Within-clutch and among-clutch variation of organochlorine residues in eggs of black-crowned night-herons

Within-clutch variability of DDE and PBC residues in eggs from 62 clutches of black-crowned night-herons (Nycticorax nycticorax) was small (12% and 17%) compared to among-clutch variability (88% and 83%). Significant correlations between concentrations of DDE median r = 0.885) and of PCBs (median r = 0.8244) occurred when 501 correlations were run on two randomly selected eggs from within the same clutch; no significant correlation occurred for either concentrations of DDE (median r = 0.0353) or PCBs (median r = 0.0843) when eggs were not restricted to the same clutch but were restricted to the same colony. The probability of finding infrequently detected organochlorine contaminants (e.g., DDT, cis-chlordane) in eggs from the same clutch varied from 43-96% and increased as the chemical became more prevalent and the number of eggs per clutch became egg technique, that the chemical residues in one egg in a clutch accurately reflect residues in the remaining eggs of the clutch.