DDE-INDUCED EGGSHELL THINNING IN WHITE-FACED IBIS: A CONTINUING PROBLEM IN THE WESTERN UNITED STATES

White-faced ibises (Plegadis chihi) nesting in Arizona in 2000 exhibited extreme eggshell thinning and possible reproductive failure associated with high egg residues of DDE. A small colony of approximately 75 pairs nested relatively late in the season, and egg laying occurred from about June 15 to June 29. Average clutch size in 19 marked nests was 2.5, which was low compared to that reported for most other ibis populations. Hatching success was 43% (13 of 30 eggs remaining in active nests). The geometric mean DDE egg residue (2.23 µg/g wet weight) was similar to those reported in other ibis populations where DDE-induced shell thinning adversely affected reproductive success. Two of 16 eggs collected from marked nests had a flexible shell that easily indented with slight finger pressure. Overall mean eggshell thickness of 23 eggs was 0.264 mm, 15% thinner than shells of museum eggs collected before the widespread use of DDT. Only 1 of 23 eggs contained mercury at potentially harmful concentrations (>2.5 µg/g dry weight). Selenium in 74% of the eggs exceeded background concentrations (8.0 µg/g dry weight).