Studies in Avian Biology No. 13:237-244, 1990. VARIATION IN THE FORAGING BEHAVIORS OF TWO FLYCATCHERS: ASSOCIATIONS WITH STAGE OF THE BREEDING CYCLE

The foraging characteristics of Hammond’s and Western flycatchers in northwestern California varied with different stages of the breeding cycle during the breeding seasons (early April-mid August) in 1984 and 1985. The species’ behaviors did not always vary in parallel nor were all foraging behaviors distributed equally during the breeding cycle. For example, the direction of aerial foraging movements for both species did not differ between stages, In contrast, the predominant type of foraging activity (either hover-glean or flycatch) differed by stage of the breeding cycle for Western Flycatchers but not for Hammond’s Flycatchers. Both birds differed in their use of foraging substrates and plant species among breeding stages. Western Flycatchers did not differ in position (height of foraging bird or distance to the canopy edge) among stages of the breeding cycle, but Hammond’s Flycatchers did. Both species foraged in trees with different structural characteristics (diameter-at-breast height, tree height, and bole height) during different stages of the breeding cycle. For both species, differences in foraging patterns within specific stages of the breeding cycle were apparent when compared with data pooled across the breeding stages. Failure to partition the data by stage of the breeding cycle may mask significant sources of variation and preclude important insights into a species’ breeding biology.