Do metal mines and their runoff affect plumage color? Streak-backed Orioles in Mexico show unexpected patterns

Metal mining causes serious ecological disturbance, due partly to heavy metal (HM) pollution that can accumulate at mining sites themselves and be dispersed downstream as runoff. Plumage coloration is important in birds' social and ecological interactions and sensitive to environmental stressors, and several local-scale studies have found decreased carotenoid-based plumage and/or increased melanin-based plumage in wild birds exposed to HM pollution. We investigated regional-scale effects of proximity to mines and their downstream rivers as a proxy of exposure to HM-contaminated mining waste on plumage coloration in Streak-backed Orioles (Icterus pustulatus) in south-central Mexico. We measured the plumage color of museum skins using reflectance spectrometry and digital photography, then used geographic information systems to estimate each specimen's distance from the nearest mining concession and river and determine whether that river's watershed contained mines. Proximity to mines and their downstream rivers was related to ventral (but not dorsal) carotenoid-based coloration; birds collected farther from mines had more vivid yellow-orange breast plumage, and belly plumage was more vivid and redder with increasing distance from rivers with upstream mines. Breast background reflectance unexpectedly decreased with mine distance and was higher among birds whose nearest river had mines upstream. The area (but not reflectance) of melanin-based plumage was also related to mines. The area of dark back streaks decreased with mine distance, while the bib patch was smaller among birds presumably more exposed to mining waste. While some of these results are consistent with predicted effects of HM pollution on plumage, most were not straightforward, and effects differed among plumage patches and variables. Further investigation is needed to understand the direct (e.g., toxicity, oxidative stress) and/or indirect (e.g., decreased availability of carotenoid-rich food) mechanisms responsible and their individual-, population-, and community-level implications.