would you be willing to expand on this hypothesis

same general body structure, has hands instead of full wings, has a slight hump, similar beaks

clearly Deinocheirus' descendants evolved to have more upright postures, a shorter tail, and forward facing eyes over the past 66 million years

This implies Big Bird slowly evolved into a carnivore.

look, it's been a while since I saw Follow that Bird, because it makes me cry every damn time, but I'm pretty sure Big Bird eats grains

that said, this means Big Bird is an herbivore that convergently evolved carnivore-like traits, which should be significantly more alarming to all of us

Ok, I want to know why it should be more alarming. Am I missing something?

Are you trying to imply that this is batsean mimicry of a big bird looking actually carnivorous species, or is this stretch?

I'm not a scientist, just an enthusiast.

So herbivores are more alarming than carnivores in general because carnivores get full. Herbivores are always on alert for predators and either have one of two responses: run (these can trample us) or fight (these will destroy us). This is why herbivores are usually much more dangerous than carnivores - for one quick example, more people are killed by the herbivorous hippopotamus each year than by sharks (yes, all species thereof)

so, an herbivore convergently evolving carnivorous traits means its an herbivore designed to take out potential dangers with the swiftness and lethality of a predator

that is extremely alarming

consider the cassowary. now imagine it more exact and capable. now imagine it the size of big bird.

we should all be glad he is a friendly presence on sesame street and not the unholy terror he should be

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.

someone needs to throw together a phylogeny and I recognize that, as a bird researcher, who has done too many phylogenies of birds, I am the prime candidate, but I have too much to do for SVP...

Leave it to the grad student to do the doctor's work

Now obviously we don't have genetic material to work with here, but based on morphology and phylogeograpgy, this seems to be a likely family tree. The common ancestor likely had a similar morphology to the North American big bird, but with a more generalized beak, and originated somewhere in Eurasia.

Caponata with her highly derived legs seems to be the biggest morphological outlier. Given her residence in Spain, I think it likely that she branched off early and evolved in isolation on some Mediterranean or Atlantic island, before eventually returning to the mainland.

The major division after Caponata is between the Eurasian and American big birds. Poupas and Minik Kuş are likely to be sister species, with Pino being an outgroup to them.

Since the big birds originated somewhere in Eurasia, the American lineage would have descended from some Eurasian migrant. While moving eastward to cross from Siberia to Alaska is possible, the basal position of the south and central American species indicates that they arrived in the south before moving north, suggesting a rafting event from Africa to South America. Garibaldo branched off first by remaining in Brasil, while the rest moved north after the great American biotic interchange. Abelardo branched off as they moved through central America, and eventually specialized on a durophagous diet. Finally, the north American taxa split between the boreal Toccata and the more temperate Big Bird.