To add more to Big Bird's biology, it should be noted that his species is either very diverse in morphology, or his clade contains many species that live in various countries' Sesame Streets. Some of them, like Bibo from Germany, look pretty much the same as American Big Bird, but others are quite different. In the Netherlands we have Pino, who could easily be the same species but has a more pronounced crest, orange beak, and blue feathers everywhere except the area around the eyes. On the rare occasions he has actually met Big Bird, he called him Cousin Jan.
Brazillian Sesame Street is inhabited by Garibaldo, who judging by beak and leg morphology is likely a different species. Between the sleeker legs and the very sharp beak, I think a stork-like lifestyle is likely for his ancestral population.
Then there's Abelardo from Mexico, who seems to come from a population that convergently evolved several parrot-like traits. An interesting detail about this is that, unlike most other vertebrates, parrots don't absorb their pigments through their diet, but make their own pigments called psittacofulvins. Given that his relatives are quite colourful themselves and can probably obtain plenty of carotenoids from their diet, I don't think that is likely to be the case in Abelardo.
Caponata from Spain looks very different from all her family members, even the more divergent ones. This could be since she is the only female member of the clade I have managed to find so far. However, the very different feet make me think she might hail from a very derived species instead. I can't even imagine what sort of evolutionary pressures could lead to a bird evolving such strange feet.
Other Sesame Street birds are a little harder to find information about, with mostly older sightings. There seem to have been sightings of one named Toccata in Quebec. His shaggier white feather coat covering more of his legs and being thicker around his neck does suggest adaptations of the harsh winters up north.
Portuguese Poupas has different colouration and seems to have some sort of feather puff going on around the ankles as well. The feathers around the head are notably swept backward as well. I think this is a somewhat derived population of the American species. Minik Kuş from Turkey may hail from this population too, or perhaps from some intermediate or interbred population, as they seem to be more morphologically similar to the more typical Big Birds.