EGGSHELL THINNING AND DEPRESSED HATCHING SUCCESS OF CALIFORNIA CONDORS REINTRODUCED TO CENTRAL CALIFORNIA

From 1997 through 2010, in collaboration with the National Park Service, we released 84 captive-reared California Condors (Gymnogyps californianus) to the wild in central California; from 2006 through 2010 we recorded 16 nestings by nine pairs and recovered eggs or eggshell fragments from 12 nests. Mean thickness of shell fragments, without membrane, was 0.46 mm, 34% lower than the average thickness of 0.70 mm of fragments recovered from nine successful nests in interior southern California, 2007-2009. Hatching success in central California was 20-40%, significantly lower than the 70-80% recorded in southern California. The outer crystalline layer was absent or greatly reduced, as in thin-shelled condor eggs laid in southern California in the 1960s. Shell thickness was not related to egg size. Weight/water loss during incubation in the wild averaged three times greater than the normal rate associated with successful hatching; the rate of loss increased significantly with decreasing shell thickness. At least four failures, three from death of the embryo, we attribute to excessive weight/water loss; two other eggs losing substantial weight hatched successfully after artificial incubation at elevated humidities. DDT/DDE from wastes of a DDT factory discharged into the Southern California Bight had previously caused extensive eggshell thinning and reproductive failures of fish-eating and raptorial birds. Feeding on carcasses of California sea lions (Zalophus californianus), reintroduced condors now occupy a higher level of the food web. Like that of other species previously affected, the thickness of condor eggshells should recover as DDE contamination continues to decline.