The Jolly Bird is a large blue bird of human height. Though a wild animal, it has anthropomorphic traits, and is capable of speaking English. The one the cast come into contact with had rather goofy, clumsy mannerisms, and also has a penchant for humming loudly.
Both the Noops and the Urpneys came into contact with a Jolly Bird (the former for one of it's feathers to cheer up Spildit, and the latter for a mineral in it's nest that works as a powerful fossil fuel). The Jolly Bird proved very boisterous, and fiercely protective of it's egg, leading to a tussle with the Urpneys when they hassled her and emptied her nest (though the Noops managed to salvage her egg for her).
The Urpneys were antagonized by a bird with a very similar appearance and mannerisms in the early episode "The Shrinking Stone", which possibly inspired the later character.