Natural soiling has a small effect on structurally-based plumage coloration

Plumage coloration is an important trait used by birds in mate choice decisions and is often an indicator of social status. The two main types of color-producing mechanisms in feathers are pigment deposition (pigmentary coloration) or the coherent scattering of light reflected from keratin microstructure (structural coloration). External factors acting on the feather surface are also hypothesized to affect structural coloration. Because preening is an energy and time demanding behavior, color variation caused by soiling deposition is generally assumed to strengthen the condition signaling function of plumage coloration. To date, studies using artificial soiling have confirmed those hypotheses. However, information about how natural soiling affects plumage color are still scarce. In this paper, I investigated the effect of natural soiling on structurally-based feather color of blue tits Cyanistes caeruleus. As a method, I applied mechanical cleaning that functionally mimicked natural preening. Removal of soiling caused a decrease of ultraviolet (UV) chroma in males and decrease in brightness in females. According to visual contrast modeling, only changes in brightness should be perceived by birds. Further, more efficient chemical cleaning resulted in a significant increase in brightness in both sexes, presumably due to preen wax removal. These results suggest that the impact of natural feather soiling is not likely to modify structural coloration signaling. One possible explanation is that, under natural conditions, the amount of soil accumulating on feathers is too small to affect coloration. (C) Koninklijke Brill NV, Leiden, 2011.