Wolfgang Winkel, Hans Hudde, Long-Term Trends in Reproductive Traits of Tits (Parus major, P. caeruleus) and Pied Flycatchers Ficedula hypoleuca, Journal of Avian Biology, Vol. 28, No. 2 (Jun., 1997), pp. 187-190
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Winkel et al. (1997) studied this question.