Thomas Alerstam, Bo Ebenman, , Staffan Tamm, Staffan Ulfstrand, Hybridization as an Agent of Competition between Two Bird Allospecies: Ficedula Albicollis and F. Hypoleuca on the Island of Gotland in the Baltic, Oikos, Vol. 31, No. 3 (1978), pp. 326-331
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Alerstam et al. (1978) studied this question.