The Olive-sided Flycatcher arrives in the Northwest Territories in late May and early June. Females incubate three to four eggs for about 15 days. The Olive-sided Flycatcher leaves the Northwest Territories in late July to early August and winters in South and Central America.
The Olive-sided Flycatcher eats flying insects. It perches on tall trees or snags and waits for insects to fly by before pursuing its prey. Typical habitat is wetlands, mature conifer forests, and areas with young trees including those created by forest fires or clear-cuts. The Olive-sided Flycatcher also uses open forests containing tall trees or snags for perching.
The Olive-sided Flycatcher has a loud song that sounds like "quick, THREE BEERS". Females will also sing when agitated or when close to their nest.