COMPARISON OF BREAKING STRENGTH AND SHELL THICKNESS AS EVALUATORS OF WHITE-FACED IBIS EGGSHELL QUALITY

Data from a 1986 field study of white‐faced ibis (Plegadis chihi) nesting at Carson Lake, Nevada, were used to compare the utility of eggshell strength measurement and eggshell thickness as indicators of eggshell quality. The ibis population had a history of reproductive failure correlated with elevated egg concentrations of p, p'‐DDE, hereafter referred to as DDE. Eggs from 80 nests (one egg/nest) were tested for shell strength and thickness. Egg contents were analyzed for organochlorines, mercury and selenium; productivity at each nest (minus one egg) was monitored in the field. DDE‐DDT concentrations in the eggs ranged from none detected (<0.1) to 29 ppm (wet weight). Shell thickness and shell strength were both negatively correlated with DDE (−0.60, −0.61, respectively), but shell strength deteriorated at a faster rate than shell thickness. Scanning electron micrographs indicated the deterioration in strength was related to changes in ultrastructure as well as to decreased thickness. Fourteen eggs with <0.40 ppm DDE were used to exemplify normal “control” eggs. Of the eggs with higher concentrations of DDE (i.e., ≥0.40 ppm), 11 of 66 were thinner (>2 SD below “control” mean) than normal, 11 of 59 were weaker than normal and 7 eggs were cracked so their strength could not be tested, although thickness was measured. Therefore, 17% of the eggs with ≥0.40 ppm DDE were thinner than normal and 27% were either weaker than normal or cracked. Further, six eggs (four with ≥15 ppm DDE) did not have abnormally thin shells, but did have abnormally weak shells. Nests with abnormal test eggs (thinner, weaker or cracked) produced fewer young than nests with normal eggs. Use of the shell strength parameter provides additional information for better evaluations of reproductive problems. The potential utility of monitoring eggshell quality goes beyond evaluating effects of organochlorines since recent work indicates that other environmental hazards can affect shell quality. Copyright © 1990 SETAC