Ecological and morphological correlates of visual acuity in birds

Birds use their visual systems for important tasks, such as foraging and predator detection, that require them to resolve an image. However, visual acuity (the ability to perceive spatial detail) varies by two orders of magnitude across birds. Prior studies indicate that eye size and aspects of a species' ecology may drive variation in acuity, but these studies have been restricted to small numbers of species. We used a literature review to gather data on acuity measured either behaviorally or anatomically for 94 species from 38 families. We then examined how acuity varies in relation to (1) eye size, (2) habitat spatial complexity, (3) habitat light level, (4) diet composition, (5) prey mobility and (6) foraging mode. A phylogenetically controlled model including all of the above factors as predictors indicated that eye size and foraging mode are significant predictors of acuity. Examining each ecological variable in turn revealed that acuity is higher in species whose diet comprises vertebrates or scavenged food and whose foraging modes require resolving prey from farther away. Additionally, species that live in spatially complex, vegetative habitats have lower acuity than expected for their eye sizes. Together, our results suggest that the need to detect important objects from far away - such as predators for species that live in open habitats, and food items for species that forage on vertebrate and scavenged prey - has likely been a key driver of higher acuity in some species, helping us to elucidate how visual capabilities may be adapted to an animal's visual needs.

Light wavelength and pulsing frequency affect avoidance responses of Canada geese

Collisions between birds and aircraft cause bird mortality, economic damage, and aviation safety hazards. One proposed solution to increasing the distance at which birds detect and move away from an approaching aircraft, ultimately mitigating the probability of collision, is through onboard lighting systems. Lights in vehicles have been shown to lead to earlier reactions in some bird species but they could also generate attraction, potentially increasing the probability of collision. Using information on the visual system of the Canada goose (Branta canadensis), we developed light stimuli of high chromatic contrast to their eyes. We then conducted a controlled behavioral experiment (i.e., single-choice test) to assess the avoidance or attraction responses of Canada geese to LED lights of different wavelengths (blue, 483 nm; red, 631 nm) and pulsing frequencies (steady, pulsing at 2 Hz). Overall, Canada geese tended to avoid the blue light and move towards the red light; however, these responses depended heavily on light exposure order. At the beginning of the experiment, geese tended to avoid the red light. After further exposure the birds developed an attraction to the red light, consistent with the mere exposure effect. The response to the blue light generally followed a U-shape relationship (avoidance, attraction, avoidance) with increasing number of exposures, again consistent with the mere exposure effect, but followed by the satiation effect. Lights pulsing at 2 Hz enhanced avoidance responses under high ambient light conditions; whereas steady lights enhanced avoidance responses under dim ambient light conditions. Our results have implications for the design of lighting systems aimed at mitigating collisions between birds and human objects. LED lights in the blue portion of the spectrum are good candidates for deterrents and lights in the red portion of the spectrum may be counterproductive given the attraction effects with increasing exposure. Additionally, consideration should be given to systems that automatically modify pulsing of the light depending on ambient light intensity to enhance avoidance.

Dog barks influence the physiological stress and behavior of a wild primate

Non-lethal impacts of dogs on primates have seldom been assessed. We used an experimental approach to determine if mantled howler monkeys (Alouatta palliata) perceive dog barks as an aversive stimulus and thus display physiological and behavioral responses toward simulated barks. For one year (1754 h of observations) we studied 16 adult males belonging to five groups in Los Tuxtlas (Mexico), and recorded the occurrence of naturally occurring dog barks, their sound pressure level (SPL), and the behavioral responses of howler monkeys to barks. We then exposed males to bark playbacks at two SPL treatments, 40 and 80 dB in a total of 50 experiments. We assayed glucocorticoid metabolite concentrations in fecal samples (fGCM) as a marker of the physiological stress response of males. We also recorded the duration of vigilance, vocalizations, and flight in relation to playbacks. Naturally occurring barks were frequent and usually elicited behavioral responses by males. fGCM concentrations increased after bark playbacks and with stimuli intensity. Time spent vigilant increased following playbacks independently of stimuli intensity but both vocalizations and flight were linked to stimuli intensity: vocalizations were the longest after barks played-back at 80 dB, but males spent more time fleeing in response to 40 dB bark playbacks. These results provide evidence that dog barks are pervasive in the habitat of mantled howler monkeys living at Los Tuxtlas and disturb males, both physiologically and behaviorally. Although the potential costs of physiological and behavioral responses could not be determined, there is sufficient evidence to assume that they do have negative impacts on individuals. Therefore, our study provides avenues for future research on dog-wildlife interactions and valuable information for the design of conservation actions aimed at mitigating the impact of dogs on mantled howler monkeys.

