The willow flycatcher (Empidonax traillii) is a small insect-eating, neotropical migrant bird of the tyrant flycatcher family. There are four subspecies of the willow flycatcher currently recognized, all of which breed in North America (including three subspecies which breed in California). Empidonax flycatchers are almost impossible to tell apart in the field so biologists use their songs to distinguish between them.

Source: Wikipedia