Which bird is in the picture?

Leaf warblers are small insectivorous passerine birds belonging to the genus Phylloscopus. The genus was introduced by the German zoologist Friedrich Boie in 1826. The name Phylloscopus is from Ancient Greek phullon, "leaf", and skopos, "seeker" (from skopeo, "to watch").

Leaf warblers were formerly included in the Old World warbler family but are now considered to belong to the family Phylloscopidae, introduced in 2006. The family originally included the genus Seicercus, but all species have been moved to Phylloscopus in the most recent classification. Leaf warblers are active, constantly moving, often flicking their wings as they glean the foliage for insects along the branches of trees and bushes. They forage at various levels within forests, from the top canopy to the understorey. Most of the species are markedly territorial both in their summer and winter quarters. Most are greenish or brownish above and off-white or yellowish below. Compared to some other "warblers", their songs are very simple. Species breeding in temperate regions are usually strongly migratory.