Wear influences mercury concentrations and stable isotope values in penguin feathers

Stable isotope (SI) values and mercury (Hg) concentrations in animal tissues are used to study migration patterns, investigate trophic ecology, and assess metal contamination. In avian studies, feathers are frequently used because they are assumed to be metabolically inert once fully grown, so their SI values and Hg concentrations remain unchanged over time, allowing investigation of retrospective information. We tested whether feather wear changes SI values (δ13C, δ15N) and Hg concentrations in penguin feathers. Newly grown (new) and recently molted (old) black back and white breast feathers from the same molt cycle were collected from captive African Spheniscus demersus and northern rockhopper Eudyptes moseleyi penguins. In both species, old feathers had a larger proportion of their barbs lacking barbules and reflected more light than new feathers, irrespective of feather color. Mercury concentrations were consistently lower in old than in new feathers in both species, decreasing by up to one third of its original content. This is likely because the highly synchronized feather growth in penguins results in Hg concentration being greatest in the feather tips, which are subject to the greatest wear. Among SI data, δ13C of black feathers tended to increase as feathers aged, whereas those of white feathers decreased with age. The δ15N increased as feathers aged in both black and white feathers. Such subtle changes in feather SI could affect inferences about molt location or trophic ecology. Our findings challenge the assumption that SI values and Hg concentrations, and potentially other biological markers, are fixed in inert tissues, such as feathers.