© Copyright: Images: Jari Peltomäki, Jouko Lehmuskallio, Kari Pihlaviita, M. & W. von Wright: Svenska fåglar (Kansalliskirjasto, The National Library of Finland). Recording: Jan-Erik Bruun. All rights reserved.
- Family: Leaf warblers – Phylloscopidae
- Appearance: Small, slender insect-eaters with thin beaks. Legs paler than those of similar Chiffchaff, wingtips more tapering, and eye-stripe more distinct. Can be distinguished from the Wood Warbler by the duller and paler yellow colouring on chest and the absence of any clear colouring boundary on chest.
- Size: Length 11–12.5 cm, weight 7–12 g.
- Nest: Oven-like nest built on the ground, made of grass and moss, lined with fine straw and white feathers.
- Breeding: 4–8 eggs laid in May–June, incubated by female for 12–13 days. Fledglings remain in nest for 10–16 days.
- Distribution: Thrives in all kinds of forest and scrubland habitats. Finland’s most numerous bird, with breeding population estimated at 7–11 million pairs.
- Migration: Nocturnal. Leaves Finland August–September, returning April–June. Winters in tropical Africa.
- Diet: Invertebrates.
- Calls: A faint “hooee”. Song a soft melancholy descending melody, starting with a few rapid beats, then slowing as pitch descends and finally fading away.
Willow Warblers have greenish brown upper parts and pale yellowish underparts. They also have a fairly distinct yellowish eye-stripe. The colouring of individual birds may vary considerably. Some birds lack any greenish tinge to their upper parts, and have whiter colouring on their bellies and eye-stripes.
It is difficult to distinguish Willow Warblers from Chiffchaffs in the field by their appearance. But Willow Warblers tend to have more greenish upper parts and more yellowish underparts, their beaks and legs are paler, and their wings are slightly longer. Best distinguished from Wood Warblers by the duller and paler yellow colouring on their chests, and the absence of any clear colouring boundary on the chest. Willow Warblers legs are light brown (or sometimes darker brown). Their beaks are brown, with a paler base to the lower mandible, and their irises are also brown.