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Wesołowski T., Tomiałojć L., Stawarczyk T. 1987. Why low numbers ofParus major in Białowieża Forest - removal experiments. Acta Ornithologica 23: 303-316. 

Parus major breeds in Białowieża Forest (E Poland) in much lower densities than elsewhere in Western and Central Europe. These Iow densities do not result from shortage of nest sites. Removal experiments were carried out to check if breeding numbers were limited by low numbers of birds present at the onset of the breeding season or by social behaviour in the spring. Birds were removed from oak-hornbeam stands in the period of nest-building and egg laying. In the spring of 1984 only males were removed. The plot was totally repopulated in a few days. In the spring of 1985, when uumbers of the tits were much lower, both males and females were removed from another plot. New birds of both sexes settled in the experimental area but in neither case was the repopulation complete. Thus, the social behaviour had only a subsidiary, to the numbers of birds present early in the spring, effect on breeding densities in 1985. Lower than elsewhere spring numbers could be due to lower production of young and/or higher mortality in the adults. Large territories in the Białowieża
P. major seem to be an anti-predator adaptation.