PS 48-140 - Effects of methylmercury and food stress on migratory activity in song sparrows, Melospiza melodia

Background/Question/Methods

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, have 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.

Results/Conclusions

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. Over the experiment, birds show a similar change in body condition with time, but during post-exposure stressed birds had stronger increase in body condition than unstressed individuals. However, body condition was not correlated with nocturnal activity. Thyroxine was not affected by treatment or sampling date, and was not correlated with nocturnal activity. Fecal corticosterone decreased in August compared to May or September, but there was no difference between migration period (September) and early exposure period (May). During the migratory season, fecal corticosterone metabolite concentrations increased in birds co-exposed to MeHg and food stress compared to control, suggesting an additive carry-over effect. Fecal corticosterone metabolite concentrations were positively correlated to the duration of nocturnal activity. 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 may affect the timing, speed, and success of fall migration.