Wood Warblers (17 June 2006)

The Wood Warbler Phylloscopus sibilatrix is a scarce bird in Cheshire, having declined greatly in the last decade or so. Breeding Bird Survey results from the BTO (British Trust for Ornithology) show that the UK population dropped by 52% from 1994 to 2004, for unknown reasons, and it is now on the Amber List of species of conservation concern. Wood Warbler is much less common than the other two Phylloscopus warblers breeding in Britain - Willow Warbler P. trochilus and Chiffchaff P. collybita - and its British breeding population is around 10,000 pairs, compared to the one million or more pairs for the other two species. The breeding population of Britain is tiny compared to that in its eastern European stronghold, where probably in excess of 14 million pairs breed.

Its habitat requirements are quite specific: climax woodland, usually deciduous, with at least a 70% closed canopy and little ground cover. Some suitable habitat is found in Delamere Forest, Cheshire, enough to support a small and variable population, from one to eight males each year. I enjoyed studying them from 1981 to 2001 but since then, there has been no Wood Warbler that has stayed in Delamere for more than an odd day. In 2006 two males set up territory, although neither was successful in attracting a mate.

They nest on the ground, and like sloping ground, with the nest often close to a small bush that provides a perch on the way in to feed their chicks. The territory of one bird is rather more open than the classic site, but, interestingly, the last pair of Wood Warblers that successfully nested in Delamere Forest (in 2001) held exactly this territory, so, five years on, it obviously still looks attractive to the species.

Structurally, Wood Warblers are the largest Phylloscopus, with noticeably long wings and short tail:

Wood Warbler Phylloscopus sibilatrixWoow Warbler Phylloscopus sibilatrix

In colour, the upperparts are strikingly yellow-green above, with pure white underparts:

Some individuals have a small hook at the end of their beak, as shown by the bird at the left below:

The Delamere birds are colour-ringed, in a project registered with the BTO, to allow the individual identity of birds to be recorded, without the need for capture, if they come back to the forest in future years. From 1981 to 1993, 8 out of 29 (28%) of birds ringed in Delamere Forest are known to have returned in a subsequent year (D Norman, ‘The return rate of adult male Wood Warblers Phylloscopus sibilatrix to a peripheral breeding area’, Ringing & Migration 15 (1994) 79-83).

Both adult and first-year birds undergo a complete moult in Africa, so there is no plumage difference between them when they arrive in Europe in spring, and there is no known way of ageing or sexing Wood Warblers by plumage. The wings and tail of these birds are in remarkably fresh condition, comparable with Garden Warbler Sylvia borin but much less abraded than any of the other warblers regularly visiting Britain, shown previously.

Needless to say, there is no connection, apart from the name that we have given them, between the Palearctic Phylloscopus sibilatrix Wood Warbler species and the North American Dendroica wood-warbler family.

Outside the breeding season, little is known about Wood Warblers in Britain: few are seen or caught on migration. They are one of the few migrants (along with Lesser Whitethroat and a few much scarcer species) that leaves Britain to the south-east in autumn, apparently moving to Italy to fatten-up then seemingly crossing the Mediterranean and the central part of the Sahara desert in one long flight. The wintering grounds probably lie from the Ivory Coast to the Congo basin, around the equator, although it is not known where British birds go within this range, as no British-ringed bird has been reported from its winter quarters. (David Norman, ‘Wood Warbler’, The Migration Atlas: movements of the birds of Britain and Ireland, (Wernham, C V, Toms, M P, Marchant J H, Clark J A, Siriwardena G M and Baillie S R, eds) T & A D Poyser, London (2002) pp.565-567).

There is much to learn about this enigmatic bird, one of my favourite species.


David Norman.


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