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P. A. Racey, S. M. Swift, Feeding Ecology of Pipistrellus pipistrellus (Chiroptera: Vespertilionidae) during Pregnancy and Lactation. I. Foraging Behaviour, Journal of Animal Ecology, Vol. 54, No. 1 (Feb., 1985), pp. 205-215
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Racey et al. (1985) studied this question.
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