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Staffan Ulfstrand, Feeding Niches of Some Passerine Birds in a South Swedish Coniferous Plantation in Winter and Summer, Ornis Scandinavica (Scandinavian Journal of Ornithology), Vol. 7, No. 1 (May 15, 1976), pp. 21-27
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Staffan Ulfstrand (1976) studied this question.
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