Colour polymorphism in owls is linked to light variability

Color polymorphic carnivores have faster speciation rates

Variation in coat color is a prominent feature in carnivores, thought to be shaped by environmental factors. As new traits could allow populations to occupy novel niches and habitats, color polymorphism may be maintained by balancing selection. Consequently, color polymorphic species may speciate more rapidly and can give rise to monomorphic daughter species. We thus predicted that, within the Carnivora, (i) speciation rate is higher in polymorphic lineages, (ii) divergence between color polymorphic lineages is more recent, and (iii) within closely related groups, polymorphic lineages are ancestral and monomorphic lineages derived. We also tested whether accelerated speciation rates relate to niche breadth, measured by the number of occupied habitats and range size. We collected data of 48 polymorphic and 192 monomorphic carnivore species, and assessed speciation rates using phylogenetic comparative methods. We found that polymorphic carnivores had higher speciation rates (λ1 = 0.29, SD = 0.13) than monomorphic species (λ0 = 0.053, SD = 0.044). Hidden and quantitative state speciation and extinction models inferred that color polymorphism was the main contributing factor, and that niche breadth was not of influence. Therefore, other selective forces than spatial niche segregation, such as predator-prey coevolution, may contribute to color polymorphism in wild carnivores.

Colour-specific diet specialization is associated with differences in owlet weight in a polymorphic owl: influence of the trophic quality variation

The niche divergence hypothesis proposes that the evolution and maintenance of colour polymorphism is based on a mechanism of disruptive selection. In a trophic context, the hypothesis predicts that individuals differing in colour vary in their trophic niche, either because they differ in foraging efficiency or feed in different habitats. A major evolutionary conundrum is how these expectations are affected by variation in trophic quality. Using an owl species with colour plumage polymorphism, the Eurasian scops owl Otus scops, we examined diet and habitat segregation during reproduction in relation to plumage colouration and trophic quality. Intensive sampling revealed that trophic quality for scops owls (i.e. abundance of grasshoppers and locusts) varied more among territories than between years, but scops owls did not segregate among territories of different quality by their colouration. However, we found that sex, plumage colouration and territory differences in trophic quality explained differences in the degree of dietary specialization. Brownish males delivered a higher diversity of prey to the nest than greyish ones in high trophic quality territories. We also found that the more diverse the diet provided by males, the heavier the owlets at fledging. Our study provides evidence for a different sensitivity to trophic quality of the colour morphs with potential fitness consequences in scops owls. We highlight the importance of studying the mechanisms leading to the persistence of colour polymorphism in patchy environments, since segregation may pass otherwise unnoticed if only habitats or years with similar conditions are considered.

The role of phenotypic plasticity in shaping ecological networks

Plasticity-mediated changes in interaction dynamics and structure may scale up and affect the ecological network in which the plastic species are embedded. Despite their potential relevance for understanding the effects of plasticity on ecological communities, these effects have seldom been analysed. We argue here that, by boosting the magnitude of intra-individual phenotypic variation, plasticity may have three possible direct effects on the interactions that the plastic species maintains with other species in the community: may expand the interaction niche, may cause a shift from one interaction niche to another or may even cause the colonization of a new niche. The combined action of these three factors can scale to the community level and eventually expresses itself as a modification in the topology and functionality of the entire ecological network. We propose that this causal pathway can be more widespread than previously thought and may explain how interaction niches evolve quickly in response to rapid changes in environmental conditions. The implication of this idea is not solely eco-evolutionary but may also help to understand how ecological interactions rewire and evolve in response to global change.

Regional variation in climate change alters the range-wide distribution of colour polymorphism in a wild bird

