With its loud and resounding song— quick, THREE BEERS! —and its position of prominence perched atop a large tree or snag, the Olive-sided Flycatcher is one of the most recognizable inhabitants of North America's coniferous forest, breeding from sea level to 3,350 m in the Rocky Mountains. This flycatcher undergoes one of the longest and most protracted migrations of all Nearctic migrants, wintering primarily in Panama and the Andes Mountains of South America.
It breeds in habitat along forest edges and openings, including burns; natural edges of bogs, marshes, and open water; semiopen forest; and harvested forest with some structure retained. Tall, prominent trees and snags, which serve as singing and foraging perches, and unobstructed air space for foraging, are common features of all nesting habitats.
One of the most tyrannical species of the tyrant flycatchers, both members of the pair aggressively defend their nest areas. Nesting territories are relatively large for a passerine bird; 1 pair may defend up to 40–45 ha. The Olive-sided Flycatcher is monogamous and produces 3–4 eggs per clutch and 1 clutch per pair per year. Nests are open-cup structures placed at various heights above ground and well out from the trunk of a coniferous tree in a cluster of needles and twigs on a horizontal branch.
Olive-sided Flycatchers prey almost exclusively on flying insects, especially bees. They often forage from high, prominent perches at the tops of snags or dead tips or uppermost branches of live trees, from which they fly out (sallying) to snatch flying insects, and then return to the same or another prominent perch (Yo-Yo flight). This typical behavior prompted Marshall ( Marshall 1988b ) to describe this species as “the Peregrine of flycatchers.”
In the past 30 years this species has experienced significant declines in populations throughout its range, causing it to be listed as a Sensitive Species or Species of Concern by several federal and state agencies and conservation groups. Until recently, most life-history information on Olive-sided Flycatchers was anecdotal. Recent species-specific research on habitat relationships and nesting ecology in Alaska ( Wright 1997a ), Colorado (Kotliar and Melcher Kotliar and Melcher 1997 , Kotliar and Melcher 1998 ), and Oregon (Altman Altman 1998 , Altman 1999b ) has advanced our knowledge of the species substantially.