This cartoon has haunted me since I last saw it at around age 5 (I'm 56). It may be a version or variation of the Emperor and the Nightingale. A king loved a little bird, who flew away, and then returned. At the end of the cartoon, the king removes a ring from his finger - which is a miniature crown - and puts it on the little bird's head. I'm guessing it was from the 1940s or 1930s. I believe this cartoon was in color, possibly an early Merrie Melody or the like. Nothing obscure, definitely produced by one of the big cartoon makers. I grew up in Brooklyn and there were numerous programs of "old" cartoons broadcast daily in the 1960s.