Everyone knows what a bird is, right? If I ask my seven-year-old son what a bird is, he 'll respond with something like "a bird is an animal that has a spine, wings, two feet, hollow bones, and can fly." Or, if her remembers the little military chant his dad made for him, he might say, "hollow bones and scaly feet, feathered wings and goes tweet tweet."
Well, penguins can not fly. They have wings, feathers, two feet, and a spell, and they swim well but they can not fly. Ostriches can not fly either, but both penguins and ostriches are considered birds. How is that possible? What's the deal?
It's all in the definition
There's a difference between the common usage of the word "bird" and the scientific use of the word. The common definition is based on features of the animal you can see with your eyes and discern with your other senses like feathers, wings, number of legs, and being warm-blooded. My seven-year-old son knows the common definition of the word "bird."
Scientists use a slightly different definition.
The scientific taxonomy of birds is a bit different than common usage. The scientific groups are made based on fossil evidence and other biological evidence such as DNA and mitochondrial DNA when the DNA can be obtained. Birds are in the Domain Eukaryotes, the Phylum Chordata meaning vertebrates, and the Class Avians. Avians have descended from theropod dinosaurs. More specifically, birds have descended from Archaeopteryx, which inserted in the late Jurassic period.
Many scientists think of birds as the only type of dinosaur that did not go extinct 65 million years ago. In fact, my daughter who is obsessed with dinosaurs, calls birds "tiny dinosaurs."
Scientifically, birds today are descended from dinosaurs, have feathers, a beak with no teeth, and they lay eggs with hard shells. Birds have a high metabolic rate, meaning they need to eat a lot to maintain their body temperature. (Some refer to them as warm blooded.) They have a four-chambered heart (like mammals), and they have lightweight, strong skeletons. Most birds can fly, but flying is not a requirement to be a bird.
And that's the crux of it. The scientific definition of "bird" does not require the ability to fly.