Coloration signals the ability to cope with elevated stress hormones: effects of corticosterone on growth of barn owls are associated with melanism

Stressful situations during development can shape the phenotype for life by provoking a trade-off between development and survival. Stress hormones, mainly glucocorticoids, play an important orchestrating role in this trade-off. Hence, how stress sensitive an animal is critically determines the phenotype and ultimately fitness. In several species, darker eumelanic individuals are less sensitive to stressful conditions than less eumelanic conspecifics, which may be due to the pleiotropic effects of genes affecting both coloration and physiological traits. We experimentally tested whether the degree of melanin-based coloration is associated with the sensitivity to an endocrine response to stressful situations in the barn owl. We artificially administered the mediator of a hormonal stress response, corticosterone, to nestlings to examine the prediction that corticosterone-induced reduction in growth rate is more pronounced in light eumelanic nestlings than in darker nest mates. To examine whether such an effect may be genetically determined, we swapped hatchlings between randomly chosen pairs of nests. We first showed that corticosterone affects growth and, thus, shapes the phenotype. Second, we found that under corticosterone administration, nestlings with large black spots grew better than nestlings with small black spots. As in the barn owl the expression of eumelanin-based coloration is heritable and not sensitive to environmental conditions, it is therefore a reliable, genetically based sign of the ability to cope with an increase in blood corticosterone level.     

Melanin‐Based Coloration in Juvenile Kestrels ( Falco tinnunculus) Covaries with Anti‐Predatory Personality Traits

Recent studies have shown that melanin‐based coloration is associated with the ability to cope with stressful environments, potentially explaining why coloration covaries with anti‐predator behaviours, boldness and docility. To investigate whether these relationships are consistent across species, we performed a study in the European kestrel ( Falco tinnunculus). Similar to our results found previously in the barn owl ( Tyto alba), nestling kestrels displaying a larger sub‐terminal black tail band stayed on their back longer (tonic immobility test) and breathed at a lower rate than individuals with a smaller black band when handled. However, in contrast to barn owls, nestling kestrels with a larger black tail band were more aggressive and more agitated. Our results strengthen the hypothesis that melanin coloration is related to stress response and in turn to the reaction to predators, a very important personality trait (i.e. boldness).     

Breeding rate is associated with pheomelanism in male and with eumelanism in female barn owls

Melanin-based coloration exists in 2 types: black eumelanismand reddish-brown pheomelanism, which both have a strong heritablecomponent. To test whether these 2 types of melanism are associatedwith alternative adaptations, we carried out a correlative studyover 8 years and an experiment in a Swiss population of barnowls, Tyto alba. This species varies in coloration from reddish-brownto white and from lightly to heavily marked with black spots.Based on the fact that plumage coloration and spottiness aremale- and female-specific secondary sexual characters, respectively,we examined whether the probability of breeding is associatedwith the degree of pheomelanism in males and of eumelanism infemales. In males, recruited nestlings were significantly lessreddish-brown than their nonrecruited nest mates. In females,individuals displaying larger black spots started to breed ata younger age and had a higher survival, and females with experimentallyreduced plumage spottiness bred less often than control females.Therefore, in the barn owl, the degree of male pheomelanismis associated with the probability of being recruited in thelocal population, whereas the degree of female eumelanism correlateswith age at sexual maturity, survival probability, and alsothe probability of skipping reproduction.     

Female plumage spottiness signals parasite resistance in the barn owl (Tyto alba)

The hypothesis that extravagant ornaments signal parasite resistancehas received support in several species for ornamented malesbut more rarely for ornamented females. However, recent theorieshave proposed that females should often be under sexual selection,and therefore females may signal the heritable capacity toresist parasites. We investigated this hypothesis in the sociallymonogamous barn owl, Tyto alba, in which females exhibit on average more and larger black spots on the plumage than males,and in which males were suggested to choose a mate with respectto female plumage spottiness. We hypothesized that the proportionof the plumage surface covered by black spots signals parasiteresistance. In line with this hypothesis, we found that theectoparasitic fly, Carnus hemapterus, was less abundant onyoung raised by more heavily spotted females and those flieswere less fecund. In an experiment, where entire clutches werecross-fostered between nests, we found that the fecundity ofthe flies collected on nestlings was negatively correlatedwith the genetic mother's plumage spottiness. These resultssuggest that the ability to resist parasites covaries with theextent of female plumage spottiness. Among females collecteddead along roads, those with a lot of black spots had a smallbursa of Fabricius. Given that parasites trigger the developmentof this immune organ, this observation further suggests thatmore spotted females are usually less parasitized. The same analyses performed on male plumage spottiness all provided non-significant results. To our knowledge, this study is the first one showingthat a heritable secondary sexual characteristics displayedby females reflects parasite resistance.     

