American lullaby. Cecil Sharp collected two voice only variations in North Carolina (1916) and Virginia (1918). "Mama's gonna buy you a mockingbird"; mockingbirds were once held as pets indeed. President Thomas Jefferson (1801-1809) had one.
Bernice Haynes [John & Ruby Lomax recording]
Elvis Presley [in film Girl Happy as Puppet On A String]
Peter, Paul & Mary [as Mockingbird]
Lyrically related with comic blackface song Hush Little Baby Don't You Cry (credited to F. Balasco - 1884), Hambone (1952), Bo Diddley ('55) and Mockingbird ('63). Not to be confused with Go To Sleep Little Baby (see there).