On one of our reptile pages we see that reptiles arose from amphibians, and that later-day reptiles are "advanced" over amphibians because they can reproduce on dry land, and have scales that help their bodies retain water.
The fossil record clearly shows that birds, in turn, arose from reptiles, so birds, as you might expect, are more "advanced" than reptiles.
So, what advancements over reptiles do birds display?
If you answer "flight," then you're forgetting about pterosaurs, which are reptiles that lived about 160 million years ago -- some 10 million years before the first birds. With wingspans of over 25 feet, pterosaurs flew very well. If you say "birds were the first animals able to keep their body temperature warm when the air got cold," then you are getting closer, but don't forget that many paleontologists believe that certain dinosaurs were also warm blooded. Neither is "eggs" a good answer, because reptiles also lay very serviceable dry-land eggs.
At this point we're left with "feathers," and that's probably the best short answer, though the next section throws a shadow even on that. Moreover, many biologists would say that feathers aren't much of an advancement since, if you look at how they're formed, feathers are really little more than glorified reptilian scales.
Fossils of a creature known as Archaeopteryx display an animal that is practically half bird, half reptile. Though Archaeopteryx is winged and covered with feathers, it also possesses reptile-like teeth, which today's birds do not, and a long, jointed, reptile-like tail. The University of California at Berkeley has a nice Web page showing the famous Archaeopteryx fossil. You may also enjoy reading a 2011 BBC News report about the ongoing controversy over whether Archaeopteryx is a bird or a dinosaur.
Fact is, from a biological point of view, there's not much fundamental difference between birds and reptiles. We usually think that birds have feathers while reptiles have scales. However, some dinosaurs also had feathers. In 2007 paleontologists at the American Museum of Natural History and the Field Museum of Natural History announced that even the dinosaur known as Velociraptor, made famous in the movie Jurassic Park, had feathers.
One way to convince ourselves of the closeness of birds to reptiles is by looking at a chicken's legs. Chicken feet bear powerful claws worthy of any small dinosaur, and they are covered with scales. One often sees in chicken yards one or more individual chickens whose lower legs sprout fluffy feathers where scales should be. This happens because genetic information for making a feather is practically the same as for making a scale, just with a tiny bit of extra information thrown in.
Therefore, in an evolutionary sense, the dawning of the class of birds was, compared to other epoch-making advancements, "no big deal." However, in a way, this makes birding even more enchanting.
For, in a real sense, when we observe birds, we're doing something awfully close to "dinosaur watching!"
You can check out books on birds-in-general at Amazon.com by clicking here.