19 



certain species in North America and Great 

 Britain since the mid- 1 940 's have been largely 

 caused by residues of p,p '-DDE or other com- 

 pounds or metabolites of the DDT group 

 (Cooke 1973). At moderate or high levels of 

 DDE, shell thinning is severe and eggs may 

 break during incubation. High DDE levels 

 have been recorded in California; species af- 

 fected there have included brown pelicans 

 (Risebrough et al. 1971), double-crested cor- 

 morants (Gress et al. 1973), great egrets, and 

 great blue herons (Faber et al. 1972). As indi- 

 cated previously, much of the DDE probably 

 originated from an insecticide manufacturing 

 plant in southern California. DDE levels asso- 

 ciated with the shell thinning of eggs of the 

 common murres (Gress et al. 1971) and ashy 

 storm petrels (Coulter and Risebrough 1973) 

 on the Farallon Islands in central California 

 may also have originated in part from this 

 particular source. 



Eggshell thinning has occurred in several 

 other species that occur in freshwater or es- 

 tuarine habitats or that nest on coastal 

 islands. In 1967, shell thickness in herring 

 gull eggs from five States decreased with in- 

 creases in chlorinated hydrocarbon residues 

 (Rickey and Anderson 1968). Comparison of 

 eggshells taken before 1946 with those taken 

 since then reveals that several species includ- 

 ing the peregrine falcon, brown pelican, 

 double-crested cormorant, black-crowned night 

 heron, bald eagle, and osprey have sustained 

 shell-thickness and shell-weight decreases of 

 20% or more, at least for brief periods (Ander- 

 son and Hickey 1972). In some of these, re- 

 gional population declines are known. How- 

 ever, in seabird species that depend upon ma- 

 rine food chains in Iceland, there was no evi- 

 dence of shell thinning in 1973 (J. A. Sproul et 

 al., unpublished manuscript). 



Shell thickness was significantly and in- 

 versely correlated with the concentration of 

 DDE in 40 great blue heron eggs from Alberta 

 (Vermeer and Reynolds 1970; Vermeer and 

 Risebrough 1972). 



In the Upper Great Lakes States, 9 of 

 13 species of fish-eating birds were found in 

 1969-70 to have sustained statistically signifi- 

 cant decreases in eggshell thickness since 

 1946 (Faber and Hickey 1973). Maximum 

 changes in a thickness index occurred in great 

 blue herons (-25%), red-breasted mergansers 

 (Mergus serrator; -15%), and double-crested 



cormorants (-15%). Heron eggs taken in Lou- 

 isiana generally displayed a smaller post- 1946 

 change than herons in the Middle West. Al- 

 though DDE was a prominent factor for most 

 groups, especially herons, in relation to the 

 eggshell thinning observed, dieldrin and 

 PCB's also were associated with thinning in 

 some species. This relationship, however, may 

 have been due to correlation in concentrations 

 of these chemicals and concentrations of 

 DDE. 



The thinning of eggshells of the brown peli- 

 can has proven to be related to the concentra- 

 tions of DDE in the eggs (Blus et al. 1971; 

 Blus et al. 1972a, 1972b). Nearly all brown 

 pelican eggs collected from 13 colonies in 

 South Carolina, Florida, and California in 

 1969 and from 17 colonies in South Carolina 

 and Florida in 1970 exhibited eggshell 

 thinning (Blus 1970; Blus et al. 1974a). Of the 

 100 eggs analyzed for residues of pollutants, 

 all eggs contained measurable quantities of 

 DDE; most eggs contained measurable quan- 

 tities of ODD, DDT, dieldrin, or PCB's. DDE 

 appears to have been responsible for virtually 

 all the eggshell thinning. 



Nest success of brown pelicans in South 

 Carolina was related to residues of DDE and 

 dieldrin in sample eggs (Blus et al. 1974b). 

 Residues of DDE seemed primarily respon- 

 sible for nest failure; however, deleterious ef- 

 fects of this pollutant on nest success was not 

 satisfactorily separated from those induced 

 by dieldrin. Significant intercorrelation of all 

 five organochlorine residues identified in the 

 eggs complicated the relationship of residues 

 to nest success. Maximum DDE residues in 

 an egg from a successful nest were 2.4 ppm 

 and in an egg from an unsuccessful nest, 

 8.5 ppm. Comparable maximum residues for 

 dieldrin in sample eggs were 0.54 ppm (suc- 

 cessful) and 0.99 ppm (unsuccessful). Resi- 

 dues of ODD, DDT, or PCB's in sample eggs 

 were not significantly related to nest success. 

 Reproductive success in the brown pelican 

 colony was subnormal in the 2 years of study 

 (1971 and 1972) but reproductive success was 

 normal in those nests in which the sample egg 

 contained either 2.5 ppm or less of DDE, or 

 0.54 ppm or less of dieldrin. 



Residues of DDE, ODD, DDT, dieldrin, and 

 PCB's exhibited a significant decline in South 

 Carolina brown pelican eggs from 1969 

 through 1973 (Blus et al. 1977a), but the de- 



