Mercury exposure of purple martins ( Progne subis ) in Brazil: a comparison between two non-breeding sites

Purple martins (Progne subis) are migratory aerial insectivores that have experienced a 37% population decline over the past five decades. One hypothesized cause for this decline is exposure to contaminants at their non-breeding grounds. However, non-breeding grounds of purple martins extend over 35 degrees latitude in South America, likely resulting in different contaminant exposure scenarios. In this study, we compare total mercury (THg) content in feathers from 247 purple martins from two roosts in Brazil: one in the Amazon Basin and another in the southeast region. We hypothesized that birds in the Amazon region would exhibit the highest feather THg content due to the combined influence of anthropogenic activities and the region's geological features. However, mean THg feather contents were significantly higher in the southeast (3.2 ± 1.6 μg g-1) than in the Amazon Basin (1.77 ± 0.81 μg g-1), likely driven by local and regional environmental factors, including industrialization and historical land-use change. Our findings highlight the importance of understanding how the non-breeding range of migratory species may influence their contaminant exposure. This information will assist in the development of conservation strategies tailored to the unique environmental conditions migratory species are exposed to throughout their annual cycles.