Wesołowski T. 1998. Timing and synchronisation of breeding in a Marsh Tit Parus palustris population from a primaeval forest. Ardea 86: 89-100.

Marsh Tits in Białowieża National Park (E Poland, 1985-96) were single brooded, laid first eggs between April 5 and May 11 (mean= April 20). Nest building overlapped partially with egg laying - on average it started 10-11 days before the first egg. Median dates of the onset of laying in different years varied between April 7 and April 26, whereas within a season laying was strongly synchronised, over 90% of birds commenced laying within ten days from the onset of season. Early laying was connected with high temperatures, Warmth-Sums in the two weeks preceding the onset of laying were possibly critical. Nesting success (72-94%) was similar in early and late seasons. Birds in different habitats commenced laying at the same time. Females breeding for the first time commenced laying two days later than in their successive seasons. Otherwise laying dates of individual females were repeatable between seasons. It is suggested that the early and synchronised breeding of Marsh Tit is not so much an adaptation to the breeding season conditions but it is to a large extent a consequence of intra-specific competition among young birds for a chance to settle, with early fledged birds having strong competitive advantage.

Key words: Białowieża Forest - nest building - egg laying dates - weather conditions - nesting success - female age - repeatability