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D. V. Weseloh, Steve Brechtel, R. D. Burns, Recent Population Changes in Double-Crested Cormorants and California and Ring-Billed Gulls in Alberta, Canada, with a Note on White Pelicans, Proceedings of the Colonial Waterbird Group, Vol. 1 (1978), pp. 10-18
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Weseloh et al. (1978) studied this question.
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