Although the sedge warbler is mainly insectivorous (3) (4) (5), it may also feed on some plant material, such as berries, during the autumn and winter (3) (4). The sedge warbler mainly hunts its prey at dawn or dusk when the temperature is lower, and it takes advantage of the insects’ sluggish behaviour at this time (8).
Typically solitary outside of the breeding season, the sedge warbler may form small, temporary groups when on migration, particularly in areas of abundant food (5). Large fat reserves are needed for the long migration, and this allows for unbroken flight for the heaviest birds (7). In autumn, the sedge warbler will mainly build its fat reserves by feeding on large numbers of plum reed aphids (Hyalopterus pruni) (5).
Male sedge warblers tend to arrive at the breeding grounds a couple of weeks before the females, and immediately select territories in which to pair and nest. Although this species is typically monogamous, the male sometimes pairs with a second female, either simultaneously or successively (5). The song of the sedge warbler is performed mainly to attract a mate, and once the male is paired it will revert to producing a short, low-intensity song (5). However, the sedge warbler’s song has been found to serve a dual function, as it can also be directed at rival males as a means of territorial defence (9).
The male sedge warbler usually perches on the top of a reed stem or bush during song (5), but a specialised song flight often separates and punctuates these performances. During this flight the bird will rise steeply into the air, then turn rapidly and make a slow spiral descent with its wings and tail outspread (5) (7). The song is generally performed from before sunrise until after sunset (5), but can also frequently be heard at night (7). Amazingly, by adding phrases into its song at random, this species is thought not to sing the same tune twice (3), and female sedge warblers are known to select males that have more complex songs and larger repertoires (10).
The nest of the sedge warbler is usually constructed near water, and is made from grass and moss with a horsehair and willow down lining (7). The female sedge warbler usually lays around 5 (3) (5) or 6 eggs (5), which are laid between May and June (5) and incubated for approximately 14 days (3). Although incubation is mostly carried out by the female, the male sedge warbler does assist in caring for and feeding the young once they have hatched. The fledglings leave the nest after 13 to 14 days, but continue to beg for food from the adult birds for a further 2 weeks (5).