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DATA SOURCES
Fox, G.A., B. Collins, E. Hayakawa, D.V. Weseloh, J.P. Ludwig, T.J. Kubiak, and T.C. Erdman. 1991. Spatial variation in the occurrence and prevalence of bill defects in young Double-crested Cormorants in the Great Lakes, 1979–87. Journal of Great Lakes Research 17(2):158–167.
Larson, J.M., W.H. Karasov, L. Sileo, K.L. Stromborg, B.A. Hanbidge, J.P. Giesy, P.D. Jones, D.E. Tillitt, and D.A. Verbrugge. 1996. Reproductive success, developmental anomalies, and environmental contaminants in Double-crested Cormorants (Phalacrocorax auritus). Environmental Toxicology and Chemistry 15(4):553–559.
Ludwig, J.P., H. Kurita-Matsuba, H.J. Auman, M.E. Ludwig, C.L. Summer, J.P. Giesy, D.E. Tillitt, and P.D. Jones. 1996. Deformities, PCBs, and TCDD-equivalents in Double-crested Cormorants (Phalacrocorax auritus) and Caspian Terns (Hydroprogne caspia) of the Upper Great Lakes 1986–91: Testing a cause–effect hypothesis. Journal of Great Lakes Research 22(2):172–197.
Ryckman, D.P., D.V. Weseloh, P. Hamr, G.A. Fox, B. Collins, P.J. Ewins, and R.J. Norstrom. In press. Spatial and temporal trends in organochlorine contamination and bill deformities in Double-crested Cormorants (Phalacrocorax auritus) from the Great Lakes. Environmental Monitoring and Assessment Journal.
DATA COLLECTED
The number of hatched Double-crested Cormorant chicks with crossed bills, deflected bills, and/or bills in which the mandibles differed in length.
GEOGRAPHICAL COVERAGE
Double-crested Cormorant colonies in the Great Lakes and reference colonies located on the Prairies and in northwestern Ontario.
DATA COLLECTION PERIOD
Cumulative total of observed defects for the period from 1979 to 1994.
DATA PRESENTATION
Prevalence of defects per 10 000 chicks examined in each region is presented in Figure 4 and Table 5. Although a number of investigators have been involved, the methods have been similar. Data presented in Larson et al. (1996) and Ludwig et al. (1996) were pooled to derive numbers for 1986–91.
For the period 1979–87, 31 168 chicks were examined in 42 Great Lakes colonies: 70 had bill defects. In the reference areas, 20 962 chicks were examined in 35 colonies: 2 had bill defects. Between 1988 and 1994, 130 024 chicks were examined in 70 Great Lakes colonies in Canadian waters: 28 had bill defects. In one reference area, 4 850 chicks were examined: none had bill defects. Statistical testing revealed that only the prevalence in Georgian Bay has declined significantly between the two periods.
The prevalence of bill defects in chicks from colonies in upper Green Bay, on the west shore of Lake Michigan, continues to be greater than in any other region.
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