Bacteria as an Agent for Change in Structural Plumage Color: Correlational and Experimental Evidence

Abstract

Recent studies have documented that a diverse assemblage of bacteria is present on the feathers of wild birds and that uropygial oil affects these bacteria in diverse ways. These findings suggest that birds may regulate the microbial flora on their feathers. Birds may directly inhibit the growth of harmful microbes or promote the growth of other harmless microbes that competitively exclude them. If keratinolytic (i.e., feather-degrading) bacteria degrade colored feathers, then plumage coloration could reveal the ability of individual birds to regulate microbial flora. We used field- and lab-based methods to test whether male eastern bluebirds (Sialia sialis) with brighter blue structural plumage coloration were better able to regulate their microbial flora than duller males. When we sampled bluebirds in the field, individuals with brighter color had higher bacterial loads than duller individuals. In the lab, we tested whether bacteria could directly alter feather color. We found that keratinolytic bacteria increased the brightness and purity, decreased the ultraviolet chroma, and did not affect the hue of structural color. This change in spectral properties of feathers may occur through degradation of the cortex and spongy layer of structurally colored barbs. These data suggest that bacteria can alter structural plumage color through degradation.

Intraspecific Variation and Interspecific Differences in the Bacterial and Fungal Assemblages of Blue Tit (Cyanistes caeruleus) and Great Tit (Parus major) Nests

“…Recent studies have shown that birds have distinctive feather bacteria that influence plumage condition and colouring [10,11,52], complex conjunctiva and nasal bacterial assemblages [53] and diet-determined differences in gut bacteria [6,23]. Microbial loads and assemblages often differ between bird species (e.g.…”

“…those that decompose keratin through keritinase production) can also be important in the Aves due to their likely role in feather degradation. Microbe-induced feather degradation can potentially hinder flight (when wing feathers are involved) and mate attraction (when plumage condition or colour is important for mate selection), as well as thermoregulation [10,11,52]; although the extent and magnitude of such effects is currently unknown [24]. Keratinolytic microbes previously associated with birds include the bacterium Bacillus licheniformis [47] and the fungi Chrysosporium tropicum and Microsporum gallinae [27].…”

Origin-related differences in plumage coloration within an island population of great tits (Parus major)

“…Not only is it sensitive to developmental stress, but it is also affected by external factors like, for example, feather abrasion, the accumulation of dirt, and the presence of feather-degrading bacteria (e.g., Ö rnborg et al 2002;Shawkey et al 2007). UV coloration is thus, just like carotenoid-based coloration, assumed to reflect phenotypic quality (Keyser and Hill 2000).…”

Differences in Culturable Microbial Communities in Bird Nestboxes According to Orientation and Influences on Offspring Quality in Great Tits (Parus major)

“…Recent research has shown that birds have distinctive plumage bacteria that can influence plumage colour and quality [10,11,59] and gut bacteria that vary according to diet both within and between species [6,18]. The importance, however, of microbial species on avian reproductive success, 56 offspring condition, and life-history traits is still poorly understood [10,35,36].…”