What kind of bird is Big Bird? A scientist explains

Mike sez, "When I was in the early stages of my PhD on the evolution of flightless birds, I was poking round in the storage cabinets of the Berlin Museum of Natural History one summer's day, and I had a revelation -- an original scientific insight. I realised what kind of bird Big Bird almost certainly is. Last month I presented my findings at the Christchurch, New Zealand, PechaKucha #8, and the audio and slides are now live. (A pecha-kucha is a talk in which 20 slides play for exactly 20 seconds each, and the speaker tries to keep up.) All the science is real, and no Big Birds were harmed in the course of this research."

What, if Anything, Is Big Bird?

(Thanks, Mike!)