Great Spotted Woodpeckers Dendrocopos major detect variation in wood hardness before excavating nest holes

Abstract

Great Spotted Woodpeckers Dendrocopos major typically excavate nest holes in trees with specific physical characteristics, namely with harder outer and softer inner wood. They excavate trial holes before excavating their nest holes, occasionally expanding old trial holes into nest holes. If trees with trial holes always share the same physical characteristics with nest trees, then Great Spotted Woodpeckers should be able to detect these characteristics before excavating trial holes and nest holes. I measured wood hardness and compared it among trees with trial holes, trees with nest holes, and sound trees. The physical characteristics of sound trees, which varied little in hardness, differed markedly from those of trees with trial holes and nest holes. Many trees with trial holes showed the same pattern of wood hardness as nest trees, although one showed the reverse pattern that is with softer outer and harder inner wood. The results of this study suggest that Great Spotted Woodpeckers are able to detect trees and positions on those trees with considerable wood hardness variation, but are unable to sense whether the hardness variation is suitable for nest excavation before they excavate trial holes. Thus, Great Spotted Woodpeckers excavate trial holes and then judge whether the positions where trial holes are excavated are appropriate for nest hole excavation.