We sampled bacteria from the plumage of 1,588 individuals of 83 species of birds. Feather-degrading bacteria, those able to extract energy and nutrients by breaking up •-keratin, were isolated from 134 individuals in 32 species. Nine of 11 samples of featherdegrading (keratinolytic) bacteria were identified as Bacillus licheniforrnis, one as B. purnilus, and one as a Bacillus of undetermined species. A strong correlation between occurrence of keratinolytic bacilli and the number of birds sampled per species suggests that feather-degrading bacilli are widespread among birds. The bacillus occurred on 6.7 to 10.7% of birds and showed little annual variation. The incidence of birds with feather-degrading bacilli was highest in late fall and winter and lowest in early spring and late summer. The bacilli occurred most frequently on the venter and less commonly on the dorsum and tail. They occurred most frequently on ground-foraging species and least frequently on aerial-foraging species. Regardless of avian species, time of year, or area of the bird from which the bacilli were isolated, the rate at which bacilli degraded feathers was similar. Because bacilli are active only when conditions are warm and humid, we suggest that they degrade feathers during the summer when the bird becomes wet, for example during thunderstorms. Such feather degradation may contribute to the deterioration of feathers and be a selective force in the evolution and timing of molt.

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's 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's rule.La Regla de Gloger, Bacterias Degradantes de Plumas y Variación de Color en Melospiza melodiaResumen. Las plumas tienden a ser de tonos obscuros en hábitats donde la humedad relativa es alta y más pálidas en hábitats donde la humedad relativa es baja. Esta correlación, conocida como la regla de Gloger, se aplica a muchas especies de aves a través del mundo. Sugerimos que la regla de Gloger es, en parte, un producto evolutivo de la selección por plumas obscuras, que son más resistentes a la degradación bacteriana que las plumas claras. La degradación bacteriana es un problema severo en hábitats húmedos donde prosperan las bacterias y un problema menor en hábitats áridos. En mayo y junio de 2000 a 2002 tomamos muestras de bacterias degradantes de plumas (Bacillus licheniformis) del Melospiza melodia fallax, que tiene plumaje pálido y reside en la parte sureste del estado de Arizona, y comparamos la incidencia y actividad de estas bacterias con las de aquellas encontradas en el plumaje obscuro de M. m. morphna, que reside de los bosques húmedos del noroeste del estado de Washington. Sin embargo, bajo condiciones estandarizadas de laboratorio, las bacterias obtenidas de M. m. morphna, degradaron las plumas más rápida y completamente que las bacterias de M. m. fallax. Las diferencias sugieren que las plumas obscuras de M. m. morphna del noroeste húmedo están sujetas a selección más intensa para resistir la degradación bacteriana que las plumas claras del gorrión del suroeste árido. La diferencia en humedad relativa produce una diferencia en la intensidad de selección, que a su vez produce la diferencia en color descrita en la regla de Gloger.

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