The Cassin’s is fairly widespread at middle elevations in the southwestern and west-central United States, where it overlaps extensively with the western kingbird. Monotypic. Length 8.4–9.2".
Adult: dark gray head and nape (mask less obvious); semiconcealed orange-red central crown patch; dark grayish olive back, brownish wings. Dark gray chest contrasts with white chin, blends to yellow belly and olive flanks. Tail squared or slightly notched; brownish black with an indistinct pale gray terminal band. Relatively small bill. Juvenile: similar but wings edged pale cinnamon and pale tail tip less obvious.
The western has paler upperparts and chest; much less chin-breast contrast; and darker wings and black tail with white outer web of outer feather pair (but beware westerns completely lacking outer tail feathers or with worn-off outer webs). The Cassin’s has pale (but not pure white) outer web. The thick-billed, tropical, and Couch’s have paler or yellower breasts, uniformly brown tails, and heavier bills; tropical and Couch’s also have paler backs and deeply notched tails.
Call: single or repeated, strident kabeer; rapidly repeated ki-dih or ki-dear. Dawn song: a repeated rruh rruh rruh-rruh rreahr, rruh ree reeuhr (possibly confused with the buff-collared nightjar’s).
Status and Distribution
Common. Breeding: open, mature woodlands, including riparian, oak, and pinyon-juniper. Migration: relatively infrequently detected away from breeding sites. In spring, mid-March–early June, peak April–May. In fall, departure late July–October, peak September. Winter: western to central-southern Mexico. Locally resident in coastal southern California. Vagrant: casual/accidental to Oregon, Ontario, Massachusetts, Virginia, Arkansas, Louisiana, and Florida.
—From the National Geographic book Complete Birds of North America, 2006