CORRELATION BETWEEN EGG SIZE AND CLUTCH SIZE IN THE PIED FLYCATCHER FICEDULA-HYPOLEUCA IN COLD AND WARM SUMMERS

The correlation between egg size and clutch size was studied in the Pi ed Flycatcher Ficedula hypoleuca in Finnish Lapland from 1975 to 1994. The hypothesis tested was that the tradeoff phenomenon is masked by w arm weather. When data for 20 years were pooled, there was no correlat ion, However, when years were analysed separately, a negative correlat ion was observed for five of the years, Over 12 years, the correlation coefficient between egg size and clutch size was not significant, and for 3 years it was significantly positive. The size and magnitude of the correlation were related to temperature during the pre-laying and egg-laying periods, Of the annual variation, 34% was explained by the mean temperature in early summer. It is suggested that trade-off betwe en egg size and clutch size may not be apparent in central (southern) parts of the distribution area of the species where climate is warmer. In the future, global warming may alter reproductive strategies of bi rds. Long-term data may thus be useful in the analysis of such effects and in testing of theoretical questions in ecology.