Colour polymorphism in owls is linked to light variability

Abstract

Owls show an astonishing variation in their degree of colour polymorphism, although the exact mechanisms driving such variation remain controversial. Here we address this fundamental question by considering information on all extant owls and recent advances in comparative methods in the frame of three mutually non-exclusive evolutionary scenarios. In addition, we study for the first time whether the evolution of influential ecological characters facilitated the evolution of colour polymorphism (or vice versa). In agreement with the niche divergence hypothesis, we found that species living under more variable luminal conditions, i.e., species with diurnal and crepuscular habits and those inhabiting in a mixture of open and closed habitats, were more likely to show colour polymorphism. Correlated evolution analyses revealed that a change in the luminal niche might be a fundamental requisite for the evolution of colour polymorphism. Moreover, polymorphism was more frequent among owl species occupying lower trophic levels, which could be explained by a particularly high selection for crypsis on small predator owls. Our results, thus, provide support for the idea that colour polymorphism in owls is an adaptive character likely maintained by the selective advantage of morphs under different environmental conditions via disruptive selection mechanisms.

Climate‐driven convergent evolution of plumage colour in a cosmopolitan bird

“…Although Gloger’s rule has been verified in a number of animal populations, spanning from insects (Mayr, ) to birds (Burtt & Ichida, ; Delhey, ; Zink & Remsen, ) and mammals (Kamilar & Bradley, ; Stoner et al, ), its proximate mechanisms are still unclear. The main hypothesis links colour polymorphism with camouflage: dark individuals can be favoured in humid habitats because their colouration is cryptic where low environmental light and dense vegetation are common (Delhey, ; Passarotto et al, ). In the case of a nocturnal predator, like the barn owl, individuals displaying a darker plumage of the ventral body side can be favoured in cloudy, rainy and vegetated environments because their prey cannot easily detect their shape against the dark sky (i.e., when the moon is covered by the clouds or by the tree canopies), as also suggested by preliminary results showing that darker individuals are more efficient in hunting when moonlight is low (our unpublished results).…”

“…Colour patterns are therefore driven by various, often opposing, selective forces, and thus represent compromises between such conflicting pressures acting locally (Cuthill et al, ). Because the strength of distinct selective pressures varies among habitats due to differences in climate and/or lighting conditions (Hill & McGraw, ; Passarotto, Parejo, Penteriani, & Avilés, ) and exposure to parasites and predators, as well as in breeding strategies and sexual behaviour (Cuthill et al, ; Hill & McGraw, ), geographical colour polymorphisms can arise.…”

Determinants of color polymorphism in the Eurasian scops owl Otus scops

“…Data available from the Dryad Digital Repository: <http://dx.doi.org/10.5061/dryad.n88k968> (Parejo et al ).…”

“…Polymorphic species belonging to the order Strigiformes seem to have wider spatial niches than monomorphic species (Galeotti and Rubolini ). Indeed, recent comparative evidence has shown that owls living under more variable luminal conditions, i.e., species with diurnal and crepuscular habits and those inhabiting in a mixture of open and closed habitats, were more likely to show color polymorphism (Passarotto et al ), suggesting that different morphs may have an advantage in different environments. In one of these species, the tawny owl Strix aluco , annual morph frequencies vary over long time in relation to annual weather conditions (Galeotti and Cesaris 1996, Roulin et al ).…”

The evolution of iris colour in relation to nocturnality in owls

“…Iris colour was scored as a binary variable as dark (including black or dark brown eyes) versus bright (including yellow, orange, red and light brown eyes); see Davidson et al () based on colour plates in del Hoyo et al (). Moreover, species were classified regarding activity rhythm as strictly nocturnal species versus species active also during the day or at dusk or dawn as in Passarotto et al (). Activity rhythm information was available for 201 species only (Supplementary material Appendix 1).…”