. The Canadian field-naturalist. 1974 Gilbertson: Pollutants in Breeding Herring Gulls 277 Anderson 1968). The thickness of 456 eggshells from the Great Lakes prior to 1947, and thus prior to the introduction of DDT, was mm (An- derson and Hie key 1972). This latter figure shows that the percentage of eggshell thinning in Lake Ontario colonies varied between 9% (Presqu'ile) and 16% (Pigeon Island). In Lake Erie it varied between (Mohawk Island) and (Port Colborne) in 1972. In all the colonies studied eggshell breakage and flaking occurred frequently (see Figure 4). These phenom
. The Canadian field-naturalist. 1974 Gilbertson: Pollutants in Breeding Herring Gulls 277 Anderson 1968). The thickness of 456 eggshells from the Great Lakes prior to 1947, and thus prior to the introduction of DDT, was mm (An- derson and Hie key 1972). This latter figure shows that the percentage of eggshell thinning in Lake Ontario colonies varied between 9% (Presqu'ile) and 16% (Pigeon Island). In Lake Erie it varied between (Mohawk Island) and (Port Colborne) in 1972. In all the colonies studied eggshell breakage and flaking occurred frequently (see Figure 4). These phenomena have not been reported for the less contaminated colonies on the Eastern Seaboard. Ludwig and Tomoff (1966), however, described these occurrences in colonies in northern Lake Michigan, and Keith (1966) similarly reported finding these in the Green Bay colony, where the mean eggshell thinning in 1964 was 15% (Ander- son et al. 1970). Figure 5 shows the distribution of the thickness of the eggshells versus their content of DDE. There was a significant inverse Spearman rank correlation coefficient (r = - , P < ) between eggshell thickness and content of DDE. The distribution suggested a non-linear relation- ship and thus product moment correlation coeffi- cients were calculated on the logarithmic trans- formations of the data. The highest correlation coefficient was between log DDE and thickness (r = ), and the corresponding least squares regression line is shown on Figure 5. This loga- rithmic regression line is curvilinear when drawn with untransformed axes. A stepwise linear regression analysis of thick- ness as a function of DDE, dieldrin, heptachlor epoxide, hexachlorobenzene, PCB, mercury and their logarithms showed a significant (Fi,24 = , P < ) regression of thickness on log DDE and a 36% reduction of the sums of squares. None of the other variables produced a significant further reduction in the sums of squares. The majority, it not all
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