Tree swallow Tachycineta bicolor is a common migratory and breeding species along the HR corridor and is predatory on emergent aquatic insects. Tissue burdens of PCBs and their effects on reproductive success were analyzed in tree swallows that occupied artificial nest boxes constructed at three sites downstream of the most PCB-contaminated section of the HR and at control sites that were up-river (Champlain Canal) of the point sources or in Ithaca, New York, a locale distant from the HR.
Despite high tissue burdens in specimens at the Champlain Canal (probably due to their migrations through the PCB contaminated HR corridor), total PCBs and TEQ burdens were still higher in eggs and nestlings from the three HR locales downriver of the PCB hotspots (Secord et al., 1999). Nestlings at the most contaminated sites exhibited two to four times greater concentrations of PCBs than the eggs from the same sites, suggesting that their diets were contaminated with PCBs. The PCB homologue pattern in insects from the HR was similar to that in the tree swallows, suggesting that prey from the HR was the primary source of contamination to the nestlings. TCDD TEQs from PCBs in tree swallows from two of the downstream sites were among the highest ever reported in the avian literature, exceeding those TCDD TEQs linked to adverse health effects in a number of bird species including total reproductive failure (Ludwig et al., 1993).
Nests built along the HR were significantly lighter and had fewer feathers than those at reference sites, indicating their lower quality. Furthermore, the number of fledglings produced was significantly correlated with the number of feathers in nests at the onset of egg-laying. This is among the first evidence of behavioral abnormalities in populations of passerines associated with exposure to PCBs. Nest abandonment and hatchability of eggs differed significantly between the contaminated and control sites resulting in significantly lower fledgling production at the contaminated sites for one of the two years of the study. Laboratory experiments would have predicted 100 percent embryo mortality in the field, which clearly is not the case. Tree swallows maybe particularly resistant to PCB-induced toxicity or that individuals in the HR population may have developed tolerance to these chronically high exposures (McCarty and Secord, 1999a,b).
Stapleton et al. (2001) used DNA fingerprinting to compare mutation rates in tree swallows from two contaminated HR sites to those in cleaner populations. Mutation rate was calculated based on the number of DNA fragments in the fingerprinting patterns of nestlings that did not match those in their parents. Mutation rates were very similar at the contaminated and cleaner sites despite a 182-fold difference in tissue burdens of total PCBs in nestlings. It is likely that the higher chlorinated PCB homologues present in the HR biota are not highly mutagenic.
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