The Willow Warbler is much less than Sparrow. The upperparts are olive-brown greenish tinged; underparts are off-white yellow tinged. It is distinguished from Chiffchaff by the paler legs, by the yellowish tinged upperparts; expressive yellow supercilium. From the Arctic Warbler, Yellow-browed Warbler and Greenish Warbler it is distinguished by the absence of the wing bars. From the Wood Warbler it is distinguished by the not so bright yellow-green color; and the absence of the expressive light edges on the tertials. Juveniles in the first plumage have the clear yellow underparts; more loose plumage. The wing form: 1st primary is longer than coverts on 2-8 mm; 2nd primary is longer than 6th or is equal it or is slightly less it. The outer web of the 6th primary isn't narrowed on tip. Weight 6-11 grams, length 12-15, wing 6,1-7,4, wingspan 18-24 cm.
The Willow Warbler is a rare breeding migrant, but at most of territory of Kazakhstan it is rare or common passage migrant. It inhabits deciduous, mixed and pine forests up to 1500 m. On migration it occurs in the forest, groves, gardens, forest-belts, reed beds, bush thickets and tall weed thickets with other Leaf Warblers often. It appears very rare in the end of March, usually in April or even early May. Majority of the birds migrate in the first half of May. On spring migration latest birds recorded in mid-June. It breeds in separate pairs. For about a week the sphere-shaped nest is built on the ground among the undergrowth; nest is built from the dry grass and thin rootless and is lined with thin grass and feathers, or dry larch needles. Clutches of 4-8 eggs is in June, incubated time is 13-15 days. Juveniles fledge at 18-19 days old, in mid – end July. The autumn migration starts early, on the late July probably. Majority of the birds migrate in August and September. Latest birds recorded in end October – early November.