Rethinking Gloger’s Rule: Climate, Light Environments, and Color in a Large Family of Tropical Birds (Furnariidae)

Ecogeographical patterns in owl plumage colouration: Climate and vegetation cover predict global colour variation

“…In addition, the pattern may be due to the response of a specific melanin type, as suggested by the complex version of Gloger’s rule. In contrast with recent research, where more rufous plumages were also associated with cool/wet conditions (Marcondes et al., 2021), our results show that plumage redness in owls clearly increases with temperature (and precipitation) and it is more frequent in warmer regions, outlining a pattern opposite to that expected under the complex version of Gloger’s rule. A body of work in owls has reported a link between melanin‐specific traits and survival along gradients of temperature.…”

“…Together these factors may have led to increased noise in analyses and unclear results (Kamilar & Bradley, 2011). In addition, recent research supports the idea that departures from Gloger's rule might be clade-specific and depend on the taxonomic level or spatial scale considered (Delhey, 2019;Friedman & Remeš, 2017;Marcondes et al, 2021). Accordingly, the effects of temperature on plumage colouration may be strong enough to be detected at different levels of organization (i.e., from species to entire owl assemblages), whereas the reinforcing effects of rainfall on tree cover may have masked an effect of precipitation on both lightness and redness.…”

“…For instance, abundant rainfall is always associated with warm areas (e.g., tropics), whereas very cold regions are characterized by very low precipitation levels (e.g., tundra). Indeed, a recent study on Furnariidae has shown that the effect of precipitation on plumage redness is contingent on temperature (Marcondes et al., 2021). Likewise, vegetation cover might be shaped by temperature and precipitation because denser forests are more frequent in warm and wet areas.…”

“…This finding may be either due to a constraining effect between predictors or to effective differences in selective pressures between species and communities. As opposed to passerines, in which social and sexual significance of plumage elaboration may complicate interpretation of spatial colour patterns (Delhey et al., 2019; Friedman & Remeš, 2017; Marcondes et al., 2021), owl cryptic plumage might be selected primarily for camouflage purposes as darker phenotypes are clearly found in more forested areas. As owl plumage colouration is mostly due to melanins, which may affect physiology and behaviour, our findings may have important implications in the context of global environmental changes and help to understand the possible disruptive effects on predator–prey interactions and/or fitness.…”

“…Gloger’s rule points out that within populations of the same species, or, between different species, more heavily pigmented forms should be found in warm and humid regions, whereas the lighter ones should be more frequent in cold dry regions (see Delhey, 2017). In contrast, ‘the thermal melanism hypothesis’, also termed Bogert’s rule (Bogert, 1949), first postulated for ectotherms but also potentially functioning for endotherms (e.g., Amar et al., 2014; Marcondes et al., 2020, 2021; Rising et al., 2009), states that animals inhabiting cold regions would benefit from having darker colourations as this would allow them to absorb more solar radiation and to improve thermoregulation performance (Clusella Trullas et al., 2007; Pinkert & Zeuss, 2018). While the mechanism causing Bogert’s rule is based on the thermal properties of colouration, the mechanisms behind Gloger’s rule could be diverse, but are still to be elucidated (Delhey, 2019).…”