Wesołowski T., Maziarz M. 2009. Changes in breeding phenology and performance of Wood Warblers Phylloscopus sibilatrix in a primeval forest: a thirty-year perspective. Acta Ornithologica 44: 69–80.

The reproductive behaviour of Wood Warblers was studied in a primeval forest area in the Białowieża National Park (E Poland). Observations carried out during twelve seasons (1976–1979, 1985–1988, 2002–2005) in deciduous and coniferous old-growth habitats spanned a 30-year period. The present paper examines whether the birds advanced their breeding dates during that time and whether any long-term shifts in fecundity or productivity were detectable. Though temperatures in the settlement period (the second half of April) rose, neither males nor females significantly advanced their dates of arrival. Wood Warblers bred earlier in 2002–2005 than in the two previous periods — the combined effect of earlier female arrival and shortening of post settlement breaks. Clutch size declined with season, was smaller in the coniferous habitat and in rodent outbreak years, but no long-term trend was perceptible. Apart from two exceptionally successful years (2003 and 2004) breeding losses remained high during the whole study. Predation was responsible for 80–95% of them and was concentrated on the nestling stage. Overall Wood Warbler phenology and breeding performance in BNP have changed relatively little during the last 30 years. These findings support the results of other studies demonstrating the remarkable resilience of this primeval forest biota to environmental change.