Local adaptation and matching habitat choice in female barn owls with respect to melanic coloration

Local adaptation is a major mechanism underlying the maintenance of phenotypic variation in spatially heterogeneous environments. In the barn owl (Tyto alba), dark and pale reddish-pheomelanic individuals are adapted to conditions prevailing in northern and southern Europe, respectively. Using a long-term dataset from Central Europe, we report results consistent with the hypothesis that the different pheomelanic phenotypes are adapted to specific local conditions in females, but not in males. Compared to whitish females, reddish females bred in sites surrounded by more arable fields and less forests. Colour-dependent habitat choice was apparently beneficial. First, whitish females produced more fledglings when breeding in wooded areas, whereas reddish females when breeding in sites with more arable fields. Second, cross-fostering experiments showed that female nestlings grew wings more rapidly when both their foster and biological mothers were of similar colour. The latter result suggests that mothers should particularly produce daughters in environments that best match their own coloration. Accordingly, whiter females produced fewer daughters in territories with more arable fields. In conclusion, females displaying alternative melanic phenotypes bred in habitats providing them with the highest fitness benefits. Although small in magnitude, matching habitat selection and local adaptation may help maintain variation in pheomelanin coloration in the barn owl.     

Sexually dimorphic melanin‐based colour polymorphism,feather melanin content,and wing feather structure in the barn owl (Tyto alba)

Feathers confer protection against biophysical agents and determine flying ability. The geometry and arrangement of the barbs, together with the keratin and pigments deposited in the feathers, determine the mechanical stability of the vane, and its stiffness and resistance to abrasive agents. In colour‐polymorphic species, individuals display alternative colour morphs, which can be associated with different foraging strategies. Each morph may therefore require specific flying abilities, and their feathers may be exposed to different abrasive agents. Feathers of differently coloured individuals may thus have a specific structure, and colour pigments may help resist abrasive agents and improve stiffness. We examined these predictions in the barn owl (Tyto alba), a species for which the ventral body side varies from white to dark reddish pheomelanic, and in the number and size of black spots located at the tip of the feathers. White and reddish birds show different foraging strategies, and the size of black feather spots is associated with several phenotypic attributes. We found that birds displaying a darker reddish coloration on the ventral body side deposit more melanin pigments in their remiges, which also have fewer barbs. This suggests that wear resistance increases with darkness, whereas feathers of lighter coloured birds may bend less easily. Accordingly, individuals displaying a lighter reddish coloration on the ventral body side, and those displaying larger black spots, displayed more black transverse bars on their remiges: as larger‐spotted individuals are heavier and longer‐winged birds also have more transverse bars, these bars may reduce feather bending when flying. We conclude that differently coloured individuals produce wing feathers of different strengths to adopt alternative behavioural and life history strategies. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 109 , 562–573.     

Selection on a eumelanic ornament is stronger in the tropics than in temperate zones in the worldwide‐distributed barn owl

Spatial variation in the pattern of natural selection can promote local adaptation and genetic differentiation between populations. Because heritable melanin‐based ornaments can signal resistance to environmentally mediated elevation in glucocorticoids, to oxidative stress and parasites, populations may vary in the mean degree of melanic coloration if selection on these phenotypic aspects varies geographically. Within a population of Swiss barn owls (Tyto alba), the size of eumelanic spots is positively associated with survival, immunity and resistance to stress, but it is yet unknown whether Tyto species that face stressful environments evolved towards a darker eumelanic plumage. Because selection regimes vary along environmental gradients, we examined whether melanin‐based traits vary clinally and are expressed to a larger extent in the tropics where parasites are more abundant than in temperate zones. To this end, we considered 39 barn owl species distributed worldwide. Barn owl species living in the tropics displayed larger eumelanic spots than those found in temperate zones. This was, however, verified in the northern hemisphere only. Parasites being particularly abundant in the tropics, they may promote the evolution of darker eumelanic ornaments.     

