The Alder Flycatcher is so similar to the Willow Flycatcher that they were thought to be the same species. Song is the only definitive way to tell them apart. However, measurements of crown color with a colorimeter, together with other measures of wing shape, bill and tail, may be able to distinguish birds in the hand that are not calling.
The Alder Flycatcher's nest is a coarse, loose cup that nearly always has material hanging off it. The nest of the Willow Flycatcher tends to be neater, with no hanging material.
Willow and Alder flycatchers do not respond to playback of recordings of each other's songs, even where their ranges overlap.
In an experiment on song learning, Alder Flycatchers were "tutored" with Willow Flycatcher song in the first two months of life. The next spring, the Alder Flycatchers sang normal Alder Flycatcher song.
The longest-lived Alder Flycatcher was over 9 years old, when it was recaptured and rereleased during a banding operation in British Columbia.