Effects of mercury exposure on the reproductive success of tree swallows (Tachycineta bicolor)

An experimental tree swallow population was established in the headwaters of the Shenandoah River, Virginia, USA to assess the accumulation and effects of mercury contamination on birds that eat emergent aquatic insects. One tributary, the South River, was contaminated with mercury before 1950. Reproductive success of swallows nesting within 50 m of this river was compared to that of three uncontaminated reference tributaries in 2005 and 2006. Female swallows on the contaminated stretch of river had significantly elevated blood and feather total mercury (blood: 3.56 +/- 2.41 ppm ww vs. 0.17 +/- 0.15 ppm reference; feather: 13.55 +/- 6.94 ppm vs. 2.34 +/- 0.87 ppm reference), possibly the highest ever reported for an insectivorous songbird. Insects collected by the swallows to be fed to nestlings averaged 0.97 +/- 1.11 ppm dw total mercury, significantly higher than on reference sites. Swallows in the contaminated area produced fewer fledglings than those in reference areas. The effect of mercury contamination on productivity was detectable only for young females in the contaminated area that were breeding for the first time in 2006, a segment of the population that may already have been stressed by inexperience. Tree swallows served as practical and effective biomonitors for mercury levels and effects and have great potential as proxy biomonitors for more logistically challenging birds such as loons or eagles.