Gloger\u27s Rule, Feather-Degrading Bacteria, and Color Variation Among Song Sparrows

Feathers tend to be darkly colored in habitats where relative humidity is high and pale where it is low. We suggest that this correlation, known as Gloger\u27s rule, results, in part, from selection for dark feathers that are more resistant than light feathers to bacterial degradation, which is a severe problem in humid habitats where bacteria thrive and a lesser problem in arid habitats. In May and June 2000–2002 we sampled feather-degrading bacteria (Bacillus licheniformis) from the plumage of Song Sparrows (Melospiza melodia) in southeastern Arizona and northwestern Washington. Under standardized laboratory conditions, feather-degrading bacteria from the plumage of sparrows in the humid Northwest degraded feathers more rapidly and more completely than feather-degrading bacteria from sparrows of the arid Southwest. The differences in feather-degrading bacteria and in relative humidity produce a difference in the intensity of selection, which in turn produces the difference in color described in Gloger\u27s rule