BIRDS: WARBLERS OF THE TREE CROWN
Three warblers of the leafy treetops come to Britain in the summer to breed. They are the willow warbler, the wood warbler and the chiff-chaff.
Three species of leaf warbler are found in Britain and Ireland: the willow warbler, the chiff-chaff and the wood warbler. Many people find warblers of any kind difficult to identify because they are invariably well camouflaged, retiring birds. Leaf warblers provide additional problems because they look so similar to one another. A leaf warbler can generally be described as a small, greenish-brown. Lightly built bird with a fine, insect-eating bill, a pale stripe above the eye and yellowish underparts, flitting about in the canopy of a tree. This description applies to virtually all the Phylloscopus (leaf warbler) species that occur in the Old World (there are about 30 of them). It certainly applies to Britain’s three species.
This, the dingiest of the three, has olive-brown upperparts and yellowish-buff underparts, an indistinct eye stripe and, usually, black legs. In keeping with its plumage and its name, its song is the least varied. Alternating between two monotonous squeaky notes -‘chiff’ and ‘chaff’, sometimes one note predominates as the bird ‘misses’ the other.
The chiff-chaff is the earliest of our summer migrants: the harbinger of spring. The first birds arrive in early March, the last in April or May. However, this pattern is complicated by the tendency of an increasing number of chiff-chaffs to overwinter in southern England. Although these hardy individuals may fly north to Scandinavia and beyond to breed, they are usually still here when the British breeders arrive.
The chiff-chaff requires both bushy under-growth for its nest, and tall trees as song-posts and feeding sites. While it can therefore live in well-grown deciduous and coniferous woodland, it is excluded from many areas where tall trees are uncommon, particularly parts of northern England and Scotland.
The willow warbler
This bird is greener above, and more yellow below, than the chiff-chaff; it has a stronger eye-stripe and, usually, pale legs. The overall impression it gives is of clearer colours and a ‘cleaner’ appearance.
The first willow warblers to arrive, usually in mid or late March, signal that the spring influx of visiting birds is under way. Their song is one of the most welcome country sounds, especially as it typically rings out from the heart of a colourful pussy willow tree on a bright sunlit morning. Starting quietly, the musical warbling gains volume and pitch and then descends back down the scale to a deliberate final flourish. In its choice of habitat, the willow warbler is the least fussy of the three leaf warblers. Almost any wooded or open bushy area is suitable, provided the canopy is not too dense. Ideal habitats range from damp areas with willows (as its name suggests) to young conifer plantations and small copses. The willow warbler is the commonest and most widespread of all our summer visitors, breeding throughout Britain and Ireland, even in sites close to built-up areas.
The wood warbler
Of the three leaf warblers, this is the most brightly coloured, having yellowish-green upperparts. A sulphur-yellow throat and breast, a white belly, a noticeable yellow eye stripe and pale legs. At 12.5cm (5in) long, it is also noticeably larger than the other two leaf warbler species, for they each measure only 11cm (4-1/2in) in length.
The wood warbler is the last of the three leaf warblers to arrive, mainly between mid-April and mid-May. Full appreciation of its song is certainly only achieved when you can both see and hear the bird in action. Starting with a steady ‘stip, stip, stip’, it accelerates to a marvellous, far-carrying trill which sets the whole bird quivering with the effort involved. The wood warbler also has a second, less well-known though equally distinctive song: a piping ‘piu’ repeated several times in quick succession.
The wood warbler is very particular in its choice of habitat: it prefers mature oak, birch, beech and chestnut woodland with little or no undergrowth. The sessile oak-woods of the valleys of western Britain are ideal, but this type of habitat is not common elsewhere. Consequently this species is the least widespread, being scarce or absent from wide tracts of England and Scotland. For every one of our breeding wood warblers, there are about ten chiff-chaffs and a hundred willow warblers.
The breeding season
Males of all three species typically arrive to set up territories a week or two earlier than their mates, often in the same area in which they were raised. Their beautiful, domed nests, almost spherical with the entrance at the side, are solely the work of the females. Leaves, grass, stalks, moss and perhaps bracken comprise the main structure, but whereas the wood warbler lines its nest with fine grass and hair, the other two use feathers. Each species hides its nest among low-growing plants and bushes. The chiff-chaff builds its nest a foot or two above the ground in most cases, while the others make theirs at ground level among grass and dead leaves.
One of the most distinct differences between our three leaf warblers, though perhaps the least obvious to the casual observer, is the timing of their feather moults. The only feature they share is that the young birds of all three species undergo a partial moult in autumn before they leave this country. (A partial moult is when the birds replace their body feathers, but not their wing or tail feathers.)
After this stage, chiff-chaffs undergo a partial moult each winter and a complete moult each autumn; wood warblers undergo a complete moult each winter and a partial moult each autumn; and willow warblers undergo two complete moults, one in winter and one in autumn. To moult the entire plumage twice in each year is highly unusual, and so the willow warbler is in a class of its own in this particular respect. Curiously, there seems to be no obvious advantage in this behaviour, for all three strategies are demonstrably successful.
End of the season
Late summer is the time when each individual bird must find extra food to fatten itself. This facilitates the growth of new feathers after the moult, and then fuels the bird’s migration journey south. Leaf warblers are quick and agile birds, hopping to and fro to pick up tiny insects and caterpillars from twigs and leaves, hovering to catch those out of reach or darting out to intercept them in flight. At this time of year, berries provide additional variety, and those chiff-chaffs that stay for the winter occasion-ally visit bird tables for fat, meat, bread and other household scraps. It is remarkable that such tiny birds, weighing some 10g (1/3oz), are able to fly to winter quarters as far away as Africa. Migration begins as early as the end of July. The first to leave are willow warblers, which migrate to tropical west Africa, some even crossing the equator. Most wood warblers migrate up the Nile valley to southern Sudan. The last to leave are the chiff-chaffs, many of which fly no further than the Mediterranean.