Wesołowski T. 1994. Variation in the numbers of resident birds in a primaeval temperate forest: Are winter weather, seed crop, caterpillars and interspecific competition involved? In: Hagemijer E. J. M. & T. J. Verstrael (eds) Bird numbers 1992. Statistics Netherlands, Heerlen, pp. 203-211.

Summary. Data on numbers of birds breeding in Białowieża National Park (eastern Poland) were collected with combined spot-mapping method during 15 years. Changes in numbers of tits, woodpeckers, Sitta europaea, Regulus regulus, Carduelis spinus and Certhia familiaris were compared with variation in numbers of leaf-eating caterpillars, acorn and hornbeam crop and fluctuations of winter temperatures, to see whether these factors influenced variation in their populations. The numbers of Dendrocopos major, Parus major and S. europaea, showed a strong tendency to increase over time, other species fluctuated without showing any clear trends. Most often numbers of different species varied independently of one another, while in about 20 % of cases their numbers changed in parallel fashion. No negative correlations were observed. The numbers of Dendrocopos medius, Parus caeruleus and R. regulus were positively correlated with the winter temperatures. This was possibly also true for Parus palustris, R. regulus, Certhia familiaris and perhaps D. medius were positively correlated with the caterpillar abundance. No relationship was found between either hornbeam or spruce crop and bird numbers. Overall, the combination of environmental variables used in analysis could account for over 50% of variation only in R. regulus, C. familiaris and D. medius. This relative independence of birds living in primaeval forest of changes in environmental factors, known to be important elsewhere, is tentatively explained as being a consequence of high diversity of resources in rich natural stands. The birds can switch over to various food sources according to their changing availability, without relying strongly on any single one.