Survival of White Ibises (Eudocimus albus) in response to chronic experimental methylmercury exposure

Although methylated mercury (MeHg) is known to have neurological, immunological, reproductive, and endocrine effects on vertebrates at low environmental exposure levels, effects on survival of exposed birds have not been demonstrated in the wild. Here, we report on survival of the same group of White Ibises (Eudocimus albus) during exposure to 4 levels of dietary MeHg in captivity and later as depurated free-ranging animals. Ibises were chronically exposed in captivity to dietary MeHg in groups at 0 (control), 0.05 (Low), 0.1 (Medium) and 0.3 (High) ppm MeHg ww for 43 months. No differences in annualized survival among captive MeHg groups were seen within age classes. Survival of all ages taken together was significantly lower for Control birds than for Low or Medium dosed birds, but was not different from High dosed birds. While this might be evidence of a hormetic effect, none of the captive results support the prediction that MeHg impairs survival. Using a mark-recapture analysis we found no effects of dose group or of Hg exposure on survival or resight probabilities during the first 99 days post-release to the wild. The latter results suggest that there is no lasting, post-depuration effect of even high MeHg exposure (0.3 ppm ww dietary) on survival. While these results agree with a variety of studies of survival of free-ranging birds, we suggest many survival studies have been confounded by seasonal depuration through molt, and variation in exposure rates. We suggest future studies concentrate on evaluating survival effects during nonmolting periods in species for which methylmercury exposure is relatively constant.