Eumelanin- and pheomelanin-based colour advertise resistance to oxidative stress in opposite ways

The control mechanisms and information content of melanin-based colourations are still debated among evolutionary biologists. Recent hypotheses contend that molecules involved in melanogenesis alter other physiological processes, thereby generating covariation between melanin-based colouration and other phenotypic attributes. Interestingly, several molecules such as agouti and glutathione that trigger the production of reddish-brown pheomelanin have an inhibitory effect on the production of black/grey eumelanin, whereas other hormones, such as melanocortins, have the opposite effect. We therefore propose the hypothesis that phenotypic traits positively correlated with the degree of eumelanin-based colouration may be negatively correlated with the degree of pheomelanin-based colouration, or vice versa. Given the role played by the melanocortin system and glutathione on melanogenesis and resistance to oxidative stress, we examined the prediction that resistance to oxidative stress is positively correlated with the degree of black colouration but negatively with the degree of reddish colouration. Using the barn owl (Tyto alba) as a model organism, we swapped eggs between randomly chosen nests to allocate genotypes randomly among environments and then we measured resistance to oxidative stress using the KRL assay in nestlings raised by foster parents. As predicted, the degree of black and reddish pigmentations was positively and negatively correlated, respectively, with resistance to oxidative stress. Our results reveal that eumelanin- and pheomelanin-based colourations can be redundant signals of resistance to oxidative stress.     

Regulation of stress response is heritable and functionally linked to melanin‐based coloration

Sexual selection theory posits that ornaments can signal the genetic quality of an individual. Eumelanin‐based coloration is such an ornament and can signal the ability to cope with a physiological stress response because the melanocortin system regulates eumelanogenesis as well as physiological stress responses. In the present article, we experimentally investigated whether the stronger stress sensitivity of light than dark eumelanic individuals stems from differential regulation of stress hormones. Our study shows that darker eumelanic barn owl nestlings have a lower corticosterone release after a stressful event, an association, which was also inherited from the mother (but not the father) to the offspring. Additionally, nestlings sired by darker eumelanic mothers more quickly reduced experimentally elevated corticosterone levels. This provides a solution as to how ornamented individuals can be more resistant to various sources of stress than drab conspecifics. Our study suggests that eumelanin‐based coloration can be a sexually selected signal of resistance to stressful events.     

Temporal variation in glucocorticoid levels during the resting phase is associated in opposite way with maternal and paternal melanic coloration

Sex‐dependent selection can help maintain sexual dimorphism. When the magnitude of selection exerted on a heritable sex trait differs between the sexes, it may prevent each sex to reach its phenotypic optimum. As a consequence, the benefit of expressing a sex trait to a given value may differ between males and females favouring sex‐specific adaptations associated with different values of a sex trait. The level of metabolites regulated by genes that are under sex‐dependent selection may therefore covary with the degree of ornamentation differently in the two sexes. We investigated this prediction in the barn owl, a species in which females display on average larger black spots on the plumage than males, a heritable ornament. This melanin‐based colour trait is strongly selected in females and weakly counter‐selected in males indicating sex‐dependent selection. In nestling barn owls, we found that daily variation in baseline corticosterone levels, a key hormone that mediates life history trade‐offs, covaries with spot diameter displayed by their biological parents. When their mother displayed larger spots, nestlings had lower corticosterone levels in the morning and higher levels in the evening, whereas the opposite pattern was found with the size of paternal spots. Our study suggests a link between daily regulation of glucocorticoids and sex‐dependent selection exerted on sexually dimorphic melanin‐based ornaments.     

Extra-pair paternity, testes size and testosterone level in relation to colour polymorphism in the barn owl Tyto alba

In many bird populations, individuals display one of several genetically inherited colour morphs. Colour polymorphism can be maintained by several mechanisms one of which being frequency-dependent selection with colour morphs signalling alternative mating strategies. One morph may be dominant and territorial, and another one adopt a sneaky behaviour to gain access to fertile females. We tested this hypothesis in the barn owl Tyto alba in which coloration varies from reddish-brown to white. This trait is heritable and neither sensitive to the environment in which individuals live nor to body condition. In Switzerland, reddish-brown males were observed to feed their brood at a higher rate and to produce more offspring than white males. This observation lead us to hypothesize that white males may equalise fitness by investing more effort in extra-pair copulations. This hypothesis predicts that lighter coloured males produce more extra-pair young, have larger testes and higher levels of circulating testosterone. However, our results are not consistent with these three predictions. First, paternity analyses of 54 broods with a total of 211 offspring revealed that only one young was not sired by the male that was feeding it. Second, testes size was not correlated with male plumage coloration suggesting that white males are not sexually more active. Finally, in nestlings at the time of feather growth testosterone level was not related to plumage coloration suggesting that this androgen is not required for the expression of this plumage trait. Our study therefore indicates that in the barn owl colour polymorphism plays no role in the probability of producing extra-pair young.     

