It is a true story. But keep in mind that when adapted for the screen or other forms of media, certain details or elements might be emphasized or modified to make it more engaging or fit the medium, while still maintaining the core truth of the events.
The 'Birdman of Alcatraz' is indeed a true story. It takes inspiration from the experiences of Robert Stroud, who was imprisoned at Alcatraz and developed his interest in birds. However, some artistic liberties might have been taken for the sake of the narrative.
Yes, it is based on a real person and events. The story has roots in the life of Robert Stroud.