Don't listen to these people, OP!

Fact: Big Bird appeared on Sesame Street in 1969.
Fact: Tzeentch's first horror models appeared in 1988.
Fact: Big Bird was 19 at this point.
Fact: Big Bird has never mentioned Tzeentch or talked about a relationship with him.

Conclusion: Big Bird and Tzeentch dated. Big Bird dumped him and doesn't even care to mention it despite his great fame. Tzeentch is jealous and deals with it by making people who have to do his bidding have bird-like features.