Big Bird was the main character from Jim Henson's Sesame Street kids TV programme.
Basically, Big Bird was a big, yellow bird with a slightly camp voice and very long eyelashes. Although he was originally the main character in Sesame Street, he eventually lost out in the popularity stakes to faster-talking, smaller, cuter, Elmo (and the 'Tickle-me-Elmo' toy parents kept buying until it drove them insane with its cute voice and disturbing vibrating giggles).
Big Bird was featured in Sesame Street less and less - perhaps because he was so large (7ft or so) it was easier to do scenes with all the little puppets. He was generally the one who did story-time though, probably because for the little kids in the show, snuggling in to Big Bird's feathers was a pretty comfortable way to go off to sleep (and his voice was so slow it was bound to send them into a deep sleep).