Corticosterone Promotes Scramble Competition Over Sibling Negotiation in Barn Owl Nestlings (Tyto alba)

In species with parental care, siblings compete for access to food resources. Typically, they vocally signal their level of need to each other and to parents, and jostle for the position in the nest where parents deliver food. Although food shortage and social interactions are stressful, little is known about the effect of stress on the way siblings resolve the conflict over how food is shared among them. Because glucocorticoid hormones mediate physiological and behavioral responses to stressors, we tested whether corticosterone, the main glucocorticoid in birds, modulates physical and vocal signaling used by barn owl siblings (Tyto alba) to compete for food. Although corticosterone-implanted (cort-) nestlings and placebo-nestlings were similarly successful to monopolize food, they employed different behavioral strategies. Compared to placebo-nestlings, cort-individuals reduced the rate of vocally communicating with their siblings (but not with their parents) but were positioned closer to the nest-box entrance where parents predictably deliver food. Therefore, corticosterone induced nestlings to increase their effort in physical competition for the best nest position at the expense of investment in sib?Csib communication without modifying vocal begging signals directed to parents. This suggests that in the barn owl stress alters nestlings?? behavior and corticosterone could mediate the trade-off between scramble competition and vocal sib?Csib communication. We conclude that stressful environments may prevent the evolution of sib?Csib communication as a way to resolve family conflicts peacefully.     

Sleep and vigilance linked to melanism in wild barn owls

Understanding the function of variation in sleep requires studies in the natural ecological conditions in which sleep evolved. Sleep has an impact on individual performance and hence may integrate the costs and benefits of investing in processes that are sensitive to sleep, such as immunity or coping with stress. Because dark and pale melanic animals differentially regulate energy homeostasis, immunity and stress hormone levels, the amount and/or organization of sleep may covary with melanin‐based colour. We show here that wild, cross‐fostered nestling barn owls (Tyto alba) born from mothers displaying more black spots had shorter non‐REM (rapid eye movement) sleep bouts, a shorter latency until the occurrence of REM sleep after a bout of wakefulness and more wakefulness bouts. In male nestlings, the same sleep traits also correlated with their own level of spotting. Because heavily spotted male nestlings and the offspring of heavily spotted biological mothers switched sleep–wakefulness states more frequently, we propose the hypothesis that they could be also behaviourally more vigilant. Accordingly, nestlings from mothers displaying many black spots looked more often towards the nest entrance where their parents bring food and towards their sibling against whom they compete. Owlets from heavily spotted mothers might invest more in vigilance, thereby possibly increasing associated costs due to sleep fragmentation. We conclude that different strategies of the regulation of brain activity have evolved and are correlated with melanin‐based coloration.     

Supplemental food increases ornamentation of male nestling Eastern Bluebirds

Although the condition‐dependence and signaling function of ornamental plumage coloration among adult males is well studied, less research has focused on the information content of ornamental coloration among juvenile birds. Eastern Bluebird (Sialia sialis) nestlings grow their nuptial plumage while in the nest and dependent on parents for food, making them an ideal species for studying the development and function of elaborate plumage. Previous research suggests that plumage brightness of Eastern Bluebirds functions, in the juvenile stage, in parent–offspring interactions as a sexually selected trait in adults. Using an experimental approach, we tested the effects of supplemental food on the structural plumage coloration (i.e., tips of primary feathers) of Eastern Bluebird nestlings in Watauga County, North Carolina, during the 2011 breeding season. We provided supplemental mealworms daily to breeding pairs from the onset of incubation through the nestling period, and measured plumage brightness, UV chroma, and mass of nestlings (N = 89 males and 71 females). Male nestlings of supplementally fed parents exhibited brighter plumage. The mass and UV chroma of young bluebirds were not significantly affected by food supplementation. However, the relationship between mass and brightness differed between male nestlings in the control and supplementally fed treatments. Males reared in food‐supplemented territories exhibited a positive relationship between color and mass. Nestlings in control territories, however, exhibited a negative relationship between size and brightness, suggesting that reduced food availability results in a tradeoff between allocating resources toward somatic growth and development of bright plumage. Our results suggest that UV‐blue structural plumage in male juvenile Eastern Bluebirds is at least partially condition‐dependent and may help to explain why plumage color can influence social interactions in Eastern Bluebirds.     

