Living off Rats
Even though he didn't turn into a cat like McGonagall, Sirius Black was living off rats. As much as he tried to pretend, for the benefit of the canine in him that was appalled to survive in such a distinctly feline fashion, he was disgusted by this fact, he had to admit that he got a thrill from stalking the vile vermin and pouncing on them. When he trapped their tails under his paws, he saw Peter unable to escape his grasp at last. Every time he gulped down the tail, he growled deep in his throat: That's for Lily. And, every time he devoured the head and body, he snarled: This is for James. Revenge, everyone said, was supposed to taste sweet, but, for Sirius, every time it tasted bitter and bloody, but no less addictive for that. Perhaps Bellatrix wasn't the only one who had inherited the insanity gene, or had been driven mad in Azkaban's dread cells.