Geoff Carey very helpfully commented on my earlier posts on Swinhoe's and Ashy Minivets, as follows:
"You mention in your blog that calls are very similar, which they are. However, with practice and assuming the call is heard clearly, they are distinguishable. I’ve attached some sonograms of the calls, three of Swinhoe’s and two of Ashy."
Swinhoe's Minivet calls.
Ashy Minivet calls.
"The call of Swinhoe’s is flatter than that of Ashy, as well as slightly shorter, on average. It’s interesting because when you look at the sonograms, you might not expect the call of Ashy to sound higher-pitched, but it does because it has a more ‘ringing’ quality. The elements of a Swinhoe’s call cover a wider frequency range, which, presumably, imparts the flatter timbre."
All sonogram images courtesy of Geoff Carey.
Calls of Swinhoe's Miniivets can be heard here:
And Ashy here: