Science · Grades 3–6

Birds

Reading comprehension lesson about birds — the only living dinosaurs, defined by feathers, hollow bones, and warm blood.

Reading Passage

Birds

Birds are warm-blooded vertebrates belonging to the class Aves. There are approximately 10,000 living species of birds, making them the most diverse group of land vertebrates on Earth. Birds are found on every continent, including Antarctica, and in nearly every habitat from deep ocean to high mountain peaks. All birds share certain defining characteristics: feathers, a beak or bill, two wings, two legs, and the ability to lay hard-shelled eggs. Birds are also the living descendants of theropod dinosaurs.

Feathers are one of the most remarkable structures in the animal kingdom. They provide insulation, enable flight, aid in camouflage, and are used in courtship displays. A single feather is made of hundreds of tiny barbs that hook together with even tinier barbules, forming a lightweight but strong surface. Birds spend a significant amount of time preening — using their beaks to realign and waterproof their feathers. Most birds molt, replacing worn feathers at least once per year.

Flight has shaped almost every aspect of bird anatomy. Birds have hollow bones that reduce weight without sacrificing strength. Their breastbones have a large keel, the attachment point for powerful flight muscles. Their respiratory systems are exceptionally efficient, with a system of air sacs that moves air through the lungs in one direction rather than in and out, allowing birds to extract oxygen even at high altitudes where the air is thin. Not all birds fly — ostriches, penguins, and kiwis are among the flightless birds that evolved in environments where flight was not necessary.

Birds show enormous diversity in diet, habitat, and behavior. Beaks are shaped for specific diets: the hooked beak of an eagle tears flesh, the flat bill of a duck filters water for aquatic plants, the long curved bill of a hummingbird probes flowers for nectar. Birds communicate through song, which serves to establish territory and attract mates. Many species migrate thousands of miles seasonally. Some birds, like albatrosses and parrots, are exceptionally long-lived, with lifespans exceeding 50 years.

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