From the Super Mario Wiki
The Stool Pigeon is a Tweeter who resides on the streets of Crime Land. Being an actual stool pigeon, he has insider information on all the crimes that are going on in the city. The Stool Pigeon only ever appeared during the events of The Super Mario Bros. Super Show! episode "The Unzappables".
Overhearing that Mario was in search of a stool pigeon, the Stool Pigeon attracted his attention, and offered to tell him about the next big robbery that Al Koopone was going to commit, for a price. Toad offered the Stool Pigeon a bag of chicken feed. While less than thrilled, the Stool Pigeon accepted the offer on account of being a bird, and told Mario's group that Al Koopone was going to rob the First Mushroom Bank, which turned out to be true.