Illinois Natural History Survey - University of Illinois

What Makes a Bird a Bird?

There are more than 9,000 species of birds in the world. More than 400 species have been recorded in Illinois. Birds are warm-blooded vertebrates. They have three characteristics which distinguish them from other animals: feathers, hard-shelled eggs and hollow bones.

WARM-BLOODED Like mammals, birds are warm-blooded. This means their body temperature stays the same no matter how hot or cold it is outside. This characteristic allows birds to maintain high levels of energy needed to fly.

FEATHERS Birds use their feathers in many ways; such as flight, regulation of body temperature (thermoregulation), protection of the body, attraction of mates and identification of species.

Contour feathers cover the body of a bird and have a strong, hollow shaft and network of hooks. Down feathers are small and are located under the contour feathers. The purpose of these feathers is to insulate the bird from the cold.

HARD-SHELLED EGGS Birds lay hard-shelled eggs. The hard shell keeps an egg from drying out and allows parents to sit on the eggs during incubation. Even though bird eggs are hard-shelled, they have microscopic pores which allow oxygen to pass into and carbon dioxide to exit the shell.

Eggs come in a variety of colors, patterns, shapes and textures. Colors and patterns on eggs vary depending on the need for camouflage. The shape of the egg depends on where the bird nests. Most eggs are oval. Birds that lay their eggs on ledges need eggs with a pointed end so they will not roll off the ledge. The texture of an egg may vary from smooth (hummingbird) to coarse (chicken).

HOLLOW BONES Simply having feathers does not permit birds to be creatures of the sky. Extremely lightweight bones are also necessary for flight. Bird bones are strong and hollow with inside supports.

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