Holes

The ability that woodpeckers have of being able to excavate holes in trees is arguably the one thing that sets them apart from other birds. In Europe woodpeckers are primary hole-excavators, that is, they are the bird family that contributes most to the process of cavity making in forests. The fact that woodpeckers can excavate their own nesting and roosting cavities is one of the main reasons why they are successful. There are many advantages for both parents and chicks in being able to raise and be raised in the weatherproof haven of a chamber inside a tree. Being able to roost in such shelters is obviously an advantage for adults, too.

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Above: Adult male Three-toed Woodpecker at nest-hole site. Finland (Jari Peltomaki). The hole is around 1 metre above ground level. Such a height is not unusual for this species though most other woodpecker species rarely nest this low.

 

When searching for woodpecker holes it should be kept in mind that it is the quality of the wood, rather than tree species, that dictates the chosen site, though some tree species are obviously preferred over others. In areas of Europe where several woodpecker species are sympatric, and in some forests in the likes of Norway, Poland, Romania and Hungary this can mean six or even more species co-existing, many of the array of holes and foraging signs created by the birds can be attributed to species.

 

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Above: Adult female feeding juvenile Great Spotted Woodpecker. England (Bill Baston). On this view the advantage of being raised in a cavity rather than in an open nest can be imagined: shelter from the elements and safety from most predators.

 

The main period of 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. Thus it is quite common in woodpecker-rich woodlands to find many apparent holes, which upon inspection prove to be just a few inches deep, merely entrances rather than chambers.

 

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Above: Juvenile Great Spotted Woodpecker. 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?

 

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.

 

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Above: Male Three-toed Woodpecker at nest-hole. Finland (Jari Peltomaki). Note that the cavity entrance is not much larger than the bird itself. Woodpeckers never excavate holes larger than they need to be. This saves time and energy when excavating and prevents larger predators from entering the nesting cavity.

 

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.

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Above: Adult male Lesser Spotted Woodpecker excavating nest-hole. 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 prop.

 

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 outlook”?. This idea, too, is linked to the need to reduce risks from predators, which can be seen more easily and earlier.

 

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Above: Adult male White-backed Woodpecker lilfordi race, Abruzzo, Italy (Paul Harris). Again, note the almost perfect round cavity entrance.

 

Below: Another adult male White-backed Woodpecker lilfordi race in the Abruzzo, Italy (Paul Harris). Note that the cavity entrance is in a natural tree hole. It is very unusual for a European woodpecker (except Wryneck) to nest in a tree-hole which it did not create itself.

 

Almost all of Europe’s woodpeckers play a role in providing nest and roost sites for other wildlife but Black Woodpecker can be regarded as a very important species in this role. Th is is because it is the only species in Europe’s forests that can excavate large holes in trees and several other species that are unable to do so, use these holes for breeding and roosting. The distribution and success of these species can depend upon the Black Woodpecker.

 

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Above: Adult male Black Woodpecker leaving the nest-hole. Nepliget, Budapest, Hungary (Gábor Vasuta). Note the oval rather than round shape of the entrance which is typical for this species.

 

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Above: Tengmalm’s Owl using an old Black Woodpecker nest-hole as a nesting site. Sumava, South Bohemia, Czech Republic (Dave Pullen). This site has been repeatably used for many years.

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