Saturday, 22 September 2007

Holes: used and unused

The main period of hole excavating activity is in early spring, in the lead up to breeding. But this is not the only time of year when such work is done. After the chicks have fledged most adults begin to excavate more holes in the home range. These, and used nest holes, serve as roosts through the autumn and winter. Depending on the species it takes up to four weeks for a new hole and chamber to be completed, though in most cases two weeks is enough. Black and Great Spotted Woodpeckers will excavate holes in live trees, the other European woodpecker species rarely do so. Black Woodpecker is the only species that can excavate cavities in the sound, living wood of trees. All other species need either dead trees with rotten wood or live trees with heart rot. For all woodpeckers the quality and condition of wood is more important than tree species. Whether good excavators or not all species seem to depend upon the heart of the tree or core of the branch being softer than the surface. Many trees which are sound on the outside may be soft at the core due to fungi such as heart-rot and/or invertebrate activity. Much of the general tapping that woodpeckers do may be a "sounding out" technique where such wood suitable for excavation is discovered. It seems that creating a complete hole (entrance, tunnel and chamber) in completely sound wood seems to be just too much for even the best excavators. It is therefore common for a bird to abandon a site soon after starting work, probably when the bird realises that the timber chosen is hard to the core. It is not unusual for woodpecker-rich woodlands to be full of many apparent holes, which upon inspection prove to be just a few inches deep, merely entrances rather than chambers. The photo here of an adult male Lesser Spotted Woodpecker excavating a nest-hole, was taken in Chemnitz, Germany (Thomas Kraft). Wood-chips can be seen flying off the tree as the bird works. Note too, how the bird's tail is pressed against the trunk and thus acts as a stable prop.

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