The possible effect of dirtiness on structurally based ultraviolet plumage

Colourful feathers are important traits in female mate choice in birds because the colour properties of the feathers are often correlated with individual condition during moult. Feather colour can change after moult, and dirt accumulation has been suggested to contribute to this variation. However, we still know little about the influence of dirt on feather colour change, possibly because it is difficult to experimentally manipulate the level of feather dirtiness. We investigated whether reflectance properties of feathers exposed to naturally deposited soil (atmospheric particle deposition such as dust, pollution and smoke) differ from feathers for which this contact was prevented. To achieve this, we compared the spectral colour of throat-breast feathers of European starling, Sturnus vulgaris, kept in the open air (dirty group) with those preserved within a plastic envelope (clean group). Before treatment and three and six weeks after the beginning of the treatment we measured the plumage reflectance of the two groups of feathers. While clean feathers did not change their reflectance spectra, the dirty group showed a reduced reflectance along the entire spectrum (300-700 nm). The reduction in reflectance was particularly pronounced in the UV range (300-400 nm). These preliminary results are consistent with the idea that feather colours are not static signals but are plastic traits. However, much work remains to determine the role of soil and air pollution in altering plumage colouration.