The five Eastern US Empidonax flycatchers are usually separable by their call notes, but only with caution, as the following examples show.
Willow Flycatchers have a contact note that is quite similar to the pip note of Alder, and it is quite possible that a non-singing Willow using this call could be mistaken for an Alder.
Alder Flycatcher pip (July, ME).
Willow Flycatcher pip (July, PA).
A second potential source of confusion is a call by the Yellow-bellied Flycatcher that is similar to Acadian Flycatcher. This is not the pewee-like call heard often on the breeding grounds, but a squeaky peer used in migration and on the wintering grounds.
Yellow-bellied Flycatcher (Sep, PA).
More examples are in the Macaulay Library:
Sound is very similar but duration less in these two examples.
Note the similarity to the pwer and pweet of the Acadian Flycatcher.
pwer (June, DE)
pweet (May, PA)
This (squeaky "peer") Yellow-bellied call doesn't have the rising sound or tone of the pweet. It's closer to pwer, but is higher, squeakier and less descending or "plaintive".
A recent forum discussion on Xeno-canto revealed to me a second Yellow-bellied Flycatcher call that is heard on the wintering grounds that is very similar to the "pwer" call, but has a richer tone and is a little lower:
As with all vocalizations, there are going to be some outliers that lead to confusion. One example is this Alder Flycatcher call that is remarkably similar to the last Yellow-bellied Flycatcher call. It was used in conjunction with a number of other vocalizations, and would be unlikely to be heard by a migrant, but is an example of the surprising variety of vocalizations Empids can make (July, ME).
Ideally call notes should be recorded for an unidentified Empid, and used in conjunction with visual and other clues to come to an identification.