Female‐ and male‐specific signals of quality in the barn owl

Most bird studies of female signalling have been confined to species in which females display a male‐ornament in a vestigial form. However, a great deal of benefit may be gained from considering phenotypic traits that are specific to females. This is because (1) sex‐specific traits may signal sex‐specific qualities and (2) females may develop a male‐ornament not because they are selected to do so, but because fathers transmit to daughters the underlying genes for its expression (genetic correlation between the sexes). We investigated these two propositions in the barn owl Tyto alba, a species in which male plumage is lighter in colour and less marked with black spots than that of females. Firstly, we present published evidence that female plumage spottiness reflects parasite resistance ability. We also show that male plumage coloration is correlated with reproductive success, male feeding rate and heart mass. Secondly, cross‐fostering experiments demonstrate that plumage coloration and spottiness are genetically correlated between the sexes. This implies that if a given trait value is selected in one sex, the other sex will indirectly evolve towards a similar value. This prediction is supported by the observation that female plumage coloration and spottiness resembled that of males, in comparisons at the level of Tyto alba alba populations, Tyto alba subspecies and Tyto species. Our results therefore support the hypothesis that sex‐specific traits signal sex‐specific qualities and that a gene for a sex‐specific trait can be expressed in the other sex as the consequence of a genetic correlation between the sexes.     

Features of begging calls reveal general condition and need of food of barn swallow (Hirundo rustica) nestlings

Altricial offspring of birds solicit food provisioning by complexbegging displays, implying acoustic and visual signals. Differentcomponents of begging behavior may function as reliable signalsof offspring state and thus reproductive value, on which parentsbase optimal parental decisions about allocation of criticalresources (e.g., food). We experimentally manipulated componentsof general condition of nestling barn swallows (Hirundo rustica)by (1) altering brood size by cross-fostering an unbalanced number of nestlings between pairs of synchronous broods andthus manipulating the level of within-brood competition forfood, (2) injecting some nestlings with a harmless immunogen,simulating an infection, and (3) preventing part of the nestlingsfrom receiving food for a short period while establishing controlgroups. We recorded rate of begging response by individual nestlings as parents visited the nest and recorded begging calls usinga DAT recorder to analyze six sonagraphic features of vocalizations.Our factorial experiment revealed that nestlings deprived offood begged more frequently when parents visited the nest comparedto their non—food-deprived nest mates. Food deprivationincreased duration of syllables forming begging calls, whereas brood size enlargement resulted in increased latency of responseto parental calls. Heavy nestlings in good body condition vocalizedat a relatively low peak frequency. To our knowledge, thisis the first study in which begging rate and sonagraphic structureof begging calls are shown to reliably reveal a diverse setof components of offspring general state, on which parental decisions may be based.     

Sex‐linked inheritance,genetic correlations and sexual dimorphism in three melanin‐based colour traits in the barn owl

Theory states that genes on the sex chromosomes have stronger effects on sexual dimorphism than genes on the autosomes. Although empirical data are not necessarily consistent with this theory, this situation may prevail because the relative role of sex‐linked and autosomally inherited genes on sexual dimorphism has rarely been evaluated. We estimated the quantitative genetics of three sexually dimorphic melanin‐based traits in the barn owl (Tyto alba), in which females are on average darker reddish pheomelanic and display more and larger black eumelanic feather spots than males. The plumage traits with higher sex‐linked inheritance showed lower heritability and genetic correlations, but contrary to prediction, these traits showed less pronounced sexual dimorphism. Strong offspring sexual dimorphism primarily resulted from daughters not expressing malelike melanin‐based traits and from sons expressing femalelike traits to similar degrees as their sisters. We conclude that in the barn owl, polymorphism at autosomal genes rather than at sex‐linked genes generate variation in sexual dimorphism in melanin‐based traits.     

