Willow warbler (Phylloscopus trochilus)
|Size||Length: 11 - 12.5 cm (2)|
|Weight||6.3 - 14.6 g (2)|
The willow warbler is classified as Least Concern (LC) on the IUCN Red List (1).
A slender, energetic bird, the willow warbler (Phylloscopus trochilus) is a rather plain species, generally olive- or grey-green to brownish on the upperparts, with slightly brighter yellow-green edges to the flight feathers and rump. The male and female willow warbler are very similar in appearance. The most distinctive feature of this species is the narrow pale or whitish-yellow band, or ‘supercilium’, which extends from the base of the bill to behind the eye (2) (3) (4). The willow warbler also has a narrow, dark olive stripe running ‘through’ the eye (2).
The underparts of the willow warbler are mostly pale yellow-white, tinged or streaked with slightly brighter yellow on the throat and breast (2) (3) (4). The underwing coverts are a brighter, more conspicuous yellow (2). The bill is fairly long and pointed, and is typically brownish except for the variable yellow-orange base of the lower mandible. The legs are usually pale brown (2) (5). The juvenile willow warbler has a pale yellow band extending above the eye and is much paler below, with a white chin and undertail and a buffish tinge to the breast (2).
The willow warbler is typically distinguished from similar looking birds, such as the chiffchaff (Phylloscopus collybita), by the long primary feathers which extend much further beyond the secondaries than in other species (2).
Three subspecies of willow warbler are recognised: Phylloscopus trochilus trochilus is smaller and more olive-green and yellow than the other two subspecies. Phylloscopus trochilus acredula is greyer and less green-tinged, becoming more grey-brown with whitish underparts in the eastern parts of its range. Phylloscopus trochilus yakutensis has a more pronounced white band above the eye, with cold grey-brown upperparts and dull white underparts, streaked or spotted greyish on the breast (2) (6).
The willow warbler is a common and widespread species that breeds in temperate Europe and Asia, and migrates during the non-breeding season to winter in sub-Saharan Africa (4) (6) (7).
Phylloscopus trochilus trochilus breeds in Britain and Ireland and southern Sweden to northern Spain, east to southern Poland and Romania (2). It winters mainly in western Africa (2) (6). Phylloscopus trochilus acredula breeds throughout Scandinavia, central and eastern Europe, and east to central Siberia (2). Non-breeding populations are found in Africa south of Sudan. Phylloscopus trochilus yakutensis breeds in central and eastern Siberia, and winters in eastern and southern Africa (2) (6).
The willow warbler inhabits a wide variety of habitats, typically breeding in deciduous or mixed forest and forest clearings, open scrubby woodland, coppice, plantations, heathland and areas of scrub and shrubs (2) (4) (6). It also occurs in overgrown gardens, orchards, hedges, railway embankments, and rough pastures with tussock grass. In montane regions, the willow warbler generally breeds on scrub-covered slopes, up to elevations of 1,000 metres (2).
In the non-breeding season, the willow warbler occurs across open countryside, in woodland and the edges of forests. In western Africa it is particularly common over winter in areas of tall grass, swamps and mangroves (2).
Small insects and spiders, as well as their eggs and larvae, form a large proportion of the willow warbler’s diet. Fruits, berries and other plant materials are also taken during autumn (2) (3) (4). The willow warbler forages by picking its insect prey from leaves, twigs and branches, usually in the canopy, but also in bushes and low vegetation (2) (4) (9). It may also make short sallies in pursuit of flying insects, and will occasionally hover over foliage in search of prey (2) (9).
The breeding season of the willow warbler typically runs between April and July (2) (4) (8). The male arrives at the breeding ground first and establishes up to two territories, which it defends aggressively against intruders (2) (6). Pair formation is initiated by the female on entering the male’s territory. During courtship, the male willow warbler approaches the female and lands close by, twittering softly and adopting a horizontal posture with its head held forward and the wings drooped. The male then chases the female, fanning its tail and shivering its wings (2).
The dome-shaped nest is built mostly by the female willow warbler, although the male may assist with collecting nest material. It is generally constructed from dry grass, leaves, stems, moss, lichen, twigs and bark woven together, and is lined with animal hair and feathers (2) (4) (6). The nest is usually placed on the ground, well concealed among grass or at the base of shrubs or trees (2) (4) (6). On rare occasions the willow warbler may place the nest up to five metres from the ground, in a tree, crevice or creeper (2). The female lays a clutch of 4 to 8 eggs, which are incubated, mostly by the female, for between 10 and 16 days (2) (4) (6) (8). The young willow warblers remain in the nest and are fed mainly by the female for around 11 to 15 days following hatching (2) (8). The chicks become independent from the adults around two weeks after leaving the nest (2) (6).
The willow warbler is fairly unusual among passerines, and unique among species in the UK, as it undergoes two complete moults each year, meaning that it has an entirely new set of feathers for each long-distance migration to and from the breeding grounds (6). This species typically leaves its breeding site in late August or early September (2), arriving in Africa from mid-October to November (9). The willow warbler may migrate up to 12,000 kilometres to reach its wintering range (4). Migration occurs mainly during the night, and the willow warbler usually covers around 100 kilometres a day, with some individuals known to average an astounding 218 kilometres per day (2).
