What feathers reveal about metal concentrations in bird tissues: An analysis of the global literature

Global bird declines necessitate close monitoring of metal contaminants in birds to understand further potential impacts. Traditionally, this monitoring requires the lethal sampling of liver or kidney so consideration of non-lethal alternatives, such as sampling feathers, is imperative. However, species-level analyses have previously found conflicting results regarding the relationship between metal concentrations in feathers and in internal tissues. In this study, we used metal concentrations collated from the global peer-reviewed avian empirical literature to determine whether feathers provide a reasonable indicator of the metal concentrations of internal tissues (liver, kidney and muscle) and how feather metals vary with life history traits. Our phylogenetically-controlled analyses suggest that feathers can be suitable indicators of the non-essential metals, mercury, lead and cadmium, being most indicative of kidney loads and least indicative of muscle loads, but less reliable indicators of the essential metals, zinc and copper. Life history moderates some feather-liver relationships. Movement strategy and trophic group predicted mercury, copper and zinc concentrations in feathers, whereas moult predicted zinc, with a trend for mercury and lead. For example, species that moulted feathers more than once a year had higher feather zinc but lower feather mercury and lead than species that only moulted feathers once a year. Our study contributes to a knowledge gap regarding correlations between metal concentrations in feathers and in internal tissues and how these correlations vary with life history. The work makes an important contribution to our understanding of the value of using feathers to monitor metal body burdens in birds.