Behavioral Responses to Body Position in Bees: The Interaction of Apis mellifera and Lithurgus littoralis in Prickly Pear Flowers

The behavior of bees is modulated by the presence of other bees and potentially by the visual information transmitted by the different body positions of bees while visiting flowers. We tested whether bee body position promoted the attraction and/or antagonistic behavior of con- and hetero-specific bees that interacted on prickly pear flowers of Opuntia huajuapensis. To test this, we placed dummy model bees of Apis mellifera and the native Lithurgus littoralis in flowers in three common body positions: alert, feeding, and horizontal. The results showed that dummy model bees in feeding and horizontal body positions attracted A. mellifera bees, while the alert position attracted native male L. littoralis. Male L. littoralis bees spent more time attacking model bees in horizontal and alert positions. The position of dummy bees also influenced response times. Bees of A. mellifera responded fastest to L. littoralis in the alert and horizontal position, male L. littoralis responded fastest to con-specific bees in the alert and feeding position, and female L. littoralis responded fastest to A. mellifera bees in the feeding position. A. mellifera reacted fastest to their con-specific bees in the alert and horizontal body positions. Our results demonstrate, for the first time in bees, that the position of individuals on a floral resource provides important visual information that modulates bee behavior, and illuminates aspects that likely have implications for bees in access to floral resources.

Social information-mediated population dynamics in non-grouping prey

Abstract

Inadvertent social information (ISI) use, i.e., the exploitation of social cues including the presence and behaviour of others, has been predicted to mediate population-level processes even in the absence of cohesive grouping. However, we know little about how such effects may arise when the prey population lacks social structure beyond the spatiotemporal autocorrelation originating from the random movement of individuals. In this study, we built an individual-based model where predator avoidance behaviour could spread among randomly moving prey through the network of nearby observers. We qualitatively assessed how ISI use may affect prey population size when cue detection was associated with different probabilities and fitness costs, and characterised the structural properties of the emerging detection networks that would provide pathways for information spread in prey. We found that ISI use was among the most influential model parameters affecting prey abundance and increased equilibrium population sizes in most examined scenarios. Moreover, it could substantially contribute to population survival under high predation pressure, but this effect strongly depended on the level of predator detection ability. When prey exploited social cues in the presence of high predation risk, the observed detection networks consisted of a large number of connected components with small sizes and small ego networks; this resulted in efficient information spread among connected individuals in the detection networks. Our study provides hypothetical mechanisms about how temporary local densities may allow information diffusion about predation threats among conspecifics and facilitate population stability and persistence in non-grouping animals.

Significance statement

The exploitation of inadvertently produced social cues may not only modify individual behaviour but also fundamentally influence population dynamics and species interactions. Using an individual-based model, we investigated how the detection and spread of adaptive antipredator behaviour may cascade to changes in the demographic performance of randomly moving (i.e., non-grouping) prey. We found that social information use contributed to population stability and persistence by reducing predation-related per capita mortality and raising equilibrium population sizes when predator detection ability reached a sufficient level. We also showed that temporary detection networks had structural properties that allowed efficient information spread among prey under high predation pressure. Our work represents a general modelling approach that could be adapted to specific predator-prey systems and scrutinise how temporary local densities allow dynamic information diffusion about predation threats and facilitate population stability in non-grouping animals.

How visual system configuration can play a role in individual recognition: a visual modeling study

Many species rely on individual recognition (i.e., the use of individual signals to identify and remember a conspecific) to tune their social interactions. However, little is known about how the configuration of the sensory system may affect the perception of individual recognition signals over space. Utilizing a visual modeling approach, we quantified (1) the threshold distance between the receiver and the signaler at which individual recognition can no longer accurately occur, and (2) the regions of the head most likely to contain the individual recognition signals. We used chickens (Gallus gallus) as our study species, as they use visual individual recognition and additionally have a well-studied visual system. We took pictures of different individuals and followed a visual modeling approach considering color vision, visual acuity, and pattern processing of the receiver. We found that distance degrades the quality of information in potential individual recognition signals. We estimated that the neighbor distance at which a receiver may have difficulty recognizing a conspecific was between 0.25 and 0.30 m in chickens, which may be related to a decrease in available features of the potential signal. This signal perception threshold closely matches the recognition distance predicted by previous behavioral approaches. Additionally, we found that certain regions of the head (beak, cheek, comb, eye) may be good candidates for individual recognition signals. Overall, our findings support that recognition in chickens occurs at short distances due to constraints imposed by their visual system, which can affect the costs and benefits associated with social spacing in groups.

