1. Birds are endothermic vertebrates.
2. Their skin is covered with feathers.
3. They have four-chambered hearts.
4. Their bones are lightweight and usually hollow.
5. Their forelimbs are modified as wings.
6. They lay eggs.
You already knew all the facts that are in bold, didn't you? Everyone knows birds have wings with feathers and lay eggs. You may not have known they are endothermic, which is Latin for "inside temperature". This means that they can maintain their own warm inside temperature, even when it is cold outside. Frogs (amphibians) and snakes (reptiles) have to hide when the weather is cold, but birds can be out and active. Let's think of the various kinds of birds at Cazadero, and think what we already know about them.
The characteristics of the class Birds are rather well-known and hardly need to be taught. We all know that birds have wings, chickens lay eggs, and that birds of a feather flock together. The only character that you might not have known is that they are warm-blooded, unless you or a friend have a pet bird at home or if you have held a chicken. Birds in general have a higher body temperature than we do, so they feel quite warm when we hold them. Their feathers are well-adapted to hold this heat in, which is why we make comforters out of down.
Jays, robins, and ducks are all birds because they are warm-blooded, have feathers, wings, and lay eggs.
