Woodpeckers Picinae are recognised as important keystone species in forest ecosystems because many species of animals find shelters or breeding places in cavities woodpeckers excavate. We evaluated the nest tree preference of a European species, the grey−headed woodpecker (GHW), Picus canus Gmel., in the semi−boreal Augustów Forest in northeast Poland, an extensive forest complex covering 114 000 ha, dominated by fresh and mixed fresh coniferous forest. Nest trees made by the GHW were sought within areas identified by the playback of territorial vocalisations and drumming in spring. We found 35 cavities on 31 trees. Twenty−one of these cavities were made in trees of poor health and 14 in dead trees. Only deciduous trees were selected for excavation, specifically aspen Populus tremula L., silver birch Betula pendula Roth., and black alder Alnus glutinosa (L.) Gaertn. Alder was chosen most frequently, but according to Ivlev’s electivity index, the most preferred species was aspen. We found important differences between forest sites used by the grey−headed woodpecker. Most cavities were found in alder forest, with fewer in broadleaf fertile forest, mixed broadleaf forest, and mixed pine forest. Cavity trees were on average 83−years−old (ranging from 45 to 127 years). The average diameter of cavity trees used by the grey−headed woodpecker was 47.4 cm (ranging from 30 to 67 cm), of which the most numerous were trees with diameters between 30 and 40 cm. All cavities were excavated in trunks. Cavity entrances were located at an average height of 8.2 metres (4.5−14 m). The mean height of the first branch on excavated trees was 10.2 metres above ground and was higher than the average height of the cavity entrance. Cavities in birch were placed higher than in other tree species. A linear mixed model explaining cavity height above ground showed important effects for first branch height, which allows for higher placement of the cavities, and tree species, because cavities in aspen and alder were excavated significantly lower than in birch. The GHW preferentially placed cavity entrances on southern and eastern exposures, with entrance aspect related to cavity height above the ground. Our results underline the importance of aspen to GHW and indicate the need to maintain this tree species in managed forests.

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