Buoys with looming eyes deter seaducks and could potentially reduce seabird bycatch in gillnets

Bycatch of seabirds in gillnet fisheries is a global conservation issue with an estimated 400 000 seabirds killed each year. To date, no underwater deterrents trialled have consistently reduced seabird bycatch across operational fisheries. Using a combination of insights from land-based strategies, seabirds' diving behaviours and their cognitive abilities, we developed a floating device exploring the effect of large eyespots and looming movement to prevent vulnerable seabirds from diving into gillnets. Here, we tested whether this novel above-water device called 'Looming eyes buoy' (LEB) would consistently deter vulnerable seaducks from a focal area. We counted the number of birds present in areas with and without LEBs in a controlled experimental setting. We show that long-tailed duck Clangula hyemalis abundance declined by approximately 20-30% within a 50 m radius of the LEB and that the presence of LEBs was the most important variable explaining this decline. We found no evidence for a memory effect on long-tailed ducks but found some habituation to the LEB within the time frame of the project (62 days). While further research is needed, our preliminary trials indicate that above-water visual devices could potentially contribute to reduce seabird bycatch if appropriately deployed in coordination with other management measures.

Risk assessment in the plateau pika (Ochotona curzoniae): intensity of behavioral response differs with predator species

Background

The ability of a prey species to assess the risk that a predator poses can have important fitness advantages for the prey species. To better understand predator–prey interactions, more species need to be observed to determine how prey behavioral responses differ in intensity when approached by different types of predators. The plateau pika (Ochotona curzoniae) is preyed upon by all predators occurring in its distribution area. Therefore, it is an ideal species to study anti-predator behavior. In this study, we investigated the intensity of anti-predator behavior of pikas in response to visual cues by using four predator species models in Maqu County on the eastern Qinghai-Tibetan Plateau.

Results

The behavioral response metrics, such as Flight Initiation Distance (FID), the hiding time and the percentage of vigilance were significantly different when exposed to a Tibetan fox, a wolf, a Saker falcon and a large-billed crow, respectively. Pikas showed a stronger response to Saker falcons compared to any of the other predators.

Conclusions

Our results showed that pikas alter their behavioral (such as FID, the hiding time and the vigilance) response intensity to optimally balance the benefits when exposed to different taxidermy predator species models. We conclude that pikas are able to assess their actual risk of predation and show a threat-sensitive behavioral response.

Personal information about danger trumps social information from avian alarm calls

Information about predators can mean the difference between life and death, but prey face the challenge of integrating personal information about predators with social information from the alarm calls of others. This challenge might even affect the structure of interspecific information networks: species vary in response to alarm calls, potentially because different foraging ecologies constrain the acquisition of personal information. However, the hypothesis that constrained personal information explains a greater response to alarm calls has not been experimentally tested. We used a within-species test to compare the antipredator responses of New Holland honeyeaters, Phylidonyris novaehollandiae, during contrasting foraging behaviour. Compared with perched birds, which hawk for insects and have a broad view, those foraging on flowers were slower to spot gliding model predators, showing that foraging behaviour can affect predator detection. Furthermore, nectar-foraging birds were more likely to flee to alarm call playbacks. Birds also assessed social information relevance: more distant calls, and those from another species, prompted fewer flights and slower reaction times. Overall, birds made flexible decisions about danger by integrating personal and social information, while weighing information relevance. These findings support the idea that a strategic balance of personal and social information could affect community function.

