No Homes for Tawny Owls

Part 1: Why there are so few cavity nest sites for tawnies

THE REASON IS VERY SIMPLE: The chain saw in the service of tree management. Cavity nest sites large enough for a Tawny Owl pair to raise young form when large branches die off and rot back into the trunk. Some form when the tree interior itself rots, as in the photo on the right. However, home owners don't want branches falling on themselves or their homes, councils must make sure that branches don't fall across the road, farmers want to protect their cattle and sheep, and the woodland manager has to keep his employer's tree crop healthy. Before the days of the chain saw and the tree surgeon many dead branches were probably left in place, but nowadays they are easily removed. The result is that holes don't form and tawnies are deprived of their traditional breeding sites.

The result of sawing off a dead branch at the trunk can be seen in the three photos across the top. The inner layer of the bark grows over the wound until there is complete closure and the tree is protected from invasion by harmful agents. A familiar sight, and it's why a tawny pair is very lucky if their territory has even one good hole for bringing up a family.

The next four photos down the side (4-7) show what happens when dead branches are left in place. The wood is invaded by fungi and rots. Fungal filaments travel back down the branch and into its "root" inside the trunk of the tree. Over time this node, from which the branch grew, rots away and a hole is left in the tree. The bigger the branch, the bigger the hole. These holes are a health hazard to the tree as they provide an open route for further invasion by insects, fungi and other microorganisms -- not to mention water. The end of the process can be a tree that is completely rotted within or even entirely hollow.

In photo 6 the lower branch is rotten and may be big enough to leave a hole for a tawny one day. More often than not, however, branches of this size are removed by the forester and the holes that form are only large enough for a woodpecker (photo 7). It's interesting that the woodpecker seems to know that it's a good idea to chip away at the perimeter of the hole to stop the inner bark growing over the entrance.

Photo 8 shows a fungal fruiting body growing on the site of an old branch node. These fruiting bodies grow from invaded wood within the trunk, so they are a sure sign that the tree has extensive fungal activity internally. Note the two woodpecker holes beneath. This is a garden tree that was badly damaged in the 1987 storm.

Photo 9 shows young oaks growing in a plantation. Here, in competition with other trees crowded around, they race upwards, concentrating their leaves in the upper parts and producing tall trees with slender trunks. There is no chance of owl-sized holes forming in these trees. This is part of the oak wood where our nestbox owls breed.

Photo 10 shows a dead oak against the canopy of an older part of the same wood. This tree may be a good resource for woodpeckers, tree creepers and other insect-eating birds, but it is still not large enough to support the formation of owl-sized cavities. I have not yet seen such cavities even in this extensive area of older oak woodland. If there were any, the local tawny pair would certainly have found them and would not have been using crows' nests, as they did until 2005.

Finally, photo 11 shows dead branch rings on a mature Scots Pine. Unlike with the oak, the decay of lower branches seems not to produce holes in the trunk, although the Scots Pine is certainly susceptible to invasion of the whole tree by fungi. These rings of branches can be lethal to flightless owl chicks when they fall from old twig nests in the crowns of the trees (the subject of part 2 of this article).

The decline in English woodland birds has been widely commented on. Here we have identified why a specific woodland species may be facing difficulties breeding, but it is likely that modern woodland management methods, which lead to a reduction in food resources, shelter and nesting sites, are responsible for the decline in other species. Below I have attached two links to pages where similar comments are made.

Piece by the Tree Council, "Managing our ancient trees"

Endangered Species Handbook, on the effects of logging old trees in rainforest

Continue to Part 2: Why Tawny Owl chicks fall from open nests

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