From Homestar Runner Wiki
The Fat Bluebird is, as the name suggests, a very fat bluebird that appears on the loading screen of Strong Bad Sings and also lands on Strong Bad's hand shortly after it begins. It's also one of Mike Chapman's favorite characters, as mentioned in the commentary of the DVD version. Matt Chapman wondered if it had his own page on this wiki, which it hadn't at the time. Despite Mike's love of the bluebird, it is not included on the Everybody Everybody Poster.
It also has a belly button, despite the fact that bluebirds are not mammals.