Microclimate of Tree Cavities Used by Great Tits ( Parus Major ) in a Primeval Forest

Abstract

Altricial birds nesting in places with a favourable microclimate would be able to maintain the proper temperature of eggs or nestlings with less effort and thus reproduce more successfully. Tree cavities provide relatively safe nesting sites, but their microclimate is hardly known. Here we present the first data on air temperature and humidity in natural cavities used by Great Tits nesting in protected primeval forest in the Białowieża National Park (Eastern Poland). We checked what microclimatic conditions existed in cavities selected by Great Tits, how the construction of a nest changed the microclimate in a cavity and to what extent cavity localisation and cavity dimensions modified the microclimate of the breeding cavities. Great Tits used cavities in the living trees which were characterised by cool but relatively stable temperatures and very high relative humidity. Such conditions appear to be typical for cavities in living trees in general. Cavities with Great Tit broods were warmer and drier that the unused ones. The microclimate did not appear to constitute the major factor determining Great Tit usage of cavities in living trees, but it could exert a modifying effect on the pattern of cavity use by them. By choosing cavities of appropriate dimensions and placing the nest at an appropriate distance from the entrance, Great Tits could modify the cavity microclimate. However, their options were probably constrained by necessity to meet other important requirements, such as nest safety and illumination.