Predator Detection is Limited in Microhabitats with High Light Intensity: An Experiment with Brown-Headed Cowbirds

Eye morphology contributes to the ecology and evolution of the aquatic avifauna

Aquatic birds are notable among the global avifauna for living in environments exposed to large amounts of light. Despite growing evidence that visual adaptations to light underly the ecology and evolution of the avian tree of life, no comprehensive comparative analysis of visual acuity as approximated by eyes size exists for the global aquatic avifauna. Here, I use Stanley Ritland's unpublished dataset of measurements for axial length collected from museum specimens to explore the ecology and evolution of eye size variation for half of the aquatic avifauna (N = 464 species). After correcting for body mass allometry and incorporating phylogenetic relationships, aquatic species had significantly smaller eyes compared to terrestrial species. Furthermore, species using hyperopic foraging manoeuvres, exhibiting carnivorous and insectivorous diets, and displaying nocturnal behaviour had larger eyes. Plunge-divers (e.g. boobies and tropic birds) and stalkers (e.g. herons) had the largest relative eye sizes, especially species identifying prey at higher altitudes or longer distances. Underwater pursuit-divers foraging at greater depths had larger eyes, likely due to the dramatic attenuation of light in the deep ocean. Overall, residual eye size was phylogenetically conserved (l = 0.94), with phylogeny alone explaining 62% of residual eye size variation. Collectively, these results suggest that the relatively bright environments found in aquatic ecosystems negate the adaptive benefits of costly metabolic investments associated with developing and maintaining larger eyes, while also reducing the potential occurrence of disability glare. Strong correlations between eye size and foraging ecology in different aquatic environments corroborate similar comparative studies of terrestrial birds and underscore the central role that vision has played in driving the ecology and evolution of the global avifauna.

Artificial light at night decreases the pupillary light response of dark-adapted toads to bright light

Artificial light at night (ALAN) is expanding worldwide. Many physiological effects have been reported in animals, but we still know little about the consequences for the visual system. The pupil contributes to control incoming light onto the retina. Sudden increases in light intensity evokes the pupil light reflex (PLR). Intrinsically photosensitive retinal ganglion cells (ipRGC) affect PLR and melatonin expression, which largely regulate circadian rhythms and PLR itself. IpRCG receive inputs from various photoreptors with different peak sensitivities implying that PLR could be altered by a broad range of light sources. We predicted ALAN to enhance PLR. Contrary to our prediction, dark-adapted cane toads Rhinella marina, exposed to ALAN (5 lx) for 12 days, exhibited a lower PLR than controls and individuals exposed to 0.04 lx, even after 1 h in bright light. We cannot conclude whether ALAN induced a larger pupil size in dark-adapted toads or a slower initial contraction. Nevertheless, the response was triggered by a light source with an emission peak (590 nm) well above the sensitivity peak of melanopsin, the main photoreceptor involved in PLR. Therefore, ALAN alters the capacity of toads to regulate the incoming light in the eye at night, which may reduce the performance of visually guided behaviors, and increase mortality by predators or road kills at night. This first study emphasizes the need to focus on the effect of ALAN on the vision of nocturnal organisms to better understand how this sensory system is altered and anticipate the consequences for organisms.

Responses of turkey vultures to unmanned aircraft systems vary by platform

A challenge that conservation practitioners face is manipulating behavior of nuisance species. The turkey vulture (Cathartes aura) can cause substantial damage to aircraft if struck. The goal of this study was to assess vulture responses to unmanned aircraft systems (UAS) for use as a possible dispersal tool. Our treatments included three platforms (fixed-wing, multirotor, and a predator-like ornithopter [powered by flapping flight]) and two approach types (30 m overhead or targeted towards a vulture) in an operational context. We evaluated perceived risk as probability of reaction, reaction time, flight-initiation distance (FID), vulture remaining index, and latency to return. Vultures escaped sooner in response to the fixed-wing; however, fewer remained after multirotor treatments. Targeted approaches were perceived as riskier than overhead. Vulture perceived risk was enhanced by flying the multirotor in a targeted approach. We found no effect of our treatments on FID or latency to return. Latency was negatively correlated with UAS speed, perhaps because slower UAS spent more time over the area. Greatest visual saliency followed as: ornithopter, fixed-wing, and multirotor. Despite its appearance, the ornithopter was not effective at dispersing vultures. Because effectiveness varied, multirotor/fixed-wing UAS use should be informed by management goals (immediate dispersal versus latency).

Adaptations to light predict the foraging niche and disassembly of avian communities in tropical countrysides

The role of light in partitioning ecological niche space remains a frontier in understanding the assembly of terrestrial vertebrate communities and their response to global change. Leveraging recent advances in biologging technology and intensive field surveys of cloud forest bird communities across an agricultural land use gradient in the Peruvian Andes, we demonstrate that eye size predicts (1) the ambient light microenvironment used by free-ranging birds, (2) their foraging niche, and (3) species-specific sensitivity to agricultural land use change. For 15 species carrying light sensors (N = 71 individuals), light intensity levels were best explained by eye size and foraging behavior, with larger-eyed species using darker microenvironments. Across the cloud forest bird community (N = 240 species), hyperopic ("far-sighted") foragers, (e.g., flycatchers), had larger eyes compared to myopic ("near-sighted") species (e.g., gleaners and frugivores); eye size was also larger for myopic insectivores that foraged in the forest understory. Eye size strongly predicted sensitivity to brightly lit habitats across an agricultural land use gradient. Species that increased in abundance in mixed intensity agriculture, including fencerows, silvopasture, and pasture, had smaller eyes, suggesting that light acts as an environmental filter when communities disassemble in a human-disturbed landscape. We suggest that eye size represents a novel functional trait contributing to terrestrial vertebrate community assembly and sensitivity to habitat disturbance.

