lol this made me laugh, because i actually had to look up what kind of bird he was for one of my preschoolers who asked the same question when we were doing a day of birds.
Truth is Big Bird is a mutt...
Big Bird himself has confessed to being a lark & giant golden condor, and he has also been refered to as a turkey, an ostrich and 'part homing pigeon'
after finding all that info (and pics of each bird species to compare) the kiddies decided very firmly that Big Bird is a bit of everything. which i think is pretty much the whole point Sesame Street was trying to get across (that whole doesnt matter where you are from, what you look like thing)
anyway, hope that helped
Ps. you know how some people have that saying 'what would jesus do?'(wwjd?) our preschool class now has a 'what would Big Bird do?' motto lol