Three days of heavy rain (15.7, 37.9, 30.7 mm) and low temperature (min. temp. 5.7°C) during the breeding season 1991 caused high brood mortality in the Great Tit (GT) and the Blue Tit (ВТ), depending on brood advancement — in both species broods which died were older. In GT the mean advancement of dying broods was 5.29 days since hatching, while that of surviving ones 0.54 (P = 0.0005). In ВТ the advancement of broods which died was 7.82 and that of broods which survived —1.71 (P = 0.0004). The mortality was higher in ВТ than in GT, the former started breeding 4 days earlier on average (mean time of 1st egg laying in ВТ was 8.09 days, for GT was 11.96 days (P<0.0006). The calculated standardized selection differentials show very strong selection on the timing of breeding both in GT (iGT = 0.366, P = 0.042) and ВТ (iBT = 1.059, P = 0.0003). The higher absolute energy demands of older broods in view of limited food resources and vulnerability of young to hypothermia seem to be proximate cause of the mortality in both species. The selection resulted in almost the unification of the time of breeding in both species. The strength of this selection implies that weather could be one of the most important factors determining the timing of breeding.
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