According to Gloger's rule, animal colouration is expected to be darker in wetter and warmer climates. Such environmental clines are predicted to occur in colour polymorphic species and to be shaped by selection if colour morphs represent adaptations to different environments. We studied if the distribution of the colour polymorphic tawny owl (Strix aluco) morphs (a pheomelanic brown and a pale grey) across Europe follow the predictions of Gloger's rule and if there is a temporal change in the geographical patterns corresponding to regional variations in climate change. We used data on tawny owl museum skin specimen collections. First, we investigated long-term spatiotemporal variation in the probability of observing the colour morphs in different climate zones. Second, we studied if the probability of observing the colour morphs was associated with general climatic conditions. Third, we studied if weather fluctuations prior to the finding year of an owl explain colour morph in each climate zone. The brown tawny owl morph was historically more common than the grey morph in every studied climate zone. Over time, the brown morph has become rarer in the temperate and Mediterranean zone, whereas it has first become rarer but then again more common in the boreal zone. Based on general climatic conditions, winter and summer temperatures were positively and negatively associated with the proportion of brown morph, respectively. Winter precipitation was negatively associated with the proportion of brown morph. The effects of 5-year means of weather on the probability to observe a brown morph differed between climate zones, indicating region-dependent effect of climate change and weather on tawny owl colouration. To conclude, tawny owl colouration does not explicitly follow Gloger's rule, implying a time and space-dependent complex system shaped by many factors. We provide novel insights into how the geographic distribution of pheomelanin-based colour polymorphism is changing.

Context-dependent coloration of prey and predator decision making in contrasting light environments

A big question in behavioral ecology is what drives diversity of color signals. One possible explanation is that environmental conditions, such as light environment, may alter visual signaling of prey, which could affect predator decision-making. Here, we tested the context-dependent predator selection on prey coloration. In the first experiment, we tested detectability of artificial visual stimuli to blue tits (Cyanistes caeruleus) by manipulating stimulus luminance and chromatic context of the background. We expected the presence of the chromatic context to facilitate faster target detection. As expected, blue tits found targets on chromatic yellow background faster than on achromatic grey background whereas in the latter, targets were found with smaller contrast differences to the background. In the second experiment, we tested the effect of two light environments on the survival of aposematic, color polymorphic wood tiger moth (Arctia plantaginis). As luminance contrast should be more detectable than chromatic contrast in low light intensities, we expected birds, if they find the moths aversive, to avoid the white morph which is more conspicuous than the yellow morph in low light (and vice versa in bright light). Alternatively, birds may attack first moths that are more detectable. We found birds to attack yellow moths first in low light conditions, whereas white moths were attacked first more frequently in bright light conditions. Our results show that light environments affect predator foraging decisions, which may facilitate context-dependent selection on visual signals and diversity of prey phenotypes in the wild.

Cold winters have morph-specific effects on natal dispersal distance in a wild raptor

Dispersal is a key process with crucial implications in spatial distribution, density, and genetic structure of species’ populations. Dispersal strategies can vary according to both individual and environmental features, but putative phenotype-by-environment interactions have rarely been accounted for. Melanin-based color polymorphism is a phenotypic trait associated with specific behavioral and physiological profiles and is, therefore, a good candidate trait to study dispersal tactics in different environments. Here, using a 40 years dataset of a population of color polymorphic tawny owls (Strix aluco), we investigated natal dispersal distance of recruiting gray and pheomelanic reddish-brown (hereafter brown) color morphs in relation to post-fledging winter temperature and individual characteristics. Because morphs are differently sensitive to cold winters, we predicted that morphs’ natal dispersal distances vary according to winter conditions. Winter temperature did not affect the proportion of brown (or gray) among recruits. We found that dispersal distances correlate with winter temperature in an opposite manner in the two morphs. Although the gray morph undertakes larger movements in harsher conditions, likely because it copes better with winter severity, the brown morph disperses shorter distances when winters are harsher. We discuss this morph-specific natal dispersal pattern in the context of competition for territories between morphs and in terms of costs and benefits of these alternative strategies. Our results stress the importance of considering the interaction between phenotype and environment to fully disentangle dispersal movement patterns and provide further evidence that climate affects the behavior and local distribution of this species.

Moonlight influence on quality of nestlings of scops owls depends on paternal coloration

Lunar phases might favor the maintenance of color polymorphism via disruptive selection if the different color variants performed differently in terms of prey capture under different moonlight levels. Moonlight, however, may affect prey capture as a side effect of its influence on prey behavior. Here we combine data of parental provisioning and quality of owlets with one ex-situ study of grasshopper activity to test whether Eurasian scops owls (Otus scops) with different plumage color and their prey are differently affected by moonlight. Food provisioning increased from new- to full-moon. However, the effect of moonlight on owlet mass gain and immune response depended on paternal coloration. On the one hand, body mass gain of nestlings of the greyest fathers increased from nights with new- to full-moon, whereas it did not change with moonlight for the brownest fathers. On the other hand, PHA response of nestlings of the brownest fathers increased with high moonlight levels during the first week of life, whereas it did not change with moonlight levels for the greyest fathers. Grasshoppers were more active at new moon than at full or waning moon. Our study provides supporting evidence that moonlight influences the behavior of both scops owls and its prey and suggests that fluctuations in environmental conditions can modulate the advantages of morphs. These results are important because they provide a general insight into a little appreciated mechanism for the maintenance of color polymorphism in natural populations based on the interactive effect of different environmental factors.

