I recently learned that some birds have been found to be able to isolate and control different parts of their vocal tract independently, allowing them to sing simultaneous double tones or alternate between frequencies very rapidly. I’m not sure about the particular species below, but I think it’s safe to say that numerous birds on this album are at least using similar ‘mad avian skills’ to sound like synthesizers.
This album, recorded by celebrated ornithologist Johan Dalgas Frisch, (and first released on the Sabia label in 1961), was once in the Top 50 of Brasil’s popular record sales. President João Goulart actually gave JFK a copy when he came to visit. Talk about a country with its musical priorities in the right place!
This collection of recordings isn’t readily available in the states, but if you can’t track down a used LP or torrent, it looks like you can buy it from this site in Brasil.