Abstract
Organisms regularly adjust their physiology and energy balance in response to predictable seasonal environmental changes. Stressors and contaminants have the potential to disrupt these critical seasonal transitions. No studies have investigated how simultaneous exposure to the ubiquitous toxin methylmercury (MeHg) and food stress affects birds' physiological performance across seasons. We quantified several aspects of energetic performance in song sparrows, Melospiza melodia, exposed or not to unpredictable food stress and MeHg in a 2×2 experimental design, over 3 months during the breeding season, followed by 3 months post-exposure. Birds exposed to food stress had reduced basal metabolic rate and non-significant higher factorial metabolic scope during the exposure period, and had a greater increase in lean mass throughout most of the experimental period. Birds exposed to MeHg had increased molt duration, and increased mass:length ratio of some of their primary feathers. Birds exposed to the combined food stress and MeHg treatment often had responses similar to the stress-only or MeHg-only exposure groups, suggesting these treatments affected physiological performance through different mechanisms and resulted in compensatory or independent effects. Because the MeHg and stress variables were selected in candidate models with a ΔAICc lower than 2 but the 95% confidence interval of these variables overlapped zero, we found weak support for MeHg effects on all measures except basal metabolic rate, and for food stress effects on maximum metabolic rate, factorial metabolic scope and feather mass:length ratio. This suggests that MeHg and food stress effects on these measures are statistically identified but not simple and/or were too weak to be detected via linear regression. Overall, combined exposure to ecologically relevant MeHg and unpredictable food stress during the breeding season does not appear to induce extra energetic costs for songbirds in the post-exposure period. However, MeHg effects on molt duration could carry over across multiple annual cycle stages.