There has long been debate about Big Bird's species, with some experts claiming he was a canary, but recent genetic analysis places him firmly in Cariamiformes.
This comic is a scene of two people, Jill and Hairy, at a museum looking at a fossil of a prehistoric terror bird. Hairy thinks out loud that Big Bird, a Muppet character from Sesame Street, must be a terror bird, and wonders whether he will be updated to be more scientifically accurate, i.e. more scary. Big Bird, however, being from a show for little kids, would almost certainly never be made to look scary, and also, he was never intended to be a terror bird. That it is Big Bird that he is thinking of becomes clear in the title text.
The title text claims that there is ongoing scientific debate about Big Bird's species; initially, he was thought to be a canary, likely due to his color, but scientists have placed him in the order Cariamiformes, which the terror birds belong to. This text is likely intended to mirror what somebody might say about the seriemas, which are modern-day birds believed to have evolved from the terror bird, as if Big Bird were the descendant of these birds, in addition to or instead of seriemas. Actual scientific analysis seems to suggest that Big Bird is in fact related to the whooping crane.