Wesołowski T., Rowiński P. 2004. The breeding behaviour of the Nuthatch Sitta europaea in relation to natural hole attributes in a primeval forest. Bird Study 51: 143-155.

Capsule Nuthatches used holes with strong walls, typically in live trees with entrances reduced by plastering, and \"oversized\" interiors filled with bark flakes. By nesting in such holes they seemed to reduce both risk of nest predation and of nest soaking.
Aims To describe patterns of nest-site utilisation by Nuthatches in primeval conditions, to examine the influence of various hole attributes on nesting success, to consider the adaptive value of nest-site choice.
Methods Observations of birds living in undisturbed conditions in a strictly protected part of the Białowieża National Park (Poland) during 27 breeding seasons coupled with measurements of hole attributes and observations of nests' fate.
Results Tree species used for breeding differed among habitats. Holes were on average 14.0 m above the ground, in trees with girth at breast height 206 cm; both parameters varied strongly among tree species. They were situated mainly in tree trunks (76%), in living trees (89%), in conical knotholes (51%); woodpecker-made holes constituted 32%. Nuthatches bred in very large (mean bottom area 325 cm2) holes, the preferred tree species (maple Acer platanoides, ash Fraxinus excelsior) had larger holes than other tree species. Nuthatches reduced entrances by \"plastering\" to mean size of 2.9 x 3.3 cm. They made the holes substantially shallower by filling them with bark flakes (mean depth to nest level - 10 cm), eggs were laid in depressions among flakes, far from the entrance (mean distance 21 cm), their \"nests\" occupied only a fraction of hole area. Broods in holes with smaller entrances were the most successful. Eggs and small young were covered with bark during absence of female. Larger nestlings, when endangered, moved to the rear wall of the hole, where they stayed tightly pressed. These behaviour patterns reduced the risk of predation.
Conclusion The features of holes used by Nuthatches (combination of strong walls, small entrances & large bottoms with copious filling) probably constitute an evolutionary solution to the need to evade predators while keeping contents of the nest dry.