The flycatcher is a summer breeder within its range in the United States. It is gone to wintering areas in Central America by the end of September. Nest territories are set up for breeding, and there is some site fidelity to nest territories.

The southwestern willow flycatcher is small, usually a little less than 6 inches in length, including the tail.

These birds have conspicuous light-colored wingbars and lack the conspicuous pale eye-ring of many similar Empidonax species. Overall, they are brownish-olive to gray-green above and have whitish throats, pale olive breasts and a yellowish belly. Their bills are relatively large and have completely pale lower mandibles.

These birds are best identified by vocalizations. The southwestern willow flycatcher call a liquid, sharply whistled whit! or a dry sprrit and have a sneezy witch-pew or fitz-bew song. While perched, the southwestern willow flycatcher characteristically flick they tail slightly upward.

For nesting, the southwestern willow flycatcher requires dense riparian riparian
Definition of riparian habitat or riparian areas.

Learn more about riparian habitats with cottonwood/willow and tamarisk vegetation and microclimatic conditions that are dictated by the local surroundings. Saturated soils, standing water or nearby streams, pools, or cienegas are a component of nesting habitat that also influences the microclimate and density vegetation component. Habitat not suitable for nesting may be used for migration and foraging. Recurrent flooding and a natural hydrograph are important to withstand invading non-native species like tamarisk. The southwestern willow flycatcher is typically found below 8,500 feet of elevation. Critical habitat was finalized on October 19, 2005 in Apache, Cochise, Gila, Graham, Greenlee, Maricopa, Mohave, Pima, Pinal and Yavapai counties (70 FR 60886). Revised critical habitat was proposed August 15, 2011 (76 FR 50542) and includes river segments in counties currently designated plus those in La Paz, Santa Cruz and Yuma counties. The 2005 critical habitat designation remains in effect until the current proposal is finalized. Training seminar/permits required for those conducting call playback surveys.

The southwestern willow flycatcher primarily eats flying insects.

The southwestern willow flycatcher arrives on breeding grounds in late April to early May. Nesting begins in late May and early June, with fledging from late June to mid-August. Southwestern willow flycatchers typically lay three to four eggs per clutch, and are laid at one day intervals that are incubated by the female for about 12 days. Young birds fledge 12 to 13 days after hatching. Typically, southwestern willow flycatchers only raise one brood per year. However, some pairs will raise a second brood, or renest after a nest failure.