Michael J. Conroy, Gregory G. Barnes, Raymond W. Bethke, Thomas D. Nudds, Increasing Mallards, Decreasing American Black Ducks: No Evidence for Cause and Effect: A Comment, The Journal of Wildlife Management, Vol. 53, No. 4 (Oct., 1989), pp. 1065-1071
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Conroy et al. (1989) studied this question.
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