Birds Are Dinosaurs
Birds are dinosaurs. At this point, that's not a very ground breaking statement, but I'm surprised by the number of people I talk to who don't know that already, or who balk at the idea.
So, let's get the basics out of the way first. Birds are definitely descended from dinosaurs. There may have been some question of this a few decades ago, but this is pretty much a settled question now. Just take a look at this figure from Donald Prothero's book, Evolution: What the Fossils Say and Why It Matters.
Here's another similar diagram from Peter Wellnhofer's Archaeopteryx: The Icon of Evolution.
Both diagrams very clearly show just how similar early birds like archaeopteryx were to their theropod cousins. Here's another diagram I like from Wikipedia. It compares the hands of a deinonychus and an archaeopteryx. If you were just shown flash cards of one these at a time without the other to compare to, would you be able to tell which was which?
And here's one more picture, also from Wikipedia, showing where birds fit into the dinosaur family tree. They're nestled right in among the theropods, in the Saurischian branch of dinosaurs.
So, that much, at least, seems to be settled. But many people still seem to have a problem with calling birds dinosaurs. For example, go read this posting on Yahoo! Answers, Are birds considered dinosaurs? (one of the answers there even linked to this website). Most of the answers there fell into the form of, birds are descended from dinosaurs, but not dinosaurs themselves.
Now, on one hand, I can appreciate this line of reasoning. After all, we are evolved from lobe-finned fish, and while I suppose that you could think of us as just very specialized fish, I don't think anybody but a cladist would actually call us fish.
So, the question becomes, are birds dinosaurs, or have they changed so much that they should no longer be considered dinosaurs? Or in technical terms, should dinosaurs be a monophyletic or paraphyletic term?
To answer this, let's leave dinosaurs behind for just a bit. Take a look at the pictures below, none of which are dinosaurs. (Click on any picture to go to the Wikimedia Commons source. Note that all have been touched up in some way, some more than others.)
That is a whole lot of variation. The first animal hops. The next walks on only one toe per foot, on a highly modified toenail only. After that is an animal that swims in the ocean and eats by using its mouth as a strainer. Next is another ocean dweller, but one that 'sees' through sound waves. And last, and perhaps most relevant to this discussion, is an animal that flies. Despite all this variation, these animals all share some common traits. For example, they all breathe air (even the ones that live in the ocean), and they all feed their offspring milk from a special organ in the females. So, these various animals all get grouped together as mammals.
Now, let's get back to dinosaurs. Here are a few more skeletons to compare. (Clicking on any of them will take you to the original source - some of these are copyrighted.)
There's still a fair amount of variation here, but not as much as in the group of mammals above. There are three bipedal animals, one of which can fly. The real outlier is the stegosaurus in the bottom right (note that the image is an outdated reconstruction, but close enough for this discussion). Just to be clear, the two skeletons on the left are modern birds. The two skeletons on the right are non-avian dinosaurs.
So, considering how much variation there is among animals that are still all classified as mammals, and considering how much more similar the gallimimus above (top right) is to the emu and eagle than it is to the stegosaurus, I don't see how you can group the gallimimus and stegosaurus in one group, yet leave the birds out. I just don't see any reason to classify birds as anything but flying dinosaurs.