Acadian Flycatcher: This is probably our earliest spring migrant Empidonax although they were still passing in numbers yesterday. Dorsally they are a greenish gray, sometimes very green. Ventrally they are the most pale below of all our empids. They have a nice even pale yellow eye ring. The bill is large and the lower mandible is fleshy orange. They often appear to have a flat angled forehead. They have a long primary extension. Down here in migration the voice is an emphatic "speet" which is often repeated. Unfortunately Yellow-bellied Flycatcher can give an identical call which I heard today at Santa Ana. The similarity of the calls is not addressed in the literature.
Least Flycatcher: Our most common Empidonax can also be an early migrant and usually a few winter in the RGV. I don't really notice them as being small but they do seem slender to me. Dorsally they are brown or grayish brown with a big white eye ring. Ventrally they have a whitish throat and and a brownish gray vest on the sides of the breast. Usually they seem round headed. The belly is whitish often with a yellowish wash. Bill is short and mostly dark. Primary projections are short giving it a long tailed look. The voice is a dry "whit" or "fwit" which can be repeated often.
Yellow-bellied Flycatcher: A fairly late spring migrant, I have only noticed them this past weekend. They are possibly the easiest to identify of our migrant Empids as they tend to be a bright green dorsally with a bill yellowish eye ring. The underparts are greenish on the breast with a greenish yellow throat and a fairly bright yellow belly. The medium length broad based bill is orange below. Usually they seem round headed. Primary extension are moderate. I do not hear the "dweeb" call that is described in the literature but rather an emphatic "speet" which is often repeated and sounds very much like the Acadian call.
Alder Flycatcher: Alder and Willow Flycatchers were for many years considered as one species, Traill's Flycatcher. To me they seem more "multicolored" than the other empids with grays, greens, browns, yellow and white. They seem like all the above empids blended together and I usually decide it's a Traill's Flycatcher by it not being any of the above. The thin and sometimes uneven eye ring is a good field mark as is the white throat against the dark head and breast. The medium length bill has an orangish lower mandible. Primary extension is moderate. I'm still learning about these guys. For Alder the somewhat musical "pep" or "perp" call is diagnostic. I heard three of them today at Santa Ana and these photos are from there.
This one from the Convention Center yesterday did not call but later I did have possibly the same one call the diagnostic "perp".
This one called yesterday but the lighting was not good.
Willow Flycatcher: If you have a Traill's Flycatcher that that make a "whit" call then you have a Willow Flycatcher. I don't know how to visually separate it from Alder. I don't know if anyone does. This Willow Flycatcher was calling at the National Butterfly Center last week.
So this is just my view of Empidonax flycatchers in spring. They can be very different in fall, all species being much more green and yellow. They will be passing through again in late July and August so we can all get more practice.