- 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.
Luckily the below bird was singing it's rreeBEEa song. On breeding territory at Meat Camp ESA near Boone, NC. There were Willows nearby too but this one bird was the most cooperative for photos.
Here is the link for a cool article about Willow and Alder songs and how they are believed to be innate and not learned. https://www.jstor.org/stable/4086218?seq=1#page_scan_tab_contents