A method to quantify the regularity of barred plumage patterns

Barred plumage is common among bird species. It is often interpreted as a form of crypsis because pigmentation patterns are less detectable against heterogeneous backgrounds. However, it has also been suggested that barred plumage could signal plumage condition, because it makes irregularities in the plumage perceptually salient. Little empirical work exists on the function of barred plumage, in part because we lack a way for quantifying the regularity of the pattern. Here, we present a method to analyze digital photographs of barred plumage and quantify the regularity of its pattern, and also provide software to perform the analysis. Using museum skins of five avian species, we tested under which conditions the method reliably captures differences among conspecific males in the regularity of their barred pattern. We found that measurements of regularity were highly repeatable when lighting conditions were standardized, unlike under variable illumination, and a relatively large area of barred plumage was analyzed, rather than small and non-representative subsections of plumage. The range of conditions under which the method performs well makes it adequate for use in the field, and we hope that it fosters research on the function of barred plumage as a communication signal.