Individual vigilance profiles in flocks of House Sparrows (Passer domesticus)

Individual vigilance against threats typically decreases with group size. However, group size often explains a small amount of variation in vigilance, suggesting that other factors such as individual differences might contribute. For instance, individuals could maintain different vigilance levels overall and also respond differently to variation in group size. We investigated individual variation in vigilance and its patterns of plasticity in flocks of House Sparrows (Passer domesticus (Linnaeus, 1758)). We carried out observations at one provisioned site and used multiple observations of the same individuals (n = 14) in flocks of different sizes over two consecutive months. The typical decline in vigilance with flock size occurred at the population level. Controlling for food density, flock size, time of year, and sex, we documented consistent individual differences in various measurements of vigilance. Plasticity of vigilance adjustments to variation in flock size occurred for the frequency of high vigilance postures. Male House Sparrows with larger bibs, which signal higher dominance status, tended to spend less time vigilant and obtained food at a higher rate, supporting a state-dependent explanation for the origin of individual vigilance profiles. Individual differences can contribute to explaining the large scatter in the relationship between vigilance and group size in many species.

Effect of initial body orientation on escape probability of prey fish escaping from predators

The kinematic and behavioral components of the escape response can affect the outcomes of predator-prey interactions. For example, because sensory perception range can have spatial bias, and because turn duration before the initiation of escape locomotion can be smaller when prey is oriented away from predators, the prey's body orientation relative to a predator at the onset of the escape response (initial orientation) could affect whether prey successfully evade predators. We tested this hypothesis by recording the escape responses of juvenile red sea bream (Pagrus major) to the predatory scorpion fish (Sebastiscus marmoratus). Flight initiation distance tended to be small when prey were attacked from behind, suggesting that prey have spatial bias in detecting attacking predators. An increase in flight initiation distance increased escape probability. An increase in initial orientation decreased turn duration and increased escape probability when the effect of flight initiation distance was offset. These results suggest that initial orientation affects escape probability through two different pathways: changes in flight initiation distance and turn duration. These findings highlight the importance of incorporating initial orientation into other studies of the kinematics of predator-prey interactions.

Summary: Our predator-prey experiments reveal that prey's initial body orientation relative to a predator affects the flight initiation distance and turn duration of prey and consequently affects escape probability.

European starlings use their acute vision to check on feline predators but not on conspecifics

Head movements allow birds with laterally placed eyes to move their centers of acute vision around and align them with objects of interest. Consequently, head movements have been used as indicator of fixation behavior (where gaze is maintained). However, studies on head movement behavior have not elucidated the degree to which birds use high-acuity or low-acuity vision. We studied how European starlings (Sturnus vulgaris) used high-acuity vision in the early stages of visual exploration of a stuffed cat (common terrestrial predator), a taxidermy Cooper’s hawk (common aerial predator), and a stuffed study skin of a conspecific. We found that starlings tended to use their high acuity vision when looking at predators, particularly, the cat was above chance levels. However, when they viewed a conspecific, they used high acuity vision as expected by chance. We did not observe a preference for the left or right center of acute vision. Our findings suggest that starlings exposed to a predator (particularly cats) may employ selective attention by using high-acuity vision to obtain quickly detailed information useful for a potential escape, but exposed to a social context may use divided attention by allocating similar levels high- and low-quality vision to monitor both conspecifics and the rest of the environment.

Visual field shape and foraging ecology in diurnal raptors

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 morphometric approaches. While the maximum binocular field width did not differ in species of 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 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.

Evolutionary analysis of vision genes identifies potential drivers of visual differences between giraffe and okapi

Background

The capacity of visually oriented species to perceive and respond to visual signal is integral to their evolutionary success. Giraffes are closely related to okapi, but the two species have broad range of phenotypic differences including their visual capacities. Vision studies rank giraffe’s visual acuity higher than all other artiodactyls despite sharing similar vision ecological determinants with many of them. The extent to which the giraffe’s unique visual capacity and its difference with okapi is reflected by changes in their vision genes is not understood.

Methods

The recent availability of giraffe and okapi genomes provided opportunity to identify giraffe and okapi vision genes. Multiple strategies were employed to identify thirty-six candidate mammalian vision genes in giraffe and okapi genomes. Quantification of selection pressure was performed by a combination of branch-site tests of positive selection and clade models of selection divergence through comparing giraffe and okapi vision genes and orthologous sequences from other mammals.

Results

Signatures of selection were identified in key genes that could potentially underlie giraffe and okapi visual adaptations. Importantly, some genes that contribute to optical transparency of the eye and those that are critical in light signaling pathway were found to show signatures of adaptive evolution or selection divergence. Comparison between giraffe and other ruminants identifies significant selection divergence in CRYAA and OPN1LW. Significant selection divergence was identified in SAG while positive selection was detected in LUM when okapi is compared with ruminants and other mammals. Sequence analysis of OPN1LW showed that at least one of the sites known to affect spectral sensitivity of the red pigment is uniquely divergent between giraffe and other ruminants.

