The Birdman

From Muppet Wiki

The Birdman is an elderly rat in the medical research lab in Muppets from Space. He trains pigeons and has a particular soft spot for Gladys.

The character is inspired by Robert Stroud, the "Birdman of Alcatraz," a prison inmate whose study of birds led to a 1955 book and a 1962 film starring Burt Lancaster.