Long-term variation in laying date and clutch size of Pied Flycatchers Ficedula hypoleuca in Central Poland

Long-term annual variation in the timing of egg laying, clutch size and relationship between clutch size and the progress of the season was analysed for the Pied Flycatcher Ficedula hypoleuca nesting in a mature deciduous woodland in central Poland in 2002-2010. The earliest mean egg laying date was 8 May (2005) and the latest 18 May (2008), resulting in the maximum difference of 10 days between averages for years. No long- term trend was found. The total average of annual mean laying dates was 12 May. For all nine years the average of annual mean clutch sizes was 6.54 [plus or minus] 0.28 (SE) eggs; for individual seasons mean clutch size ranged from 6.0 to 7.1 eggs but differences among years were not significant. Clutch size clearly tended to decline with the progress of the breeding season within years, with some variation between years; correlation for pooled standardized data was -0.49. This supports the idea that in long-distance single-brooded passerine birds clutch size should decrease with the course of the breeding season due to progressively deteriorating food conditions.