Variation in Egg Sizes of Pied FlycatchersFicedula hypoleucain Central Poland: A Long-Term Decreasing Trend

Abstract

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 Łodź, 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 Łodź, 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 i...