Discussion

By taking a systemic approach to gene function in vision, the results provide the first molecular clues associated with giraffe and okapi vision adaptations. At least some of the genes that exhibit signature of selection may reflect adaptive response to differences in giraffe and okapi habitat. We hypothesize that requirement for long distance vision associated with predation and communication with conspecifics likely played an important role in the adaptive pressure on giraffe vision genes.

Infection reduces anti-predator behaviors in house finches

Infectious diseases can cause host mortality through direct or indirect mechanisms, including altered behavior. Diminished anti-predator behavior is among the most-studied causes of indirect mortality during infection, particularly for systems in which a parasite's life-cycle requires transmission from prey to predator. Significantly less work has examined whether directly-transmitted parasites and pathogens also reduce anti-predator behaviors. Here we test whether the directly-transmitted bacterial pathogen, Mycoplasma gallisepticum (MG), reduces responses to predation-related stimuli in house finches (Haemorhous mexicanus). MG causes conjunctivitis and reduces survival among free-living finches, but rarely causes mortality in captivity, suggesting a role for indirect mechanisms. Wild-caught finches were individually housed in captivity and exposed to the following treatments: 1) visual presence of a stuffed, mounted predator (a Cooper's Hawk (Accipiter cooperii)) or control object (a vase or a stuffed, mounted mallard duck (Anas platyrhynchos)), 2) vocalizations of the same predator and non-predator, 3) approach of a researcher to enclosures, and 4) simulated predator attack (capture by hand). MG infection reduced anti-predator responses during visual exposure to a mounted predator and simulated predator attack, even for birds without detectable visual obstruction from conjunctivitis. However, MG infection did not significantly alter responses during human approach or audio playback. These results are consistent with the hypothesis that predation plays a role in MG-induced mortality in the wild, with reduced locomotion, a common form of sickness behavior for many taxa, as a likely mechanism. Our results therefore suggest that additional research on the role of sickness behaviors in predation could prove illuminating.

Avian binocular vision: It's not just about what birds can see, it's also about what they can't

With the exception of primates, most vertebrates have laterally placed eyes. Binocular vision in vertebrates has been implicated in several functions, including depth perception, contrast discrimination, etc. However, the blind area in front of the head that is proximal to the binocular visual field is often neglected. This anterior blind area is important when discussing the evolution of binocular vision because its relative length is inversely correlated with the width of the binocular field. Therefore, species with wider binocular fields also have shorter anterior blind areas and objects along the mid-sagittal plane can be imaged at closer distances. Additionally, the anterior blind area is of functional significance for birds because the beak falls within this blind area. We tested for the first time some specific predictions about the functional role of the anterior blind area in birds controlling for phylogenetic effects. We used published data on visual field configuration in 40 species of birds and measured beak and skull parameters from museum specimens. We found that birds with proportionally longer beaks have longer anterior blind areas and thus narrower binocular fields. This result suggests that the anterior blind area and beak visibility do play a role in shaping binocular fields, and that binocular field width is not solely determined by the need for stereoscopic vision. In visually guided foragers, the ability to see the beak-and how much of the beak can be seen-varies predictably with foraging habits. For example, fish- and insect-eating specialists can see more of their own beak than birds eating immobile food can. But in non-visually guided foragers, there is no consistent relationship between the beak and anterior blind area. We discuss different strategies-wide binocular fields, large eye movements, and long beaks-that minimize the potential negative effects of the anterior blind area. Overall, we argue that there is more to avian binocularity than meets the eye.

Modeling scan and interscan durations in antipredator vigilance

Many prey species alternate between bouts of foraging and bouts of antipredator vigilance. Models of vigilance typically predict how much total time prey animals should allocate to vigilance but do not specify how that time should be scheduled throughout foraging. Here, we examine how the scheduling of vigilance pays off in terms of food intake and predator detection. Specifically, we study how changes in ecological factors affect the expected duration of scans to look out for predators and the duration of interscan intervals dedicated to foraging. Our framework includes factors like the risk of attack, how difficult it is to locate food and predators, and the distance to protective cover. Our individual-based model makes several predictions about scan and interscan durations, which are discussed in relation to the available empirical evidence in birds and mammals. This model of antipredator vigilance is a first step in incorporating constraints related to food gathering and the detection of predators. Adding such constraints adds a novel dimension to vigilance models and produces a variety of predictions that await empirical scrutiny.