Insularity and the evolution of melanism,sexual dichromatism and body size in the worldwide‐distributed barn owl

Island biogeography has provided fundamental hypotheses in population genetics, ecology and evolutionary biology. Insular populations usually face different feeding conditions, predation pressure, intraspecific and interspecific competition than continental populations. This so‐called island syndrome can promote the evolution of specific phenotypes like a small (or large) body size and a light (or dark) colouration as well as influence the evolution of sexual dimorphism. To examine whether insularity leads to phenotypic differentiation in a consistent way in a worldwide‐distributed nonmigratory species, we compared body size, body shape and colouration between insular and continental barn owl (Tyto alba) populations by controlling indirectly for phylogeny. This species is suitable because it varies in pheomelanin‐based colouration from reddish‐brown to white, and it displays eumelanic black spots for which the number and size vary between individuals, populations and species. Females are on average darker pheomelanic and display more and larger eumelanic spots than males. Our results show that on islands barn owls exhibited smaller and fewer eumelanic spots and lighter pheomelanic colouration, and shorter wings than on continents. Sexual dimorphism in pheomelanin‐based colouration was less pronounced on islands than continents (i.e. on islands males tended to be as pheomelanic as females), and on small islands owls were redder pheomelanic and smaller in size than owls living on larger islands. Sexual dimorphism in the size of eumelanic spots was more pronounced (i.e. females displayed much larger spots than males) in barn owls living on islands located further away from a continent. Our study indicates that insular conditions drive the evolution towards a lower degree of eumelanism, smaller body size and affects the evolution of sexual dichromatism in melanin‐based colour traits. The effect of insularity was more pronounced on body size and shape than on melanic traits.     

Evolutionary trade‐off between naturally‐ and sexually‐selected melanin‐based colour traits in worldwide barn owls and allies

Natural selection typically constrains the evolution of sexually‐selected characters. The evolution of naturally‐ and sexually‐selected traits can be intertwined if they share part of their genetic machinery or if sex traits impair foraging success or increase the risk of depredation. The present study investigated phenotypic correlations between naturally‐ and sexually‐selected plumage traits in the Tytonidae (barn owls, grass owls, and masked owls). Phenotypic correlations indicate the extent to which selection on one trait will indirectly influence the evolution of another trait. In this group of birds, the ventral body side varies from white to dark reddish, a naturally‐selected pheomelanin‐based colour trait with important roles in predator–prey interactions. Owls also exhibit eumelanin‐based black spots, for which number and size signal different aspects of individual quality and are used in mate choice. These three plumage traits are strongly heritable and sexually dimorphic, with females being on average darker reddish and more spotted than males. Phenotypic correlations were measured between these three plumage traits in 3958 free‐living barn owls in Switzerland and 10 670 skin specimens from 34 Tyto taxa preserved in museums. Across Tyto taxa, the sexually‐selected plumage spottiness was positively correlated with the naturally‐selected reddish coloration, with redder birds being more heavily spotted. This suggests that they are genetically constrained or that natural and sexual selection are not antagonistically exerted on plumage traits. In a large sample of Swiss nestlings and within 34 Tyto taxa, the three plumage traits were positively correlated. The production of melanin pigments for one plumage trait is therefore not traded off against the production of melanin pigments for another plumage trait. Only in the most heavily‐spotted Tyto taxa do larger‐spotted individuals display fewer spots. This indicates that, at some threshold value, the evolution of many spots constrains the evolution of large spots. These analyses raise the possibility that different combinations of melanin‐based plumage traits may not be selectively equivalent.     

Barn owls display larger black feather spots in cooler regions of the British Isles

Although, in many organisms, genotypes are adapted to specific environmental conditions, the identification of the ecological factors explaining patterns of local adaptation is not a trivial task. In relation to the cosmopolitan barn owl (Tyto alba), its plumage varies from white to dark pheomelanic and shows a difference in the number and size of black spots located at the tip of ventral feathers. The expression of these traits is strongly heritable and weakly sensitive to variation in body condition. Therefore, if owls located in cold or rainy regions are differently plumaged compared to owls living in warm or dry regions, this may not be a result of climate affecting the expression of plumage traits. Instead, different plumages might be selected under different environmental conditions. We have found that, on the British Isles, comparatively larger spots are present on barn owls found in regions that are cooler in summer. This is similar to the findings of a previous study performed in North America and on continental Europe, raising the possibility that larger‐spotted barn owls better cope in cold temperatures during the rearing period or that they are better adapted to some environmental factors prevailing in cooler summers.