This book [1] is fascinating, and there's not much out there with the same info that isn't buy the same author or his circle of friends. There are a couple youtube videos.
Humans (who's livelyhood depend on it) can understand what birds are talking about. There's a story in there about an african safari tour guide driving along slowly with tourists and when he hears a certain sound, pulls over and shows his customers a rad snake or a lion.
One of the themes of the book is (re)learning how to do this, and how it can be therapeutic(?) and is a great form of _meditation_
The book's site had bird call recordings (probably not the same bird as the OP!) for those that are looking for them.