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Use of Chemical Tracer to Detect Floaters in a Tree Swallow ( Tachycineta bicolor ) Population

In many species of birds, a non-breeding portion of the population spends the breeding season on or near the nesting grounds. This floating subset of the population is often comprised of young or lowquality individuals that can subsequently recruit into the breeding population. However, floaters are difficult to study and most documentation has been the result of nest site manipulation or experimental removal of residents. Here, using a contaminant as a tracer, we show unequivocal recruitment of floater Tree Swallows (Tachycineta bicolor) into a breeding population without experimental manipulation. In 2005, a nest box trail was established along the South River (Virginia, USA) to monitor breeding Tree Swallows exposed to aquatic mercury. Because feather mercury levels reflect exposure during the previous breeding season, we were able to use mercury to detect prior occupancy among new recruits in the population. Through this technique, we estimated that 58% of 79 new breeders had been present in our contaminated study area during the previous breeding season. Future studies could profit from incorporating floater abundance and behavior into evaluations of contaminant impact because floaters at contaminated sites receive more exposure than would be estimated based only on their longevity in the breeding population.