The influence of habitat structure and abiotic factors on winter bird distribution was studied at the within-habitat level in the montane Pyrenean oakwoods of central Spain. Abiotic factors associated with thermal stress were estimated based on altitude and solar radiation received by woodlands (calculated by the steepness and orientation of the terrain). This paper demonstrates the great importance of abiotic factors in influencing bird distribution. Several bird community parameters related to density and species richness decreased with altitude, while they increased with radiation incidence of oakwood plots (i.e., birds avoided northern orientations where solar radiation is minimal in winter). The most important habitat structure variables related to bird distribution were the density of young and mature oaks. A thick undergrowth of thin oaks negatively influenced total bird abundance and species richness and the number of species of the ground searchers guild. Conversely, oak maturity played a positive role on total bird density and species richness and on the number of species of tree canopy gleaners and trunk foragers. Bird density and species richness were better explained by tree regression models considering complex interactions between variables than by general linear regression analyses. To enhance winter survival and habitat suitability for birds, forest management in these mediterranean endemic oakwoods should preserve the most mature forests at lower altitudes exposed to the south.
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Vol. 13 • No. 1