Oculomotor strategy of an avian ground forager: tilted and weakly yoked eye saccades

Many bird species are capable of large saccadic eye movements that can result in substantial shifts in gaze direction and complex changes to their visual field orientation. In the absence of visual stimuli, birds make spontaneous saccades that follow an endogenous oculomotor strategy. We used new eye-tracking technology specialized for small birds to study the oculomotor behavior of an open-habitat, ground-foraging songbird, the European starling (Sturnus vulgaris). We found that starlings primarily move their eyes along a tilted axis 13.46 deg downwards anteriorly and upwards posteriorly, which differs from the axis parallel to the horizon employed by other species. This tilted axis could enhance foraging and anti-predator strategies while starlings are head-down looking for food, allowing them to direct vision between the open mandibles to visually inspect food items, and above and behind the head to scan areas where predators are more likely to attack. We also found that starlings have neither fully conjugate saccades (as in humans, for example) nor independent saccades (as in chameleons, for example). Rather, they exhibit weakly yoked saccades where the left and right eyes move at the same time but not at the same magnitude. Functionally, weakly yoked saccades may be similar to independent saccades in that they allow the two eyes to concomitantly perform different tasks. The differences between the oculomotor strategies of studied species suggest eye movements play variable but important roles across bird species with different ecological niches.

Vision in avian emberizid foragers: maximizing both binocular vision and fronto-lateral visual acuity

Avian species vary in their visual system configuration, but previous studies have often compared single visual traits between 2-3 distantly related species. However, birds use different visual dimensions that cannot be maximized simultaneously to meet different perceptual demands, potentially leading to trade-offs between visual traits. We studied the degree of inter-specific variation in multiple visual traits related to foraging and anti-predator behaviors in nine species of closely related emberizid sparrows, controlling for phylogenetic effects. Emberizid sparrows maximize binocular vision, even seeing their bill tips, which may enhance the detection of prey and facilitate food handling. Sparrows have a single retinal center of acute vision (i.e., fovea) projecting fronto-laterally (but not into the binocular field). The foveal projection close to the edge of the binocular field may shorten the time to gather and process both monocular and binocular visual information from the foraging substrate. Contrary to previous work, we found that species with larger visual fields had higher visual acuity, which may compensate for larger blind spots (i.e., pectens) above the center of acute vision, enhancing predator detection. Finally, species with a steeper change in ganglion cell density across the retina had higher eye movement amplitude likely due to a more pronounced reduction in visual resolution away from the fovea, which would need to be moved around more frequently. The visual configuration of emberizid passive prey foragers is substantially different from that of previously studied avian groups (e.g., sit-and-wait and tactile foragers).

Do American goldfinches see their world like passive prey foragers? A study on visual fields, retinal topography, and sensitivity of photoreceptors

Several species of the most diverse avian order, Passeriformes, specialize in foraging on passive prey, although relatively little is known about their visual systems. We tested whether some components of the visual system of the American goldfinch (Spinus tristis), a granivorous bird, followed the profile of species seeking passive food items (small eye size relative to body mass, narrow binocular fields and blind areas, centrally located retinal specialization projecting laterally, ultraviolet-sensitive vision). We measured eye size, visual field configuration, the degree of eye movement, variations in the density of ganglion cells and cone photoreceptors, and the sensitivity of photoreceptor visual pigments and oil droplets. Goldfinches had relatively large binocular (46°) and lateral (134°) visual fields with a high degree of eye movement (66° at the plane of the bill). They had a single centrotemporally located fovea that projects laterally, but can be moved closer to the edge of the binocular field by converging the eyes. Goldfinches could also increase their panoramic vision by diverging their eyes while handling food items in head-up positions. The distribution of photoreceptors indicated that the highest density of single and double cones was surrounding the fovea, making it the center of chromatic and achromatic vision and motion detection. Goldfinches possessed a tetrachromatic ultraviolet visual system with visual pigment peak sensitivities of 399 nm (ultraviolet-sensitive cone), 442 nm (short-wavelength-sensitive cone), 512 nm (medium-wavelength-sensitive cone), and 580 nm (long-wavelength-sensitive cone). Overall, the visual system of American goldfinches showed characteristics of passive as well as active prey foragers, with a single-fovea configuration and a large degree of eye movement that would enhance food searching and handling with their relatively wide binocular fields.

