Turns out the consensus of songwriters is correct: The nightingale is the most musical of birds. At least it has, by far, the most extensive repertory of birdcalls.
Researchers at the University of Bath and Cornell University, studying 49 species of songbirds from the US, Europe and South Africa, found that the common nightingale produces 1,160 “syllables,” compared with 341 for the Eurasian skylark and 108 for the common blackbird. (Condolences to Johnny Mercer and Paul McCartney.)
Jamie Doward and Amy Moore of The Guardian report on the study:
http://www.theguardian.com/environment/2014/may/04/nightingale-best-birdsong-brain-research