At Sesame Place the Big Bird walk-around character wears a necktie. The purpose for this is because the performer's head is in Big Bird's neck and the performer looks out through mesh material in the tie.
This is different than how Big Bird is operated on the TV series (Sesame Street performer Caroll Spinney looks at a small monitor inside the suit.)
Image from MuppetWiki
Currently Big Bird wears a striped multicolored tie.
At some point he had a solid purple tie:
And at some point he had yellow spots on a purple tie:
I don't know if there were other variations.