another dumb question: Terror Birds are not dinosaurs?

[Paula Goodman <paulawilder@mac.com>]

With all this talk about birds-are-dinosaurs, I'm curious to know what, 
if anything, distinguishes the bird status of Phorusrhacidae from that 
of the recently discovered Gigantoraptor, for example. Why do we call 
one a big bird fossil, and the other a dinosaur fossil?

Paula Goodman
bemused lay-person
Berkeley