Hammond's Flycatcher, Empidonax hammondii is a small insect-eating bird. It is a small Empidonax flycatcher, with typical size ranging from 12.5-14.5 cm. "Adults have greyish-olive upperparts, darker on the wings and tail, with whitish underparts; they have a conspicuous white eye ring, white wing bars, a small bill and a short tail. The breast is washed with grey and the sides of the belly with yellow. Many species of Empidonax flycatchers look closely alike. The best way to distinguish species is by voice, by breeding habitat and/or range." -Wiki
Their preferred breeding habitat is coniferous forests in highlands of the western United States, Alaska and Canada. They make a cup nest on a fork in a tree, usually high in a horizontal branch.
I found them in the edge of a small forest (Mahan Trail) in Gibsons, BC (Sunshine Coast) I played their song and at least 3 of them flew around me.