Bark Roosting Birds

Tree creepers are quiet unassuming small brown birds camouflaged with the bark colors of their arboreal haunts.

They explore tree trunks probing for a multitude of insects and grubs with their thin curved beaks. Stiff tails help as a prop on the trunk and aided with long clasping claws they patrol the bark with the busyness of rooks on a newly plowed field. They very rarely walk down the trunk like the acrobatic nuthatch, instead the birds work systematically from the tree’s bole ascending usually for most of the main trunk and then flying to work from the base of another tree. During flight or when viewed from the side on the trunk, the silvery white underparts of the bird may be detected.

Of the six species, two occur in Europe. The common tree creeper, Certhia familiaris, is found mainly in northern coniferous forests and the short-toed tree creeper, Certhia brachydactyla, prefers the deciduous forests of central and southern Europe.

In Britain the northern species inhabited the isolated pine forests that survived in the last ice age and the adaptable birds spread southwards adopting the deciduous woodlands. These evolutionary trends have probably been altered due to the introduction of many species of conifers over the past 150 years. Tree creepers nest and roost in bark crevices and the adaptability of these birds can be noticed in large gardens and parks where they often roost for the winter in the bark of exotic conifers. Their favored tree is usually the giant or Sierra redwood, Sequoiadendron giganteum, in which they excavate small hollows, shaped to fit one or sometimes two birds, their bodies when roosting complete the normal contours of the trunk, while warmth is conserved by the spongy dark bark acting as insulation. The hollows can usually be found above the height of two meters where a number are made in varying aspects around the trunk to enable selection on the sheltered side of the tree away from the erratic driving rains and cold winter winds. Tree hollows which are in regular use can be detected during the daytime by the presence of bird droppings.

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A tree creeper roosting in the bark with its head tucked under a wing.

These shy woodland birds are adapting quickly to the suburban and village environment.

The time scale can be estimated by considering that the first seed of Sequoiadendron sent to Britain dates back to 1853 when it was introduced from California to nurserymen in Devon which suggests that the birds have only adopted their new roosting homes during the last hundred years. The fast-growing Sequoiadendron attains a very large size in Britain, many are over 30 meters in height, a puzzle for many who may wish to guess their age, only to find that there are no trees in this country over 129 years old.

At Merrist Wood in Surrey the birds roost in the bark of a 30-meter tall Sequoiadendron which is in the busy and sometimes noisy center of the college. Nearby is a large oak which has had some past remedial tree surgery work involving the filling of large cavities with urethane foam which can be capped with hard metal body filler. The soft rigid foam-filled cavities which are not capped have been partly hollowed out by the tree creepers for roosting and a new home found for the 20th Century.

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