Wesołowski T., Czeszczewik D., Rowiński P., Walankiewicz W. 2002. Nest soaking in natural holes - a serious cause of breeding failure? Ornis Fennica 79: 132-138.

Nest loss rates of four secondary hole-nesters: Pied (Ficedula hypoleuca) and Collared (Ficedula albicollis) Flycatchers, Marsh Tit (Parus palustris) and European Nuthatch (Sitta europaea) due to soaking recorded during a long-term study in a primaeval temperate forest (Białowieża National Park, Poland) were low. Even during extremely heavy rains (> 74 mm in one day) they did not exceed 11%, far less than the loss due to predation. Soaking of nests occurred significantly less often in the late (flycatchers) than in the early breeding species (Nuthatches and Marsh Tits). In the latter species partial brood losses due to nest soaking were observed as well, the partial nest loss being more frequent in the rainy seasons. Interior walls of 8-40% holes were moist due to sap drain, but this only exceptionally led to the total nest failure. We suggest that the main function of bulky nest foundations in all these species may be protection against moisture.