Darker where cold and wet: Australian birds follow their own version of Gloger's rule

Abstract

Gloger's rule is usually interpreted as predicting darker coloured animals in warmer and more humid/vegetated regions. The relative importance of temperature and rainfall or vegetation is however unclear, and often only one variable is tested at a time, mainly through proxies. Here, I assess the predictions of Gloger's rule for interspecific achromatic plumage variation (dark to light variation) for an entire avifauna (551 species of Australian landbirds). I tested the effects of climatic variables (temperature and rainfall) and vegetation structure on plumage reflectance at species and assemblage level (100 × 100 km cells), controlling for phylogenetic relatedness and spatial autocorrelation. To assess the robustness of these results I compared observed results with those of a null distribution of effects obtained from repeatedly simulating random plumage reflectance evolution on the phylogeny. At both the species and assemblage level, darker coloured birds were found in wetter and colder regions and in more densely vegetated habitats. Simulations confirm results at the species level and the effect of temperature at the assemblage level, but rainfall and vegetation effects at the assemblage level fall within the distribution of simulated effects and should be interpreted with care. Interspecific colour variation in Australian birds supports Gloger's rule for rainfall/vegetation, but shows the opposite pattern for temperature. Darker colours in wet and vegetated environments are consistent with the role of melanin pigmentation in preventing feather degradation by bacteria, but also with background‐matching for camouflage. Darker plumage might be beneficial in colder regions or detrimental in warmer regions if it affects thermoregulation, a selective force often only assumed to be of importance for ectotherms. The data highlight the need to test the generality of biogeographic rules across levels and at broad scale. Experimental work is needed to confirm the mechanisms linking plumage achromatic variation to climate.

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

“…Indeed, a significant effect of the interaction between these climatic variables was not found in any of the barn owl lineages studied here. A similar pattern was recently shown in two studies of the Australian avifauna (Delhey, ; Friedman & Remeš, ), thus indicating that both rainfall and precipitation can concomitantly act as selective pressures that affect plumage colour traits. The combined effects of these climatic factors, which vary in space depending on latitude, elevation and distance from oceans, which in turn considerably differ among the ranges of distribution of the three barn owl lineages, can thus explain why Western, American and Eastern barn owls show different geographical distributions of their colour morphs (Table ; Figure ; Supporting Information Figure S3; see also Roulin et al, ; Roulin & Salamin, ).…”

“…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, ).…”

“…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).…”

Revealing the colourful side of birds: spatial distribution of conspicuous plumage colours on the body of Australian birds

“…These differences were smaller and not statistically significant in females (Fig. Less conspicuous dorsal regions, together with the already described countershading Théry 2007, Cuthill et al 2016), which also occurs in this sample of Australian birds (Delhey 2018), probably contribute to make birds harder to detect by aerial predators such as birds of prey. Head and ventral regions also showed higher average levels of sexual dichromatism (Fig.…”

Abiotic and biotic predictors of macroecological patterns in bird and butterfly coloration

“…Support for Gloger's rule has been found in birds, mammals, and butterflies (Hovanitz , Zink and Remsen , Kamilar and Bradley , Vanderwerf , Zheng et al. , Delhey ), as well as in plants (Koski and Ashman ). These results suggest that the amount of rainfall in a habitat is also a key aspect of resource availability that will affect the coloration of birds and butterflies, but the relative importance of rainfall remains unknown.…”