Wesołowski T. 1996. Natural nest sites of Marsh Tits Parus palustris in a primaeval forest (Białowieża National Park, Poland). Die Vogelwarte 38: 235-249.

Data on the location of 413 and dimensions of 198 Marsh Tit nesting holes gathered in 1975-95 in a strictly protected forest reserve are analysed. Breeding was confined to deciduous stands. In riverine stands over 96% of the holes were in Alnus and Fraxinus, in oak-hornbeam stands over 83% in Tilia and Carpinus. Holes were on average 5.6 m above the ground, in trees with girth at breast height 107 cm, both parameters varied strongly among tree species. Holes were situated mainly in tree trunks (89%), in living trees (92%), and the share of woodpecker-made holes was <2%. Entrances were mostly (74%) in vertical plane. Their minimum diameter was 20 mm in circular openings and 18 mm in elongated ones, median - 30 mm. Openings were usually elongated, in 65% of cases they were <20 cm2. Hole bottoms were approximately circular, with median diameter of 9 cm, and minimum diameter of 5.5 cm, median bottom area was 73 cm2. Depth of holes (to nest level, median 14 cm) was positively correlated with the least entrance diameter (rS = 0.48). In comparison with other areas Marsh Tits bred higher, in holes with larger bottoms, the holes having, however, similar depth and entrance diameter. None of the hole dimension variables depended on tree species, thus the observed variation in hole placement was probably to a large extent a by-product of differences in distribution of holes with adequate qualities. Small openings of holes could serve to avoid competition for holes and/ or to evade predators, but non-usage of woodpecker-made holes and nesting in holes in living trees can be best explained as an anti-predator tactic.

Key words: Parus palustris, tits, primaeval temperate forest, natural holes, hole dimensions, hole placement, anti-predator tactics