Willow flycatchers?
We record all birds present (and their breeding status) along the Lower Colorado River, including tributaries such as the Virgin River. But we are to be "extra" vigilant regarding certain "focal" species, which definitely does include the southwestern race (subspecies) of the willow flycatcher (conveniently known as the southwestern willow flycatcher).
Some other focal species include Bell' vireo, summer tanager, Gila woodpecker, gilded flicker, and the southwestern race of the yellow warbler, I believe.