Selective attention in peacocks during predator detection

Predation can exert strong selective pressure on the evolution of behavioral and morphological traits in birds. Because predator avoidance is key to survival and birds rely heavily on visual perception, predation may have shaped avian visual systems as well. To address this question, we examined the role of visual attention in antipredator behavior in peacocks (Pavo cristatus). Peacocks were exposed to a model predator while their gaze was continuously recorded with a telemetric eye-tracker. We found that peacocks spent more time looking at and made more fixations on the predator compared to the same spatial location before the predator was revealed. The duration of fixations they directed toward conspecifics and environmental features decreased after the predator was revealed, indicating that the peacocks were rapidly scanning their environment with their eyes. Maximum eye movement amplitudes and amplitudes of consecutive saccades were similar before and after the predator was revealed. In cases where conspecifics detected the predator first, peacocks appeared to learn that danger was present by observing conspecifics' antipredator behavior. Peacocks were faster to detect the predator when they were fixating closer to the area where the predator would eventually appear. In addition, pupil size increased after predator exposure, consistent with increased physiological arousal. These findings demonstrate that peacocks selectively direct their attention toward predatory threats and suggest that predation has influenced the evolution of visual orienting systems.

A novel mechanism for a survival advantage of vigilant individuals in groups

In many animal species, vigilance is crucial for avoiding predation. In groups, however, nonvigilant individuals could benefit from the vigilance of others without any of the associated costs. In an evolutionary sense, such exploitation may be compensated if vigilant individuals have a survival advantage. The novelty in our model is that the probability to detect a predator is "distance dependent." We show that even if nonvigilant individuals benefit fully from information produced by vigilant individuals, vigilant individuals nevertheless enjoy a survival advantage. This happens because detection of predators is more likely when vigilant individuals happen to be targets of predation. We expect this distance-dependent mechanism to be compatible with previously reported mechanisms.

An alternative theoretical approach to escape decision-making: the role of visual cues

Escape enables prey to avoid an approaching predator. The escape decision-making process has traditionally been interpreted using theoretical models that consider ultimate explanations based on the cost/benefit paradigm. Ultimate approaches, however, suffer from inseparable extra-assumptions due to an inability to accurately parameterize the model's variables and their interactive relationships. In this study, we propose a mathematical model that uses intensity of predator-mediated visual stimuli as a basic cue for the escape response. We consider looming stimuli (i.e. expanding retinal image of the moving predator) as a cue to flight initiation distance (FID; distance at which escape begins) of incubating Mallards (Anas platyrhynchos). We then examine the relationship between FID, vegetation cover and directness of predator trajectory, and fit the resultant model to experimental data. As predicted by the model, vegetation concealment and directness of predator trajectory interact, with FID decreasing with increased concealment during a direct approach toward prey, but not during a tangential approach. Thus, we show that a simple proximate expectation, which involves only visual processing of a moving predator, may explain interactive effects of environmental and predator-induced variables on an escape response. We assume that our proximate approach, which offers a plausible and parsimonious explanation for variation in FID, may serve as an evolutionary background for traditional, ultimate explanations and should be incorporated into interpretation of escape behavior.

Sensory basis of vigilance behavior in birds: synthesis and future prospects

Birds gather visual information through scanning behavior to make decisions relevant for survival (e.g., detecting predators and finding food). The goal of this study was (a) to review some visual properties involved in scanning behavior (retinal specialization for visual resolution and motion detection, visual acuity, and size of the blind area), and (b) hypothesize how the inter-specific variability in these properties may lead to different scanning strategies. The avian visual system has a high degree of heterogeneity in visual performance across the visual field, with some sectors providing higher levels of visual resolution and motion detection (e.g., retinal specializations) than others (e.g., peripheral retina and blind area). Thus, information quality will vary in different parts of the visual field, which contradicts some theoretical assumptions on information gathering. Birds need to move their eyes and heads to align the retinal specializations to different sectors of visual space. The rates of eye and head movements can then be used as proxies for scanning strategies. I propose specific predictions as to how each of the visual properties studied can affect scanning strategies in the context of predator detection in different habitat types and with different levels of predation risk. Establishing the degree of association between sensory specializations and scanning strategies can enhance our understanding of the evolution of anti-predator behavior.