Posted by: Dan | January 22, 2009

Desert Warbler and Gloger’s Rule

The bird of the month for me was seeing a Desert Warbler, Sylvia nana, with it being the only the eighth recorded bird of its species in Cyprus to date, as I’ve been told. Quite cool, as it simply wanted to hide in the desert shrubs of the area in which it had been hanging around for over a week (in Larnaca), and would allow us to approach these shrubs very closely before flying off to a bush 20m or so away.

As its name implies, this species is an Old World warbler which specializes in semi-desert and other arid environments. Its sand-colored body reflects this habitat preference, making it an excellent representative of Gloger’s rule (wish I had a better reference than Wiki for this, but it’ll do).

Gloger’s Rule is a zoological rule which states that within a species of endotherms, 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 avian plumage color. Gloger found out that birds in more humid habitat tended to be darker than their relatives from regions with higher aridity. Over 90% of the species researched conform to this rule.

An explanation of Gloger’s rule in the case of birds appears to be the increased resistance of dark feathers to feather-degrading bacteria (such as Bacillus licheniformis). Feathers in humid environments have a greater bacterial load and humid environments are more suitable for microbial growth, but dark feathers or hair are more difficult to break down1. More resilient eumelanins – dark brown to black – are deposited in hot and humid regions, whereas in arid regions, pheomelanins – reddish to sandy color – predominate due to the benefit of crypsis.

  1. Burtt, Edward H. Jr. & Ichida, Jann M. (2004): Gloger’s Rule, feather-degrading bacteria, and color variation among Song Sparrows. Condor 106(3): 681-686. DOI

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