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Barney A. Schlinger, Robert H. Slotow, Arthur P. Arnold, Plasma Estrogens and Brain Aromatase in Winter White-Crowned Sparrows, Ornis Scandinavica (Scandinavian Journal of Ornithology), Vol. 23, No. 3, Proceedings of the Symposium Hormones, Physiology and Non-Reproductive Behaviour in Birds, 21-23 August, 1991, Göteborg, Sweden (Jul. - Sep., 1992), pp. 292-297
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Schlinger et al. (1992) studied this question.
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