Western Bonelli's warbler | Phylloscopus bonelli

Voice

The song lasts barely a second and is entirely colourless: a dry, flat rattle on one pitch, srrrrrrrrr, resolving at close range into a run of separate ticks, tsi-tsi-tsi-tsi-tsi-tsi-tsi. Every note sits at the same height, the tempo never changes, and the phrase stops as abruptly as it starts; the next follows after five to ten seconds. Anyone who knows Wood Warbler hears the difference at once: Wood Warbler accelerates from loose ticks into a shivering trill and dies away, while Bonelli's rattles at one speed from the first note and simply ends. The sound carries far through open pine and oak wood and is densest from sunrise until about ten, from late April to late June. The call is the second character and every bit as important: a soft, disyllabic, rising hoo-eet, warm and weak — exactly what a Willow Warbler says. In southern Europe that causes no confusion, because Willow Warbler is only a passage bird there; in north-west Europe it is the classic way a vagrant goes unnoticed.

Identification

A pale, low-contrast leaf warbler that looks greyer and whiter than any other. The upperparts are grey-green to grey-brown, the underparts silky white with no trace of yellow on throat, breast or flanks. Three things do carry colour, and those three make the bird: a bright yellow rump, a yellow-green panel on the closed wing formed by the yellow fringes of primaries and secondaries, and yellow-green edges to the tail feathers. The head looks strikingly open and expressionless — a short, diffuse whitish supercilium, a large dark eye in a blank face, no real eye-stripe. The lower mandible is orange to flesh, the legs grey-brown. The primary projection is long, about three-quarters of the tertial length. Sexes alike; juveniles are warmer brown with a duller rump.

Confusion species

Wood Warbler is bigger and far more strongly marked: vivid yellow throat and breast against a snow-white belly, where Bonelli's is white right up to the chin. Chiffchaff is browner and darker-legged, lacks the yellow rump and wing panel, and constantly dips its tail; Willow Warbler is more suffused with yellow and has pale legs. The real problem is Eastern Bonelli's Warbler — see its own species page, where the same difference is set out from the other side. That bird is a shade greyer and whiter still, with less green in wing and rump and a slightly longer, heavier bill, but in the field those differences overlap completely and settle nothing. The voice does settle it: Western gives a soft, disyllabic hoo-eet, Eastern a hard monosyllabic chip. Mind the trap: that hoo-eet sounds very like a Willow Warbler.

Habitat

Open, dry, elevated woodland on sunny slopes: Pyrenean and downy oak woods that hold their dead brown leaves through winter, mixed oak and pine, old pinewood with a thin low understorey, sweet chestnut, and warm holm-oak hillsides. The signature is tall, well-spaced trees over a sparse shrub layer; dense, damp, closed woodland is avoided. In Spain the bulk of the population sits between 500 and 2,000 metres, in the Alps and Pyrenees up to the highest woodland, and in the Maghreb in the cedar and oak forests of the Atlas. The nest is a domed ball on the ground, tucked against a bank or under a tussock.

Range and movements

A breeding bird of the western Mediterranean: Iberia, France as far north-east as a line from the Seine estuary to the Adriatic, Italy and the Maghreb. It winters in the Sahel, from Senegal to Chad. Spain is the core: the Spanish breeding population is put at 1.1 to 2.7 million pairs, roughly two-thirds of the European total, spread across almost the whole northern half of the country and the mountains of Andalusia. In Britain it is a scarce but near-annual vagrant, mostly singing males in May and coastal birds in late August and September; in the Netherlands there are sixty-four accepted records to the end of 2022, on the same seasonal pattern.

• Breeding (summer)
• Passage
• dark = core range, pale = thinly spread

Where and when to look

The second and third weeks of May, in an old pine or oak wood on a sunny slope in the Spanish sierras, the French Cévennes or the Italian foothills, from sunrise onward. Do not search — stand and listen. The dry rattle carries, while the bird itself stays high and restless in the canopy. Follow the song until you get it in sunlight, then watch for two things at once: the clean white throat and the yellow rump, which only flares as the bird turns or flies. In north-west Europe, May is the month to listen in coastal pine and dune woodland: a Willow-Warbler-like hoo-eet repeated for hours from the same treetop is worth walking towards.

Conservation status

Not threatened, and stable over most of its range. No decline has been detected in Spain, where the species has if anything gained from large-scale pine planting. On the northern edge, in France and Switzerland, numbers have fallen, most likely because open slope woodland closes over once grazing stops. Drought in the Sahel remains, as for every trans-Saharan migrant, the great unknown.