Effects of Mercury Contamination on The Physiology of Purple Martins ( Progne subis )

Abstract

Purple Martins (Progne subis) are insectivorous birds that spend their breeding season in North America before migrating to South America, where they molt. Many individuals migrate to the Amazon basin, a region of high mercury (Hg) contamination, which raises the possibility that observed declines in Purple Martins could be linked to mercury exposure. Hg is a toxic heavy metal; exposure to and bioaccumulation of Hg can result in numerous and systemic negative health outcomes, including disruption of hormones like Corticosterone (B) and triiodothyronine (T3). Both play important roles in metabolic regulation in migratory birds. Feathers, tissues that accumulate Hg and hormones during growth and can thus be used as a proxy to assess a bird's Hg load and endocrine profile. The objective of this study is to assess interrelationships among total Hg (THg), B and T3 concentrations in feathers grown in the Amazon and reproductive (productivity) and body condition (mass, fat score) endpoints. We assayed tail feathers from 80 Purple Martins from their breeding grounds in Florida, Virginia, and Wisconsin. The concentration of THg in Purple Martins’ feathers ranged from 1.103 ug/g dw to 8.740 ug/g dw, which is above the level at which physiological impacts have been observed in studies of other avian species. Hg concentration did not significantly differ by sampling locations or correlate with either B, T3 or nest productivity. However, THg concentration negatively correlated with both mass and fat score. Both B and T3 concentrations were significantly higher in Florida than in the other two states, but neither showed correlations with biometric data or nest productivity. Furthermore, B concentrations in three birds from Florida were much higher than the basal range and may indicate a high stress condition during feather-growth in South America. Taken together, our findings suggest that concentrations of THg in Purple Martins’ feathers are high enough to be a cause of concern. The results show that THg concentration negatively impacts the ability of Purple Martins to accumulate fat, which can present a deleterious effect for a migratory bird. This result also indicates carryover effects of Hg contamination at the wintering grounds to the summer breeding grounds, where fat score was measured. The impact on fat accumulation, however, did not appear to be caused by Hg disruption of T3 or B at the time of feather-growth. Nevertheless, endocrine disruption by Hg could still have happened at a later moment. These physiological impacts suggest a possible mechanism by which high THg exposure in the nonbreeding grounds could negatively impact purple martins and have contributed to observed population declines over the last several decade.