ORGANOCHLORINES AND BREEDING SUCCESS IN CATTLE EGRETS FROM THE MEXICALI VALLEY, BAJA-CALIFORNIA, MEXICO

I examined breeding success in the Cattle Egret (Bubulcus ibis), and analyzed organochlorine residues in eggs collected during 1987 and 1988 in the Mexicali Valley, Baja California, Mexico. DDE was found in all 50 of the eggs (geometric mean 3.2 ppm ww); other organochlorines also were detected but at lower concentrations. Eggshell thickness was negatively correlated with DDE (N = 50, P < 0.01, r = -0.42), and with Aroclor 1260 (N = 40, P < 0.01, r = -0.51). The mean eggshell thickness in 1987-88 was 9.3% thinner than the mean for pre-1953 museum clutches, but 8.8% thicker than the mean for Cattle Egret eggshells collected in 1974 from the Salton Sea, California. The mean clutch size for 1988 was similar to that of the Salton Sea heronry in 1974, but 14% to 23% lower than the mean clutch size observed in Texas and the eastern United States. The productivity (number of young raised to three weeks of age per breeding adult) was 32% higher than for the Salton Sea area, but 34% to 58% lower than for Texas and Florida. Minor shell thinning and moderately low egg failures indicated that hatching success was probably not significantly affected by DDE or other organochlorines, thus, the relatively high nestling mortality observed in marked nests (49%) might be associated with unmeasured ecological factors rather than with high DDE levels.