Birds are now generally believed to represent descendents of dinosaurs, and so are classed with them by those who believe groups should be monophyletic.
Can someone point to a reputable source that claims birds are a kind of dinosaur? - Tim
Just type "bird dinosaur site:.edu" into Google. Here's one that popped up: http://www.ucmp.berkeley.edu/diapsids/avians.html. "Kind of dinosaur" is of cause a matter of definition, but there is no questions that birds are the closest living relatives of the dinosaurs, closer still than the crocodiles and other reptiles. --AxelBoldt
The point is more than that. Birds are not just the closest living relatives of the dinosaurs, but are in fact closer to some dinosaurs than they are to others. As there has recently been a big push for monophyletic groups, the Dinosauria would either have to be broadened as a formal category or extended to include birds. In practice the latter approach seems to be the one taken. I can't name any particular source, though UCMP and the tree of life would be some of the first I check, but I have seen it in a few more recent books.
On the main page there had been a comment that only the most adaptable animals survived the K-T extinction, like mammals. I don't believe this is concensus, and there is a lot of evidence against this. Dionsaurs were highly adaptable and diverse, as were ammonites and other groups that were devastated or destroyed, while crocodiles and other such forms managed to pull through. Indeed the placental mammals had not shown much diversity before the Cretaceous. If anything, then, I would say that the more conservative groups were the ones that faired better. If there is a kernel of truth in the statement, it will need considerable modification before it is true enough to replace.