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Wesołowski T. 2011. Blackcap Sylvia atricapilla numbers, phenology and reproduction in a primeval forest—a 33-year study. Journal of Ornithology 152: 319–329.

Data on Blackcaps Sylvia atricapilla gathered in 1975–2007 in an old-growth primeval forest (Białowieża National Park, Poland) are analysed to see whether the species numbers, breeding phenology or reproduction showed any long-term trends. Neither the clutch size (mean 4.8) nor the nesting success (c. 38%) changed, but a number of directional changes during 33 years were observed. In comparison with the 1970s, in the twenty-first century, Blackcaps arrived over 2 weeks earlier, but advanced egglaying by only 8 days. The breeding numbers increased threefold; they had been increasing slowly before 2000, but almost exponentially from 2001 onwards. The densities raised to c. 10 pairs/10 ha in luxuriant riverine habitats. Due to structural changes in the oak-hornbeam habitat, in the twenty-first century, Blackcaps situated nests lower and less frequently in conifers. The numerical increases were only partially accountable by structural changes in local habitats, but they were concordant with large-scale increases of Blackcaps across W Europe. It is hypothesised that the forest has recently been invaded by Blackcaps from strongly increasing, SW migrating, populations.