Saturday, 22 September 2007

Holes: location

Holes are generally placed as high as possible and two factors probably contribute to this. Firstly, a high location means more protection from general disturbance and predators. Secondly, wood is newer and hence softer in higher sections of trunk than it is lower down and thus excavation is easier. When a pair of woodpeckers nest at low levels, in stumps for example, it is probably because of a lack of suitable high locations. The softest parts of a tree are usually chosen, though the outer layers of the tree must be strong and rigid enough to support the cavity. Thus, the perfect location will combine a hard outer protective shell and a soft, rotten core that can be easily excavated. And another trade-off exists: the hardness of wood decreases as trees die thus live trees may provide a sound shell but are harder to excavate, whilst rotten trees are easier to excavate but may not provide a strong enough outer structure to support a cavity. Strong winds snap trees off at the weakest point and this is often the spot where a woodpecker cavity has been excavated. It is unclear how woodpeckers identify suitable nesting trees, that is, how they know which trees contain soft heartwood. It is probable that a combination of visual clues, such as fungal growths and already existing holes and the sounding out of wood with taps are combined. Whenever possible soft tree species like pine and aspen are used. Despite the formidable chisel-like bill that woodpeckers have, they seemingly do not like to do more excavating than is necessary. In hilly landscapes Europe’s woodpeckers usually excavate their nests on south-facing slopes. Holes in trees situated in open areas are more likely to be placed on the north facing side than those inside woodland. It has been suggested that this is to minimize overheating by the sun. Besides compass direction, other factors influence the location of holes on trees. A very important factor seems to be height. In my experience, when a nest tree is on a slope, holes placed in it will face away from the slope, regardless of compass direction. This may be because a hole facing downhill will always be higher than one on the opposite side of the trunk facing uphill and this offers better protection from predators. Indeed, I have never found a woodpecker hole of any species where the entrance faced uphill or inclined upwards. It has also been suggested that the reason why holes usually face outwards from slopes is because of a need for a "good lookout". This idea, too, is linked to the need to reduce risks from predators, which can be seen more easily and earlier. This photo of a juvenile Great Spotted Woodpecker was taken in England (Bill Baston). This is a rather worn entrance hole, and does not seem as tight a fit as is usual for woodpecker nest-hole entrances. Perhaps the rather rugged bark, and angle of the trunk made creating a close-fit difficult?

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