When looking at the similarities between birds and any other creature, including dinosaurs, it is first necessary to define the characteristics of birds. This isn’t quite as easy as it sounds, because while most bird species may have some things in common, a few may not. However, this is a good starting point, since the most common traits for most birds will be alike.
For instance, ostriches, emus, kiwis, and penguins don’t fly through the air. Most other birds do have at least limited flight capability, and penguins move through water in way that is much like flight through the air. It isn’t the ability for flight that is important, though. Nearly all birds except those previously mentioned have a heavily keeled breast bone. The flight muscles are attached to this, so it needs to be strong and pronounced.
Dinosaurs also had a great variation. However, though not as pronounced as the breastbone of birds, many dinosaurs did have a keeled breastbone. In a few of these cases, there didn’t appear to be a reason for it. This indicates that dinosaurs and birds may have had a common ancestor.
Many people can point out that birds have feathers. This may seem dissimilar to dinosaurs, since dinosaurs are thought to have had scaly skin like the reptiles of today. However, is it that dissimilar?
Scales, hair, and feathers all serve the initial purpose of protecting the animal and helping it to maintain its body temperature. This is even true of cold-blooded reptiles of today’s world. Feathers may in fact be modified hair. Hair is something that doesn’t fossilize well. This means that though the thought might be unusual, we don’t know just how many dinosaurs had hair.
There is a growing amount of evidence that at least some dinosaurs may have been warm-blooded, just as birds are. Through most of the time the dinosaurs were living, the earth was a great deal warmer than it is now, and polar ice caps didn’t even exist. It might make one wonder why it was necessary for some dinosaurs to be warm-blooded.
Then again, there are birds in the tropics that are warm blooded, despite higher and nearly constant temperatures. This could lead us to postulate that perhaps this gave certain species the advantage of being able to adapt when temperatures fluctuated. Either way, it is a similarity between dinosaurs and birds.
Many or most birds have very low weight bones. This is from having air pockets in them, and it helps to make them light enough for the bird raise its mass off the ground. Surely there can’t be a similarity here, right? Many dinosaurs did have similar bone structure, though. We are still guessing at all the purposes and advantages this would give.
The skull structure of birds and dinosaurs is also similar, and apparently, the same areas of the brain were of the greatest importance, for instance those responsible for eyesight and the sense of smell. Even the eye sockets were similar in shape and in relative size to the skull.
Certainly, they were similar in feeding habits. Some birds are predatory, some eat plants, some eat carrion, and some will eat almost anything they can. The same can be said of dinosaurs.
Dinosaurs and birds also tend or tended to have a nest and to lay eggs. As similar as many birds are to mammals, this is unusual in that mammals develop the young within the body. A few reptiles do this today, though it is usually still in the form of eggs. The eggs simply hatch while still within the mother.
There is no conclusive proof that birds evolved from dinosaurs. However, there are so many similarities between the two that it is becoming progressively more likely that they had a common ancestor.