In Europe, the willow warbler is thought to have undergone a moderate decline in its population since the 1980s (10) (11). In the UK, this species has decreased by almost 70 percent over the past 25 years (6), with declines particularly apparent in young woodland and coppice (12).
The decline of the willow warbler throughout its range may be due to habitat loss, as well changes in management practices in many woods which lead to the development of a ‘high canopy’ and a lack of diversity in woodland structure (13) (14). Habitat deterioration in its wintering grounds in Africa may also be a contributing factor, but very little is currently known about its wintering requirements (14) (15).
In the UK, willow warblers have undergone a more serious decline in the south compared with populations in the north. In general, the willow warbler is more typical of birch woods in the north and oak in the south, and as such, declining oak tree health in the southern UK since 1989 may have impacted on the willow warbler, as oak is typically a major source of invertebrates for insect-gleaning birds (15).
In the UK, the willow warbler is considered to be an Amber List bird of conservation concern due to its population decline over the past 25 years (3). Recommendations for managing the decline of this species include restoring neglected coppice, developing scrubby woodland edge and creating new woodland patches (12). Management of grazing, by deer in particular, would also be beneficial to this species as excessive grazing removes understorey and low level vegetation favoured by the willow warbler (6) (12).
Find out more about the willow warbler and other bird species:
BirdLife International - Willow warbler:
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- Coppice: coppicing is a traditional form of woodland management in which trees are cut close to the base of the trunk. Re-growth occurs in the form of many thin poles. Coppiced woodlands are cut in this way on rotation, producing a mosaic of different stages of re-growth.
- Coverts: small feathers concealing the bases of larger flight feathers, usually on the wings or tail.
- Deciduous forest: forest consisting mainly of deciduous trees, which shed their leaves at the end of the growing season.
- Flight feathers: the feathers at the end of the wing, involved in flight.
- Incubate: to keep eggs warm so that development is possible.
- Invertebrates: animals with no backbone, such as insects, crustaceans, worms, molluscs, spiders, cnidarians (jellyfish, corals, sea anemones), echinoderms, and others.
- Larvae: stage in an animal’s lifecycle after it hatches from the egg. Larvae are typically very different in appearance to adults; they are able to feed and move around but usually are unable to reproduce.
- Lichen: a composite organism made up of a fungus in a co-operative partnership with an alga. Owing to this partnership, lichens can thrive in harsh environments such as mountaintops and polar regions. Characteristically forms a crustlike or branching growth on rocks or tree trunks.
- Mandible: in birds, the lower jaw and beak, but the term is also used to denote the two parts of the beak.
- Montane: of mountains
- Passerines: a group of more than 5,000 species of small to medium-sized birds which have widely varied plumage and shape. They all have three toes pointing forward and one directed backward which assists with perching, and are sometimes known as perching birds or song birds.
- Primary feathers: in birds, the main flight feathers projecting along the outer edge of the wing.
- Secondaries: in birds, the shorter flight feathers projecting along the inner edge of the wing.
- Subspecies: a population usually restricted to a geographical area that differs from other populations of the same species, but not to the extent of being classified as a separate species.
- Temperate: referring to the geographical region that lies between the polar and tropical regions, characterised by a moderate climate with distinct seasons.
- Territory: an area occupied and defended by an animal, a pair of animals or a colony.
IUCN Red List (March, 2011)
- del Hoyo, J., Elliott, A. and Christie, D. (2006) Handbook of the Birds of the World. Volume 11: Old World Flycatchers to Old World Warblers. Lynx Edicions, Barcelona.
RSPB - Willow warbler (March, 2011)
Avibirds European Birdguide Online - Willow warbler (March, 2011)
- Peterson, R.T., Mountfort, G. and Hollom, P.A.D. (1993) Birds of Britain and Europe. HarperCollinsPublishers, London.
Congleton Birding -Willow warbler (March, 2011)
- Barriocanal, C. and Robson, D. (2007) Spring passage of willow warbler Phylloscopus trochilus across the western Mediterranean: comparing islands with the mainland. Ardea, 95(1): 91-96.
- Cramp, S. (1955) The breeding of the willow warbler. Taylor & Francis, 2(3): 121-135.
- Salewski, V., Bairlein, F. and Leisler, B. (2002) Different wintering strategies of two Palearctic migrants in West Africa - a consequence of foraging strategies? Ibis, 144: 85–93
BirdLife International - Willow warbler (March, 2011)
European Bird Census Council - Willow warbler (March, 2011)
Forestry Commission: Reversing Woodland Bird Decline (March, 2011)
Countryside Council for Wales (March, 2011)
British Garden Birds - Willow warbler (March, 2011)
- Peach, W.J., Crick, H.Q.P., and Marchant, J.H. (1995) The demography of the decline in the British willow warbler population. Journal of Applied Statistics, 22(5&6): 905-922.