You can’t teach a caged bird to sing

This is a blend of “you can’t teach an old dog new tricks” (can’t get people to change their habits or acquire a new skill) and “I know why the caged bird sings”, the title of the wonderful 1969 Maya Angelou autobiography.  While the latter perhaps is not literally a cliche or idiom, I think this still constitutes as a malaphor.  By the way, caged birds can be taught to sing, just as old dogs can learn new tricks.  A shout out to Eric Soller for blurting this one out and Jen Soller for sending it in!