Birds drop off their old feathers, and regrow a new set, in a periodic process known as molting. This makes sense; feathers, though resilient and strong, are not invulnerable, and gradual abrasion causes them to wear and even break. Worn and broken feathers can have negative effects on the fitness of birds by reducing insulation and aerodynamic efficiency. A reduction in insulation or a reduction in aerodynamic efficiency would tend to increase energy expenditure in a bird, and thus could disrupt its energy balance, potentially lowering its survivorship and reproductive success. Thus, abrasion of feathers may have contributed to the evolution of molting because when a bird molts, it gets rid of old, compromised feathers and replaces them with feathers with improved insulation and aerodynamic properties. There is yet another factor, other than simple abrasion, that may cause degradation of feathers, a factor that may make the selective pressure favoring molting all the more powerful . . .

Edward Burtt Jr. and Jann Ichida, scientists at Ohio Wesleyan University, recently discovered that many birds harbor bacteria that release keratin-degrading enzymes that break down feathers [see Burtt and Ichida, 1999. Auk 116:364-372]. From 1993 to 1996, Burtt and Ichida sampled the plumage of 1,588 birds of 83 eastern North American species and they detected feather-degrading bacteria in 7.4, 6.7, 9.4, and 10.7% of the birds sampled in successive years. The bacteria species most commonly found was Bacillus licheniformis. The prevalence of feather-degrading bacteria varied significantly by season and by the foraging ecology of the birds. The bacteria were most abundant in the winter, and least abundant in the spring and summer. Also, the bacteria were least prevalent in aerial foraging insectivores, and were most abundant in ground-foraging birds. These differences raise many interesting questions. For example, are bacterial infestations lower in the summer because the birds are spending more time preening at that time? Are infestations higher in ground-foraging birds because they have more opportunities to pick up the bacteria when on the ground, or because of differences in preening, or differences in exposure to ultraviolet radiation, or because of other reason(s)?

Although infestations of feather-degrading bacteria are higher in the winter than in the summer, the effect of the bacteria may be much more pronounced in the summer. The bacteria occur in two stages, spores, which can remain dormant for long periods, and vegetative cells, which become active in warm, moist conditions. Because warm, moist conditions are more common in the summer, Burtt and Ichida hypothesize, feather-degrading bacteria may have a more pronounced effect on feathers during the summer months. Their data show that the proportion of birds with feather-degrading bacteria drops considerably in September, and then again in March, after birds perform their prebasic and prealternate molts. They suggest that because molting sheds feather-degrading bacteria along with feathers, the existence of these bacteria could have contributed to the evolution of molting in birds. The questions raised by Burtt and Ichida's interesting study will undoubtedly be the subject of exciting future work in this area.