Reconciling ecogeographical rules: rainfall and temperature predict global colour variation in the largest bird radiation

Abstract

Ecogeographical rules that associate climate with organismal form and function can reveal patterns of climatic adaptation. Two rules link animal coloration with climate: Gloger's rule (darker coloration where wet and warm), and Bogert's rule (darker coloration where cold). Whereas Gloger's rule was proposed for endotherms, and Bogert's rule for ectotherms, both rules may apply more broadly, despite their seemingly opposing effects. Here, we test this contradiction on a global scale across passerine birds. Consistent with Gloger's rule, birds were darker in wetter areas and, following Bogert's rule, lighter where warm, although birds became lighter again at very low temperatures. Rainfall and temperature had antagonistic or additive effects depending on their pattern of covariation, and this predicted whether birds followed the rules. We integrate both rules into a general framework to explain heterogeneity in climatic effects on coloration, which has implications to understand patterns of diversification, climatic adaptation and climate change impacts.

Climate, crypsis and Gloger’s rule in a large family of tropical passerine birds (Furnariidae)

“…Our Models 1 and 2 showed a positive effect of temperature on brightness, particularly in rainy climates (see below). This is diametrically opposite to Gloger’s (1833) formulation, but in accordance with intraspecific findings in the Black Sparrowhawk Accipiter melanogaster (Amar et al 2014), Savannah Sparrow Passerculus sandwichensis (Rising et al 2009) and Variable Antshrike Thamnophilus caerulescens (Marcondes et al in review), as well as comparative results from analyses of the Australian avifauna (Delhey 2018) and the world’s passerines (Delhey et al 2019). These findings are consistent with Bogert’s rule, a lesser known ecogeographical rule usually considered to apply only to ectothermic animals (Clusella-Trullas et al 2018, Delhey 2018, 2019).…”

“…The test of time—and of modern quantitative techniques—have validated Rensch’s (1936) emphasis on humidity. Intra-(e.g., Rising et al 2009, Amar et al 2014, Marcondes et al in review) and interspecific (e.g., Delhey 2018, Delhey et al 2019) comparisons, including this study, have consistently failed to find support for a tendency for birds to be darker in warmer places. Our Models 1 and 2 showed a positive effect of temperature on brightness, particularly in rainy climates (see below).…”

“…In fact, interspecific comparative analyses are crucial to revealing how evolutionary processes operating within species can be generalized across macroevolutionary scales (Meiri 2011, Stoddard et al 2019). The predicted negative correlation between brightness and humidity has been supported in phylogenetic comparative studies of the world’s primates (Kamilar and Bradley 2011), a large clade of Holartic shrews (Stanchak and Santana 2018), the entire Australian avifauna (Delhey 2018), the world’s woodpeckers (Miller et al 2019), and the world’s passerine birds (Delhey et al 2019). The latter two studies also supported the prediction of Bogert’s rule that animals are lighter in warmer regions.…”

Evolutionary drivers of seasonal plumage colours: colour change by moult correlates with sexual selection, predation risk and seasonality across passerines

“…The illustrations in the HBW have been used to objectively estimate colour in several comparative studies (e.g. Owens & Hartley ; Dey et al ; Dale et al ; Delhey et al ) and correlate well with values obtained from plumage of museum specimens using reflectance spectrometry (Dale et al ; Delhey et al ).…”

“…Sexual dichromatism was estimated using scanned illustrations of passerines from the HBW and the R package ‘colorZapper’ (Valcu & Dale ). We obtained RGB values for nine plumage patches (forehead, crown, nape, back, throat, upper breast, lower breast, belly and vent) for males and females of each species ( N = 5809 species; for detailed methods see Delhey et al ). These plumage patches were chosen as they are considered important for visual signalling and are consistently illustrated for all species (Dale et al ).…”