Specialist predation covaries with colour polymorphism in tawny owls

Abstract

Understanding intraspecific phenotypic variation in prey specialisation can help to predict how long-term changes in prey availability affect the viability of these phenotypes and their persistence. Generalists are favoured when the main food resources are unpredictable compared to specialists, which track the availability of the main prey and are more vulnerable to changes in the main food resource. Intraspecific heritable melanin-based colour polymorphism is considered to reflect adaptations to different environments. We studied colour morph-specific diet specialisation in a generalist predator, tawny owl (Strix aluco), during offspring food provisioning in relation to mammal prey density. We hypothesised that the grey morph, with higher fitness than the brown in Northern boreal conditions, is more specialised in mammalian prey than the brown morph, which in turn has higher fitness than the grey in the temperate zone. We found a higher diversity of prey delivered to the nest by brown fathers compared to grey ones, which also depended on the overall mammalian prey availability. Brown fathers provided proportionally fewer mammalian prey than grey in poor, but not in favourable mammal prey years. Our results suggest that the brown morph is more generalistic and reacts more strongly to variations in food supply than the grey morph, which may be a beneficial strategy in an unpredictable environment caused by environmental degradation.

Significance statement

Diet choice of a species may vary depending on fluctuations in the abundance of their food resource, but also within a population, there can be adaptations to use different food resources. The tawny owl exhibits a grey and a reddish-brown colour morph and is considered a generalist predator eating both mammal and bird prey. We find that the diet of the reddish-brown morph is more diverse than that of the grey. When the tawny owls’ main prey, small mammals, are abundant both colour morphs prey on mammals, but in years with less small mammals, the reddish-brown morph is more prone of switching to small bird predation than the grey. The generalist strategy of the brown morph is likely to be more favourable than a stricter specialisation in small mammals of the grey under recently reoccurring irregularities in small mammal dynamics.

Different ontogenetic trajectories of body colour, pattern and crypsis in two sympatric intertidal crab species

Animals frequently exhibit great variation in appearance, especially in heterogeneous habitats where individuals can be concealed differentially against backgrounds. Although background matching is a common anti-predator strategy, gaps exist in our understanding of within- and among-species variation. Specifically, the drivers of changes in appearance associated with habitat use and occurring through ontogeny are poorly understood. Using image analysis, we tested how individual appearance and camouflage in two intertidal crab species, the mud crab Panopeus americanus and the mottled crab Pachygrapsus transversus, relate to ontogeny and habitat use. We predicted that both species would change appearance with ontogeny, but that resident mud crabs would exhibit higher background similarity than generalist mottled crabs. Both species showed ontogenetic changes; the mud crabs became darker, whereas mottled crabs became more green. Small mud crabs were highly variable in colour and pattern, probably stemming from the use of camouflage in heterogeneous habitats during the most vulnerable life stage. Being habitat specialists, mud crabs were better concealed against all backgrounds than mottled crabs. Mottled crabs are motile and generalist, occupying macroalgae-covered rocks when adults, which explains why they are greener and why matches to specific habitats are less valuable. Differential habitat use in crabs can be associated with different coloration and camouflage strategies to avoid predation.

Genomic Evidence for Sensorial Adaptations to a Nocturnal Predatory Lifestyle in Owls

Owls (Strigiformes) evolved specific adaptations to their nocturnal predatory lifestyle, such as asymmetrical ears, a facial disk, and a feather structure allowing silent flight. Owls also share some traits with diurnal raptors and other nocturnal birds, such as cryptic plumage patterns, reversed sexual size dimorphism, and acute vision and hearing. The genetic basis of some of these adaptations to a nocturnal predatory lifestyle has been studied by candidate gene approaches but rarely with genome-wide scans. Here, we used a genome-wide comparative analysis to test for selection in the early history of the owls. We estimated the substitution rates in the coding regions of 20 bird genomes, including 11 owls of which five were newly sequenced. Then, we tested for functional overrepresentation across the genes that showed signals of selection. In the ancestral branch of the owls, we found traces of positive selection in the evolution of genes functionally related to visual perception, especially to phototransduction, and to chromosome packaging. Several genes that have been previously linked to acoustic perception, circadian rhythm, and feather structure also showed signals of an accelerated evolution in the origin of the owls. We discuss the functions of the genes under positive selection and their putative association with the adaptation to the nocturnal predatory lifestyle of the owls.

