Colors of birds feathers and climate: is there a connection?
Gloger’s rule states that birds living in humid climates tend to have darker pigmentation. Darker feathers are believed to be useful for defense against parasites, temperature regulation, and camouflage.
However, researchers are uncertain about the validity of these explanations. The specifics of how and why feather coloration relates to geographical distribution are not fully understood.
And in fact, popular notions, such as that tropical birds are more colorful than those living in dry climates, may be erroneous.
What is Gloger’rule?
Gloger’s rule is an ecogeographical rule which states that within a species of more heavily pigmented forms tend to be found in more Humid environments, e.g. near the equator. It was named after the zoologist Constantin Wilhelm Lambert Gloger, who first remarked upon this phenomenon in 1833 in a review of covariation of climate and plumage color.
Gloger found that birds in more humid habitats tended to be darker than their relatives from regions with higher aridity. Over 90% of 52 North American bird species studies conform to this rule.
A recent scientific study appearing in the journal Global Ecology and Biogeography, by Nicholas R. Friedman and Vladimír Remeš, sheds some light on our questions about feathers’ colors.
To better understand the reasons for our feathered friends’ colors, the researchers studied two closely related but distinctive bird families: Honeyeaters (Meliphagidae) and Australasian Warblers (Acanthizidae).
The researchers wanted to compare plumage coloration across these two families to find out if and how environmental factors affect the coloration of their feathers.
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By Sarah Kaiser – 12 jan 2017
The original study
Animal coloration often shows high degrees of evolutionary lability, producing variation among species that is easily apparent. This variation may produce consistent geographical patterns as species converge on adaptive phenotypes in similar environments. Some such geographical patterns in colour variation have been recently predicted as a response to the light environment in different habitats. Others like Gloger’s rule – the negative relationship between brightness and humidity – have long been observed but still demand explanation. Finally, the conventional wisdom that tropical birds are more colourful remains largely untested.
Here, we compared plumage coloration across two families of Australian birds (Meliphagidae, n = 97 species; Acanthizidae, n = 40 species) in a combined spatial and phylogenetic framework. We assessed the extent to which environmental variables extracted from species ranges explain variation in colour traits, while correcting for the autocorrelation inherent in spatially structured data using extensive simulations.
We found several strong effects of environment on plumage coloration. Inland species with ranges marked by high aridity and temperature seasonality showed greater colour span among acanthizids, and greater saturation among meliphagids. Gloger’s rule was supported in both clades, but more strongly for dorsal plumage. The most consistent correlate in this relationship was vegetation: birds in regions with more vegetation had markedly darker plumage. Ornament hue showed no significant associations with vegetation or climate.
Birds living close to the equator were not more colourful, but species inhabiting arid regions were. Species may respond to the shorter and less predictable breeding seasons of arid environments by evolving increased ornamentation. The consistent relationship we observed between vegetation and dorsal brightness supports a primary role for countershading and background matching in Gloger’s rule.