Birdman of Alcatraz (1962), directed by John Frankenheimer.
The core of this film has marvelous elements:
Burt Lancaster's tightly wrapped, restrained performance as a hard man, in prison most of his life, in solitary most of that.
The "escape within" motif, finding freedom and dignity within the prison walls.
A main character who develops: not into a softy, but into someone with a purpose who can sometimes be generous. It demonstrates how self-discipline comes from caring for others.
The remarkable true story of a man without education who became a published scientist and recognized expert in his field.
...while still being a rebel and irritant to the prison system.
The downside:
It is overlong and padded with some soapy elements involving Mom and a wife.
The prison reform Message takes us out of the drama.
A late segment with a prison riot and siege seems like another movie, although something like that really happened.
I think the "birds" themselves are easy to accept. If you've ever lived alone with only a pet for company you know how deeply important they become to you. It always ends in tears.
I note that the prisons are stark but seem humane and well-run for the time. The guards are professional and we have no beatings or abuse.
Neville Brand has perhaps his most sympathetic role as a guard who grows old with the inmates. Telly Savalas provides the only comic relief as another prisoner. Karl Malden, as always, is excellent as a warden who believes he is doing the right thing, but probably isn't.
This is based on a true story, but the real Robert Stroud was a less appealing character, a high-IQ psychopath, dangerous to guards and inmates alike and a sexual predator. That's why he was in solitary so much: everyone was afraid of him, with good reason.
Twilight Time Blu-ray with an excellent image. The usual crew provide an excited non-stop commentary track.