Mercury exposure and survival in free-living tree swallows (Tachycineta bicolor)

Mercury has become a ubiquitous contaminant in food chains worldwide. A large body of literature detailing bioaccumulation and effects on birds has revealed the potential for mercury to adversely impact avian physiology and reproduction. However, the extent to which these effects impair survival remains poorly understood. The objective of this study was to determine whether mercury exposure was associated with reduced annual survivorship in tree swallows (Tachycineta bicolor) breeding at a site with legacy industrial contamination. From 2005 to 2008, we captured and marked 932 adult swallows. We used Cormack-Jolly-Seber models and an information-theoretic approach to test our hypotheses that adult survival varied by sex, breeding location, and cumulative individual mercury exposure. Blood mercury was significantly elevated on contaminated sites (2005-2007 combined mean ± SE: 2.84 ± 0.09 μg/g; reference: 0.17 ± 0.01 μg/g). Model-averaged estimates of female apparent survival ranged from 0.483 to 0.488 on reference sites and 0.473 to 0.477 on contaminated sites. For males, apparent survival ranged from 0.451 to 0.457 on reference sites and 0.444 to 0.448 on contaminated sites. Thus, we observed approximately a 1% difference in survival between mercury-contaminated and reference sites. Such a small difference is unlikely to impact population viability in this short-lived species; however, some songbirds accumulate mercury to a greater degree than tree swallows and do not possess the migratory behavior that removes swallows to less contaminated areas for the majority of the year. Identifying whether such species are at risk of suffering biologically significant reductions in survival should become a focus of future research.