It's easy to "know" what a bird is. It has feathers, it flies. Or does it? Penguins are birds, they don't fly. Hummingbirds look like large insects. What defines a bird?

In the days of the Bible, the word we equate to bird just meant 'flying thing'. In one quote, it says Lev. 11:13, 19 And these are they which ye shall have in abomination among the fowls...And the stork, the heron after her kind, and the lapwing, and the bat.

As our scientific knowledge grew, we began to make finer distinctions. Bats are now mammals - in essence flying mice. But why say it's a mammal and not a bird? Mostly because bats and flying squirrels all had roots in one line of evolution - things that crawled and then learned to glide. Birds had roots in separate line of evolution - from reptiles that glided, that then evolved to have feathers.

One of the oldest dinosaur-birds is the Archaeopteryx. Fossilized skeletons clearly show its feathers and wings. From this root and ones similar to it grew the 10,000 or more current day species of birds.

Over the years, birds changed to meet different environmental needs. Ostriches and emus no longer need to fly - their large size protects them from harm. Penguins swim. Peacocks have very showy tail feathers, cardinals have very bright feathers all over. Each bird fits into its niche beautifully. Even in modern day times, we have seen birds change subtly over the years, becoming better adapted as their environment has been altered. Their songs change, their coloring changes, their size changes.

The Scientific Organization of Birds - the Actual Classification Tree
Bird Classification Example - Chickadee
The Orders of Aves Birds