Survival of Adult Tree Swallows (Tachycineta bicolor) at a Site Contaminated by Mercury

Mercury is a heavy metal that has become a ubiquitous contaminant in aquatic and terrestrial ecosystems worldwide, primarily as a result of human activity. While several studies have documented the serious physiological and neurological impairments caused by mercury exposure in birds, few have attempted to examine the effects of mercury on fundamental demographic parameters such as survival and reproductive success. The short time frame in which many ecotoxicological studies are undertaken cannot capture long-term changes in population structure and dynamics. To address these shortcomings, I conducted a multi-year mark-recapture study (2005-2008) to examine whether adult Tree Swallows (Tachycineta bicolor) breeding along the contaminated South River in Virginia, USA suffer reduced annual survival as a result of exposure to mercury. Over the course of the study, I individually marked 932 swallows and monitored their presence on our breeding sites in each year. I used Cormack-Jolly-Seber models for live recaptures of marked individuals to estimate apparent survival and probability of recapture. I evaluated 11 a priori linear models representing my hypotheses about the potential factors affecting survival, including sex, mercury treatment level, and mercury exposure for each individual. Three models containing univariate effects of these variables were supported in the data; the most parsimonious model assumed constant survival over time. Overall, survival declined with increasing individual mercury levels, although the effect of mercury exposure was not statistically significant. Post-hoc analyses suggested that females breeding on contaminated sites tended to exhibit reduced survival at older life stages relative to swallows breeding on reference sites. Because the number of individuals surviving to these older life stages is relatively small, it may be difficult to detect similar age-related differences in population-wide analyses of survival. In some passerine species, reproductive potential increases with age. Thus, reduced survival of the oldest birds could have serious consequences for population dynamics of songbirds in contaminated areas. Future research should attempt to integrate multiple demographic parameters and life history traits to effectively address how contaminants, such as mercury, may affect population dynamics of avian species over longer time scales than have traditionally been considered