The diet of the alder flycatcher includes a variety of arthropods, including bees, wasps, flies, moths, butterflies, grasshoppers and crickets (2). An individual may search for prey from a perch and pursue it through the air, or may take it directly from within foliage (2) (4). In winter, this species may also eat small amounts of fruit (3) (4).
A long-distance migrant (2) (4), the alder flycatcher leaves its wintering grounds between March and early May, moving northwards to breed (2). This species usually remains at its breeding grounds until late August, when most individuals will have begun their southward migration (4).
The relatively short breeding season of the alder flycatcher runs from mid-June to early August (2). The nest is constructed solely by the female and is a loose cup of coarse grass which is lined with wiry grass and conifer needles (2). The nest is usually completed after around 36 hours (4) and the female then produces a clutch of 3 or 4 cream-white eggs (2) (3) (4), which may be unmarked or have an irregular dark pattern on the surface (3) (4). The female incubates the nest for between 11 and 15 days, and once the eggs have hatched (2) (4) the young are fed by both adults (4). The young then fledge the nest after 14 or 15 days (2).