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

Abstract

Aim Ecogeographical rules link animal colours, especially those produced by melanin pigments, with variation in environmental conditions over wide geographical scales. In particular, Gloger’s rule, coined for endothermic animals in two versions, suggests that tegument darkness should increase at high temperature, as well as in highly humid environments. On the other hand, the thermal melanism hypothesis predicts that darker colourations should be more frequent in colder areas given their thermoregulation benefits. Location Global. Time period Contemporary. Major taxa studied Strigiformes. Methods Here, we provide a global comparative test of these contrasting expectations in all extant owls (n = 198 species), a group of nocturnal birds displaying huge variability in the degree of melanin‐based plumage colouration and environmental specialization. Combining analyses at both species and assemblage level, we assessed the climatic and environmental variables explaining variation in plumage lightness and redness across broad geographical gradients. Results Darker and redder owl phenotypes are more likely found near the equator. Species and assemblage level analyses reveal that owls have more frequently darker and redder plumages in warmer regions. In addition, owl species living in more vegetated areas are darker, and owl assemblages show darker colours in wetter areas. Main conclusions Global patterns of colour variation in owls do not fit expectations from the thermal melanism hypothesis but support Gloger’s rule. Our findings also stress that several alternative selective forces may explain climatic effects on colouration over large geographical scales. Experimental work is urged to uncover the possible mechanisms behind the detected associations between owl colour and environmental variables.

Deep learning image segmentation reveals patterns of UV reflectance evolution in passerine birds

“…According to Gloger's rule, darker colours are associated with higher temperature and humidity and provide protection from solar radiation 53 . For example, Passarotto et al 54 found higher degrees of melanism (darker and redder colours) in owl plumage towards the equator, consistent with Gloger's rule. Other studies have found support for a role of pigmentation in protection from UV radiation (e.g.…”

Regional variation in climate change alters the range‐wide distribution of colour polymorphism in a wild bird

“…Our study adds a spatiotemporal complement to otherwise scarce studies linking microevolutionary patterns associated with climate change (Gienapp et al, 2008) and the emergence of Gloger's rule (Delhey, 2019;Delhey et al, 2020;Passarotto et al, 2021).…”

Ecological factors drive the divergence of morphological, colour and behavioural traits in cactus wrens (Aves, Troglodytidae)

“…The complex Gloger's rule has been explored insufficiently in birds and mammals. The few studies to date support patterns of geographical colour variation in crimson rosella ( Platycercus elegans ) [15], the variable antshrike ( Thamnophilus caerulescens ) [9], Furnariidae [8], Meliphagidae and Acanthizidae [78], Strigiformes [79] and feral pigs ( Sus scrofa ) [14]. A possible explanation in birds is that in warm and wet environments, dark feathers and pelage confer resistance to microbial degradation as melanin increases the abrasion resistance and keratin thickness [16,79].…”

“…The few studies to date support patterns of geographical colour variation in crimson rosella ( Platycercus elegans ) [15], the variable antshrike ( Thamnophilus caerulescens ) [9], Furnariidae [8], Meliphagidae and Acanthizidae [78], Strigiformes [79] and feral pigs ( Sus scrofa ) [14]. A possible explanation in birds is that in warm and wet environments, dark feathers and pelage confer resistance to microbial degradation as melanin increases the abrasion resistance and keratin thickness [16,79]. Darker colours are also selected in low-light environments due to dense vegetation favouring crypsis [8,80,81].…”