Social information affects Canada goose alert and escape responses to vehicle approach: implications for animal-vehicle collisions

BACKGROUND

Animal-vehicle collisions represent substantial sources of mortality for a variety of taxa and can pose hazards to property and human health. But there is comparatively little information available on escape responses by free-ranging animals to vehicle approach versus predators/humans.

METHODS

We examined responses (alert distance and flight-initiation distance) of focal Canada geese (Branta canadensis maxima) to vehicle approach (15.6 m·s-1) in a semi-natural setting and given full opportunity to escape. We manipulated the direction of the vehicle approach (direct versus tangential) and availability of social information about the vehicle approach (companion group visually exposed or not to the vehicle).

RESULTS

We found that both categorical factors interacted to affect alert and escape behaviors. Focal geese used mostly personal information to become alert to the vehicle under high risk scenarios (direct approach), but they combined personal and social information to become alert in low risk scenarios (tangential approach). Additionally, when social information was not available from the companion group, focal birds escaped at greater distances under direct compared to tangential approaches. However, when the companion group could see the vehicle approaching, focal birds escaped at similar distances irrespective of vehicle direction. Finally, geese showed a greater tendency to take flight when the vehicle approached directly, as opposed to a side step or walking away from the vehicle.

CONCLUSIONS

We suggest that the perception of risk to vehicle approach (likely versus unlikely collision) is weighted by the availability of social information in the group; a phenomenon not described before in the context of animal-vehicle interactions. Notably, when social information is available, the effects of heightened risk associated with a direct approach might be reduced, leading to the animal delaying the escape, which could ultimately increase the chances of a collision. Also, information on a priori escape distances required for surviving a vehicle approach (based on species behavior and vehicle approach speeds) can inform planning, such as location of designated cover or safe areas. Future studies should assess how information from vehicle approach flows within a flock, including aspects of vehicle speed and size, metrics that affect escape decision-making.

Response time of an avian prey to a simulated hawk attack is slower in darker conditions, but is independent of hawk colour morph

To avoid predation, many species rely on vision to detect predators and initiate an escape response. The ability to detect predators may be lower in darker light conditions or with darker backgrounds. For birds, however, this has never been experimentally tested. We test the hypothesis that the response time of avian prey (feral pigeon Columbia livia f. domestica) to a simulated hawk attack (taxidermy mounted colour-polymorphic black sparrowhawk Accipiter melanoleucus) will differ depending on light levels or background colour. We predict that response will be slower under darker conditions, which would translate into higher predation risk. The speed of response of prey in relation to light level or background colour may also interact with the colour of the predator, and this idea underpins a key hypothesis proposed for the maintenance of different colour morphs in polymorphic raptors. We therefore test whether the speed of reaction is influenced by the morph of the hawk (dark/light) in combination with light conditions (dull/bright), or background colours (black/white). We predict slowest responses to morphs under conditions that less contrast with the plumage of the hawk (e.g. light morph under bright light or white background). In support of our first hypothesis, pigeons reacted slower under duller light and with a black background. However, we found no support for the second hypothesis, with response times observed between the hawk-morphs being irrespective of light levels or background colour. Our findings experimentally confirm that birds detect avian predators less efficiently under darker conditions. These conditions, for example, might occur during early mornings or in dense forests, which could lead to changes in anti-predator behaviours. However, our results provide no support that different morphs may be maintained in a population due to differential selective advantages linked to improved hunting efficiencies in different conditions due to crypsis.

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.

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.

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).

Foraging patch selection in winter: a balance between predation risk and thermoregulation benefit

In winter, foraging activity is intended to optimize food search while minimizing both thermoregulation costs and predation risk. Here we quantify the relative importance of thermoregulation and predation in foraging patch selection of woodland birds wintering in a Mediterranean montane forest. Specifically, we account for thermoregulation benefits related to temperature, and predation risk associated with both illumination of the feeding patch and distance to the nearest refuge provided by vegetation. We measured the amount of time that 38 marked individual birds belonging to five small passerine species spent foraging at artificial feeders. Feeders were located in forest patches that vary in distance to protective cover and exposure to sun radiation; temperature and illumination were registered locally by data loggers. Our results support the influence of both thermoregulation benefits and predation costs on feeding patch choice. The influence of distance to refuge (negative relationship) was nearly three times higher than that of temperature (positive relationship) in determining total foraging time spent at a patch. Light intensity had a negligible and no significant effect. This pattern was generalizable among species and individuals within species, and highlights the preponderance of latent predation risk over thermoregulation benefits on foraging decisions of birds wintering in temperate Mediterranean forests.