Check this out:
http://news.yahoo.com/s/livescience/20070412/sc_livescience/trexrelatedtochickens
Are dinosaurs related to birds? And will the Jurassic Park senario actually become a reality?
The Dinosauria reference book has a whole section on this, goes into minute detail, compares bone and skull shape, structure, and placement, hollow bones or bones with air sacs, muscle use etc. etc. After ploughing through this I must admit that I could only understand the simple stuff, not the finer points, but it did convince me that indeed birds and dinosaurs share a common ancestry. I find it fascinating to think that the blood of the mighty Allosaurus may yet flow through the bodies of the Humming birds that visit our feeders.
(http://i135.photobucket.com/albums/q122/DENNISDURWARDHAMMOND/allosaurus_p1.jpg)