Birds: Tree Trunk Climbers
The treecreeper can be seen in woodland, spiralling up tree trunks and clinging to the bark with sharp claws. It pauses to seize insects with its fine, down-curved beak.
The birdwatcher’s first impression of a treecreeper is of a small, brownish bird, fluffy in appearance and furtive and jerky in its movements. It is often seen feeding in its characteristic manner-it creeps up the trunk of a tree in short stages, searching for insects, and when it cannot usefully hunt any higher it flies across and down, to start again at the base of another trunk.
Adult treecreepers are brown above, with various darker and paler streaks, and off-white below: the sexes are alike in plumage and general appearance. In good lighting conditions and during dry weather, this white can appear almost silvery, shading to fawn on the flanks, but in damp weather, when the belly has been pressed close against the bark of damp tree trunks, it can appear brownish or even greenish, smeared with material such as algae rubbed off the bark. The tail is long and brown, and the wings are short and rounded, with a conspicuous pale crescent-shaped wingbar in flight. Climbing up a tree trunk, treecreepers seem long, slender birds, but seen in flight-which is deeply undulating and looks rather feeble in character-they appear much fatter and almost moth-like, so rounded are the wings.
Young birds are if anything slightly fluffier than their parents, particularly around the white under-tail coverts, and the feathers of the head and back each have a yellow-buff fringe, giving the upper parts a scaly appearance. This lasts only from when the chicks Hedge in the summer until September, when the moult of body feathers is completed.
Head, tail and toes
Treecreepers feed mostly on small insects, spiders, other bark-living invertebrate animals, and their eggs and larvae. Their long, slender, finely pointed beaks are down-curved, and are ideal for extracting such tiny items from the crevices in the bark in which they are hidden. The eyes are large, and protected by unusually prominent eyebrows for a small, slow-flying bird. The advantage of these ‘beetling’ eyebrows is not known, but a close-up view through binoculars reveals that they give to the otherwise docile-looking treecreeper an extremely bad-tempered expression, additionally emphasised by the long, bold white eyestripe.
The long tail closely resembles that of the woodpeckers, with the shafts of the central feathers specially strengthened. As in the case of the woodpeckers (to which, however, tree-creepers are not related), the tail is used as a prop, the treecreeper normally moving head-uppermost on trunks and branches.
Compared with the powerful claws, toes and legs of the woodpeckers, treecreepers seem poorly adapted to their arboreal habitat. Unlike the woodpeckers’ toe arrangement (two facing forward, two behind for maximum grip), the treecreepers have the same pattern as almost all other passerine birds-three in front, one behind. The toes and legs seem slender and feeble, but the long and sharp claws can cling easily to the bark.
Song and display
The birds are most vocal, and most easily seen, in late winter and in early spring, when there are few leaves on the trees. The song, high-pitched and silvery in tone, is a descending trill with a small last flourish of three clear notes, recalling the songs of willow warblers and chaffinches. It is at this time of year that treecreeper pairs display: a novel performance as the birds chase each other in spirals up tree trunks.
The first eggs are laid early in April, but the peak laying period is in late April and early May. It seems that the majority of pairs are single-brooded, and that the eggs found freshly laid in early June are replacements for clutches lost to predators.
A hollow in the bark
An intriguing feature of the treecreeper is its use of winter roosting sites in enormous Wellingtonia trees. The bark of these North American introduced trees-relatives of the redwood- is thick and papery, and easy even for the slender beak of a treecreeper to excavate. Close inspection of almost any Wellingtonia anywhere will show some of these roosting sites a few feet from the ground. They are cavities, roughly the size and shape of a hard-boiled egg cut lengthways. And usually there is a give-away trickle of white droppings running down from them. The treecreeper snuggles down into this hollow, fluffy feathers bristling for the best insulation, and becomes almost invisible, so good is its camouflage.
Change of habitat
In Britain and Ireland, treecreepers seem to be most numerous in deciduous woodland and quite scarce among conifers. On the Continent of Europe, there are two treecreeper species: the one that is common in Britain and Ireland, and the short-toed treecreeper. There are very small differences in some anatomical measurements, and equally small differences in plumage, but these are not reliable for identification. Their songs, though, are quite different, and so too are their habitats. Astonishingly, it is the short-toed treecreeper that is found in deciduous woodland on the Continent, while ‘our’ tree-creeper is almost confined to upland conifers.
The explanation for this is thought to be that after the last Ice Age (which ended about 10.000 years ago) the two species recolonized Europe at different rates. ‘Our’ treecreeper spread northwards with the coniferous forest, which was its natural habitat, and which grew in Britain and Ireland some centuries before the deciduous woodland.
The English Channel and the North Sea formed during the period of recolonization, isolating Britain and Ireland before the deciduous woodland – and the short-toed treecreeper arrived. When, subsequently, the conifers were mostly replaced by deciduous species, the treecreeper was able to adapt itself to the changed habitat in the absence of competition, as the Channel proved an effective barrier to the short-toed treecreeper.
- Birding on the Net: Nesting Behaviour (birdwatchtips.info)