Population Trends and Environmental Contaminants in Herons in the Tennessee Valley, 1980-81

Abstract

-Great Blue Heron (Ardea herodias) eggs (N = 40) collected in 1980 from four of the largest colonies in the Tennessee Valley contained organochlorine pesticide, polychlorinated biphenyl (PCB), and chromium concentrations below those associated with reduced productivity. Low concentrations of organochlorine pesticide and PCB residues also were found in eggs (N = 31) from three of the larger Black-crowned Night-Heron (Nycticorax nycticorax) colonies in the Tennessee Valley. However, DDE concentrations in two of the Black-crowned Night-Heron eggs exceeded levels associated with reduced nesting success. Mercury was found in all eggs of both species with residues ranging to a high of 2.0 ppm; residues of this magnitude have not been identified as reducing nesting success of either of these two species. Green-backed Heron (Butorides striatus) eggs collected in 1981 near a former DDT manufacturing site in the Tennessee Valley had the highest DDE concentrations ever reported for this species; the effect of these high concentrations on productivity is not known. Eggshell thickness of Great Blue Heron, Green-backed Heron, and Black-crowned NightHeron eggs averaged 7.5%, 7.6%, and 3% thinner, respectively, than shell thickness of eggs collected before 1947. This amount of thinning is not deleterious; intraclutch variation in shell thickness is often this high. Shell thickness in all three species was correlated (P < 0.1) with DDE concentrations. The number of nesting pairs of Great Blue Herons and Black-crowned Night-Herons at each of the colonies studied has been stable or increasing during the previous decade. These population data, combined with the residue data, suggest that organochlorine pesticides, PCBs, mercury, and chromium are not adversely affecting these populations. However, we did not assess nesting success which would be a requisite for confirming this.