RESUMEN
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 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 of exposure during the breeding season, followed by 3 month post-exposure. Birds exposed to food stress had reduced basal metabolic rates 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-1 ratio of some of their primary feather. 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 delta AICc lower than two but the 95% CI of these variables overlapped zero, we found weak support for MeHg effects on all measures except BMR, and for food stress effects on MMR, factorial metabolic scope and feather mass.length-1 ratio. This suggests that MeHg and 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.
RESUMEN
Avian migration is a challenging life stage susceptible to the adverse effects of stressors, including contaminants like methylmercury (MeHg). Although birds often experience stressors and contaminants concurrently in the wild, no study to date has investigated how simultaneous exposure to MeHg and food stress affects migratory behavior. Our objectives were to determine if MeHg or food stress exposure during summer, alone or combined, has carry-over effects on autumn migratory activity, and if hormone levels (corticosterone, thyroxine) and body condition were related to these effects. We tested how exposure to dietary MeHg and/or food stress (unpredictable temporary food removal) affected migratory behavior in captive song sparrows, Melospiza melodia. Nocturnal activity was influenced by a 3-way interaction between MeHg × stress × nights of the study, indicating that activity changed over time in different ways depending on prior treatments. Thyroxine was not affected by treatment or sampling date. During the migratory season, fecal corticosterone metabolite concentrations increased in birds co-exposed to MeHg and food stress compared to controls, suggesting an additive carry-over effect. As well, during the period of behavioral recording, body condition increased with time in unstressed birds, but not in stressed birds. Fecal corticosterone metabolite concentrations were positively correlated to duration of nocturnal activity, but thyroxine levels and body condition were not. The differences in nocturnal activity between groups suggest that food stress and MeHg exposure on breeding grounds could have direct and indirect carry-over effects that have the potential to affect the fall migration journey.
