Purple Martins and the concern of environmental mercury contamination

Purple Martins and the concern of environmental mercury contamination
Jonathan Branco, University of Sao Paulo

Purple Martins (Progne subis) are declining migratory birds that breed in North America and overwinter and complete their molt in South America. The eastern North American subspecies of Purple Martins (P. subis subis) comprises over 90% of all Purple Martins and overwinter and molt in the Amazon Basin. This region is high in mercury (Hg) contamination, which raises the possibility that observed declines in Purple Martins could be linked to Hg exposure. Exposure to Hg results in numerous and systemic negative health outcomes, including endocrine disruption. Corticosterone (CORT) is a primary modulator of the stress and metabolic axes of vertebrates; thus, it is important in meeting metabolic and other challenges of migration. Because feathers accumulate Hg and hormones while growing, quantification of Hg and CORT in provide an opportunity to retrospectively assess Hg exposure and adrenal activity of birds using minimally invasive methods. We evaluated interrelationships among total Hg (THg) and CORT concentrations in feathers that grew in the Amazon and body condition (mass, fat score) of these birds in North America. Concentration of THg in Purple Martin feathers ranged from 1.103 to 8.740 µg/g dw, levels associated with negative physiological impacts in other avian species. Concentrations of CORT did not correlate with THg concentration at the time of feather growth. However, we found evidence that THg concentration may negatively impact the ability of Purple Martins to accumulate fat, which could impair migratory performance and survivorship due to the high energy requirement of migration. This finding suggests potential carryover effects of Hg contamination at the wintering grounds in the Amazon to the summer breeding grounds in North America.