Variation in Egg Sizes of Pied Flycatchers Ficedula hypoleuca in Central Poland: A Long-Term Decreasing Trend

Abstract. Sequentially laid eggs of small songbirds are built from the current income of resources and therefore can be subject to different environmental influences on the short-term and long-term time scales. In this paper we study variation in egg length, breadth, volume and shape in Pied Flycatchers Ficedula hypoleuca in Łódź, central Poland, during 2002–2012, in relation to climatic and habitat-related factors. We found a significant decreasing trend in egg length, breadth and volume across the study years and a slight tendency for eggs to be a little more elongated in a forest site than in an urban parkland site. Even though mean monthly temperature in May is known to have increased since the 1970s in the area of Łódź, the average ambient temperature during the time of formation of eggs in particular clutches tended to decrease, with increasing variability at the same time, in 2002–2012. Egg size traits showed a positive association with average temperature and a negative relation with variability in temperature during egg formation. In spite of a general trend for spring temperature to increase, mean clutch-specific temperature during egg formation decreased and variation in this temperature increased over the years of this study, which explains the overall decreasing trend in egg sizes. We also found that fledging success was positively correlated with egg length, but not any other trait. Therefore, a decrease in egg sizes resulting from climate changes may influence nestling performance, with potentially negative consequences for the Pied Flycatcher population.