Seasonal and geographical variation of organochlorine residues in birds from northwest Mexico.

Eight species of birds (129 individuals) were collected from three agricultural areas with long histories of pesticide use in northwestern Mexico. Plucked carcasses were analyzed for organochlorine (OC) pesticides and polychlorobiphenyls (PCBs). DDE was found in all of the samples and at higher levels than other OCs. Mean (geometric) DDE concentrations varied from 0.04 (microgram/g) ppm in mourning doves (Zenaida macroura) to 5.05 ppm in double-crested cormorants (Phalacrocorax auritus). Hexachlorocyclohexane (HCH) was detected in 95% of the samples, but at lower levels than DDE. Hexachlorobenzene (HCB) residues were detected more frequently in birds from Mexicali (62%, p less than 0.05) than in those from Yaqui and Culiacan. HCH and HCB concentrations were significantly higher in birds from Mexicali during the winter than in the summer (p less than 0.05), indicating accumulation of these compounds during that period. Other OCs such as DDT, DDD, dieldrin, oxychlordane, heptachlor expoxide, endosulfan, and endrin were found at lower levels and less frequently. PCBs (quantitated as Aroclor 1260) were found mostly in cattle egrets (Bubulcus ibis) and cormorants at the three locations. Overall, concentrations of OCs were higher for Mexicali than for Yaqui and Culiacan (p less than 0.01). In a few cases, DDE levels were above those that might adversely affect birds.