging ecology. We tested whether predators (actively hunting mobile prey) and carrion eaters (eating dead prey) from the order Accipitriformes differ in eye size, foveal depth, and retinal thickness using spectral domain optical coherence tomography and comparative phylogenetic methods. We found that (1) all studied predators (except one) had a central and a temporal fovea, but all carrion eaters had only the central fovea; (2) eye size scaled with body mass both in predators and carrion eaters; (3) predators had larger eyes relative to body mass and a thicker retina at the edge of the fovea than carrion eaters, but there was no difference in the depth of the central fovea between the groups. Finally, we found that (4) larger eyes generally had a deeper central fovea. These results suggest that the visual system of raptors within the order Accipitriformes may be highly adapted to the foraging strategy, except for the foveal depth, which seems mostly dependent upon the eye size.

Visual field shape and foraging ecology in diurnal raptors

ABSTRACT Birds, particularly raptors, are believed to forage primarily using visual cues. However, raptor foraging tactics are highly diverse – from chasing mobile prey to scavenging – which may reflect adaptations of their visual systems. To investigate this, we studied the visual field configuration of 15 species of diurnal Accipitriformes that differ in such tactics, first focusing on the binocular field and blind area by using a single-traits approach, and then exploring the shape of the binocular field with a morphometric approach. While the maximum binocular field width did not differ between species with different foraging tactics, the overall shape of their binocular fields did. In particular, raptors chasing terrestrial prey (ground predators) had a more protruding binocular field and a wider blind area above the head than did raptors chasing aerial or aquatic prey and obligate scavengers. Ground predators that forage on mammals from above have a wide but short bill – which increases ingestion rate – and a large suborbital ridge to avoid sun glare. This may explain the protruding binocular field and the wide blind area above the head. By contrast, species from the two other groups have long but narrow bills used to pluck, flake or tear food and may need large visual coverage (and reduced suborbital ridges) to increase their foraging efficiency (e.g. using large visual coverage to follow the escaping prey in three dimensions or detect conspecifics). We propose that binocular field shape is associated with bill and suborbital ridge shape and, ultimately, foraging strategies. Highlighted Article: Raptors from Accipitriformes family differ in the shape of their binocular field according to their foraging tactics.

High resolution of colour vision, but low contrast sensitivity in a diurnal raptor

Animals are thought to use achromatic signals to detect small (or distant) objects and chromatic signals for large (or nearby) objects. While the spatial resolution of the achromatic channel has been widely studied, the spatial resolution of the chromatic channel has rarely been estimated. Using an operant conditioning method, we determined (i) the achromatic contrast sensitivity function and (ii) the spatial resolution of the chromatic channel of a diurnal raptor, the Harriss hawk Parabuteo unicinctus. The maximal spatial resolution for achromatic gratings was 62.3 c deg−1, but the contrast sensitivity was relatively low (10.8–12.7). The spatial resolution for isoluminant red-green gratings was 21.6 c deg−1—lower than that of the achromatic channel, but the highest found in the animal kingdom to date. Our study reveals that Harriss hawks have high spatial resolving power for both achromatic and chromatic vision, suggesting the importance of colour vision for foraging. By contrast, similar to other bird species, Harriss hawks have low contrast sensitivity possibly suggesting a trade-off with chromatic sensitivity. The result is interesting in the light of the recent finding that double cones—thought to mediate high-resolution vision in birds—are absent in the central fovea of raptors.

Sight or smell: which senses do scavenging raptors use to find food?

Raptors are usually considered to be mainly visually dependent, and the use of other sensory modalities has rarely been studied in these birds. Here, we investigated experimentally which senses (vision and/or olfaction) Turkey vultures (Cathartes aura) and Southern caracaras (Caracara plancus) use to find hidden food. First, two identical stainless-steel perforated balls, one containing a putrefied piece of meat and the other an odorless control, were presented to birds in binary choice experiments. Both species interacted more with the smelling ball than with the control, suggesting that they were attracted by the odor of the hidden meat. In a second experiment, individuals were accustomed to eat in one specifically colored ball (blue or green). In the test phase, the meat was hidden in the opposite color with respect to the one each bird had become accustomed to. Vultures still interacted more with the smelly ball disregarding the color, while caracaras interacted equally with the two balls. The prevalence of olfaction in Turkey vultures may partly explain why they are the first raptors to find carcasses in tropical forests. In contrast, caracaras forage on the ground opportunistically, a strategy where both olfaction and sight may be involved. Our experiments suggest that both species are able to use olfactory cues for foraging. However, olfaction could be the predominant sense in Turkey vultures while olfaction and sight could play an equivalent role in Southern caracaras.