Some years ago, I just happened to be at the Powdermill Nature Reserve in southwestern Pennsylvania where, among other things, ornithologist Mike Lanzone was recording flight calls of migrating songbirds. Mike had figured out then that captured migrants (from year -round, daily mist-netting and banding at Powdermill) would give flight calls when prompted by the sound of conspecifics flying overhead. So Mike applied his ingenuity to develop the technology to record flight calls from these migrants, and he began building an auditory library.
In 2006, the Cornell Lab of Ornithology‘s Andrew Farnsworth developed the “Rosetta Stone for Warblers” that compiled and spectrally compared flight calls of 48 species of warblers in North America. It’s very cool . . .