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).