Regional variation and relationships between the contaminants dde and selenium and stable isotopes in swallows nesting along the Rio Grande and one reference site, Texas, USA.Isotopes Environ Health Stud. 2005 Mar; 41(1):69-85.IE
Cave swallows (Petrochelidon fulva) and cliff swallows (P. pyrrhonota) nest in numerous colonies throughout the Texas portion of the Rio Grande along the U.S. border with Mexico. We collected swallows during 1999 and 2000 from eight locations along the Rio Grande to determine if delta15N and delta13C values could be used to predict 1,1-di-(p-chlorophenyl-)2,2-dichloroethene (DDE) and selenium (Se) contaminant burdens in insectivorous birds nesting across a geographic gradient in the Texas-Mexico border and to discern if stable isotopes could help discriminate between local versus nonlocal acquisition of contaminants. We analysed delta15N and delta13C in liver and muscle and DDE and Se in swallow carcasses. Within individuals, delta15N was higher in liver than in muscle of both species by an average of 1.34%, whereas delta13C was 0.145% higher in muscle than in liver. Significant differences occurred among locations in delta15N and delta13C values in liver and muscle of both species. Cave swallows from three locations in the Lower Rio Grande Valley were more enriched in delta15N than swallows from other sites. In general, swallows nesting in more northern latitudes along the Rio Grande had lower delta15N and delta13C values than those nesting farther south. Concentrations of DDE were significantly greater in swallows from El Paso, Llano Grande, and Pharr than in those from Brownsville, Falcon Lake, Laredo, Del Rio, and a reference site outside the Rio Grande. All swallows (n = 21) from El Paso, Llano Grande, and Pharr had DDE concentrations > or = 3 microg g(-1) wet weight (ww), a value three times greater than the estimated threshold in avian prey that could cause potential reproductive failures in raptors. Concentrations of Se also were significantly greater in El Paso and Del Rio than at other locations. Most Se concentrations were not of concern for direct effects on birds or their predators. Principal component analysis indicated some positive correlations between delta15N and delta13C values in tissues and contaminant concentrations in carcass; however, analysis of covariance suggested a stronger effect of location on concentrations of DDE and Se. At the local level (Llano Grande and Pharr) there was a significant positive correlation between delta15N in liver and DDE concentrations in swallow carcasses; however, Se concentrations were not significantly correlated with isotopes even at the local level. Our results provide a good database of delta15N and delta13C values for insectivorous birds nesting along the Rio Grande. Similar ecologies for cave and cliff swallows and their abundance and wide distribution along the Rio Grande make them ideal indicators of environmental pollution of this portion of the Texas-Mexico border.