Flaws and pitfalls in the chemical analysis of feathers: bad news-good news for avian chemoecology and toxicology.

Ecologists have frequently used biochemical assays as proxies for processes or phenomena too difficult to explore by traditional means of investigation. Feathers have been subjected to a number of chemical analyses to study such things as their elemental composition, contaminants, and hormones. The reliance on standard methodology of using concentrations to express quantities of chemical substances is seriously problematic because it creates artifacts by ignoring the physiology of feathers. Some elements and compounds are incorporated into the feather as part of the very building blocks of the keratin. However, others that are less functionally important to feathers (but not necessarily to the bird) enter the developing cells in proportion to their abundance in the bloodstream; in other words, feathers are merely receptacles, and deposition of chemicals is time dependent. In the latter case, one that applies to much of the work done on feather chemistry, data expressed as concentrations are meaningless because the varying mass across the feather alters concentrations in a way that has no biological significance. I discuss this problem and various pitfalls in the chemical analysis of feathers, and offer solutions that ultimately will offer a better understanding of the mechanisms influencing feather composition and, thus, the ecological patterns and processes they were meant to study.