Contrasting parental color morphs increase regularity of prey deliveries in an African raptor

Disassortative mating in color-polymorphic raptors is a proposed mechanism for the maintenance of color polymorphism in populations. Selection for such a mating system may occur if there are fitness advantages of mating with a contrasting morph. In the black sparrowhawk (Accipiter melanoleucus), mixed-morph pairs may have a selective advantage because they produce offspring that have higher survival rates. Two hypotheses, which may explain the mechanism, are the “avoidance-image” and “complementarity” hypotheses. The first suggests that, within a predator’s territory, prey develop a search image for the more commonly encountered parental morph, for example, the male morph during incubation and brooding. Females of a contrasting morph to their partner would then have higher capture rates once they commence hunting in the later nestling phase. Thus, the “avoidance-image” hypothesis predicts higher provisioning rates for mixed-morph pairs. Alternatively, the “complementarity” hypothesis posits that different color morphs exploit different environmental conditions, allowing mixed-morph pairs to hunt under a wider range of conditions and predicts that food is delivered more consistently. We test these hypotheses using nest cameras to record prey delivery rates during the late nestling phase when both parents are hunting. We found support for the “complementarity” hypothesis, with mixed-morph pairs delivering food more consistently but not at a higher rate. This higher consistency in prey deliveries may explain the improved survival of the offspring of mixed-morph pairs and could, therefore, play a role in maintaining the stability of color polymorphism in this system.

Gray plumage color is more cryptic than brown in snowy landscapes in a resident color polymorphic bird

Camouflage may promote fitness of given phenotypes in different environments. The tawny owl (Strix aluco) is a color polymorphic species with a gray and brown morph resident in the Western Palearctic. A strong selection pressure against the brown morph during snowy and cold winters has been documented earlier, but the selection mechanisms remain unresolved. Here, we hypothesize that selection favors the gray morph because it is better camouflaged against predators and mobbers in snowy conditions compared to the brown one. We conducted an online citizen science experiment where volunteers were asked to locate a gray or a brown tawny owl specimen from pictures taken in snowy and snowless landscapes. Our results show that the gray morph in snowy landscapes is the hardest to detect whereas the brown morph in snowy landscapes is the easiest to detect. With an avian vision model, we show that, similar to human perceivers, the brown morph is more conspicuous than the gray against coniferous tree trunks for a mobbing passerine. We suggest that with better camouflage, the gray morph may avoid mobbers and predators more efficiently than the brown morph and thus survive better in snowy environments. As winters are getting milder and shorter in the species range, the selection periods against brown coloration may eventually disappear or shift poleward.

Differential fitness effects of moonlight on plumage colour morphs in barn owls

The Moon cycle exposes nocturnal life to variation in environmental light. However, whether moonlight shapes the fitness of nocturnal species with distinct colour variants remains unknown. Combining long-term monitoring, high-resolution GPS tracking, and experiments on prey, we show that barn owls (Tyto alba) with distinct plumage colourations are differently affected by moonlight. The reddest owls are less successful hunting and providing food to their offspring during moonlit nights, which associates with lower body mass and survival of the youngest nestlings and with female mates starting to lay eggs at low moonlight levels. Although moonlight should make white owls more conspicuous to prey, hunting and fitness of the whitest owls are positively or un-affected by moonlight. We experimentally show that, under full-moon conditions, white plumages trigger longer freezing times in the prey, which should facilitate prey catchability. We propose that the barn owl’s white plumage, a rare trait among nocturnal predators, exploits the known aversion of rodents to bright light, explaining why, counterintuitively, moonlight impacts less the whitest owls. Our study provides evidence for the long-suspected influence of the Moon on the evolution of colouration in nocturnal species, highlighting the importance of colour in nocturnal ecosystems.

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.