This species is extremely similar to the eastern wood-pewee and is best identified by range and voice. Vagrants should be identified with great care and preferably documented with recordings of vocalizations, photos, or video. This is the only exclusively western-breeding passerine that winters almost entirely in South America. Polytypic (5 ssp. north of Mexico). Length 6.3".
Adult: extremely similar to eastern wood-pewee, with long wings that extend one-third of the way down the tail. Average differences listed below, but plumage somewhat variable. western wood-pewee tends to be slightly darker, browner, and less greenish than eastern wood-pewee, with complete grayish breast band, darker centers to undertail coverts, less pale nape contrast, and duller back. Wing bars, on average, are narrower and more grayish, contrasting less with wings. The base of the lower mandible is primarily dark (usually darker than the eastern wood-pewee’s) but usually shows some dull orange. Adults molt on the wintering grounds, and worn summer birds (and fall birds in North America) are essentially identical to the eastern wood-pewee in appearance. Juvenile: during fall separated from worn adults by fresh plumage, buff-gray wing bars, and brownish wash to the upperparts. Many have more extensive pale coloration on lower mandible than adults (i.e., more like the eastern wood-pewee’s). On average, the wing bars contrast less than on the juvenile eastern wood-pewee; also note that the lower wing bar is broader and more defined than the upper: the paler tips on the greater coverts are broader and more defined than those on median coverts.
Differences minor and clinal. Compared to widespread veliei, coastal breeders from southeastern Alaska through central Oregon (saturatus) usually have more of a yellow wash to flanks, a browner breast, and a duskier crown that contrasts more with brownish or olive back. Mexican and Central American subspecies are darker and larger, except for southern Baja California’s peninsulae, which are paler with a larger bill.
Extremely similar to the eastern wood-pewee and best separated by range and voice. (Average visual differences compared above.) Most often confused with the willow flycatcher; note the willow’s (and the alder’s) relatively short primary projection, smaller size, and tendency to wag tail. Compare with the greater pewee, the olive-sided flycatcher, and the eastern phoebe.
Call: a harsh, slightly descending peeer; a short, even, rough brrt; and clear descending whistles similar to the eastern’s upslurred pwee-yee. Song: has 3-note tswee-tee-teet, usually mixed with peer notes; heard mostly on breeding grounds.
Status and Distribution
Common. Breeding: open woodlands. Migration: In spring, Arizona and California mid-April–mid-June. From Colorado to Oregon, first individuals generally appear in early May, with peak in mid- to late May. In fall, primarily August and September. Most have left United States by early October. Winter: mostly northwestern South America. No valid United States winter records. Vagrant: casual in East to Iowa, Minnesota, Wisconsin, Illinois, Indiana, Louisiana, Mississippi, Florida, Maryland, Massachusetts, Ontario, and Quebec, and north to western and northern Alaska and Yukon.
Breeding Bird Survey trends show widespread declines throughout most of range north of Mexico, but species not classified as threatened, vulnerable, or of “special concern.” Causes likely include loss and deterioration of habitat on both breeding and wintering grounds.
—From the National Geographic book Complete Birds of North America, 2006