Some woodpecker species select nest trees that have sustained an infectious disease that produces soft, decayed cores and hard sapwood. In the study described in this paper, I measured the wood hardness of nest trees of the Great Spotted Woodpecker Dendrocopos major without destructive sampling to describe the physical characteristics of the wood. The woodpeckers started excavating vertical cavities immediately after digging horizontal holes through the hard sapwood, although the thickness of the sapwood varied widely among nest trees. The depth of the vertical cavities was negatively correlated with the hardness of the cores. The woodpeckers selected heights for nest cavities where the hardness of the core wood and the ratio of core hardness to sapwood hardness were relatively low. They also selected nest trees that had relatively large variations in wood hardness within the stem. These results suggest that the birds can identify variations in wood hardness among trees and among positions within a tree. Thus, wood hardness appears to be an important factor in the selection of nest trees and nest heights within a tree. The woodpeckers do not necessarily excavate their nests in trees with a specific diameter, but instead select trees with specific physical characteristics, irrespective of tree species.
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