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Bird Evolution

Birds likely descended from a group of theropod dinosaurs that evolved during the Mesozoic era.

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Bird Evolution

Did birds really evolve from dinosaurs?

One is an ostrich, the other is a mononykus dinosaur. The structural relationship is obvious. Mononykus moved about on two legs, was very nimble, and could run at high speeds, something that would have been useful in the open desert plains where they lived. It had a small skull, and its teeth were small and pointed, suggesting that it ate insects and small animals, such as lizards and mammals.

Evolution of Birds

Birds are thought to have evolved from a group of bipedal dinosaurs called theropods. The ancestor of birds was probably similar to the theropod called Deinonychus, which is represented by the sketch in Figure below. Fossils of Deinonychus were first identified in the 1960s. This was an extremely important discovery. It finally convinced most scientists that birds had descended from dinosaurs, which had been debated for almost a century.

Deinonychus illustration: extinct bird relative

Extinct Bird Relative: Deinonychus. Deinonychus shared many traits with birds. What similarities with birds to you see?

What was Deinonychus?

Deinonychus is the genus name of an extinct dinosaur that is considered to be one of the closest non-bird relatives of modern birds. It lived about 110 million years ago in what is now North America. Deinonychus was a predatory carnivore with many bird-like features. For example, it had feathers and wings. It also had strong legs with clawed feet, similar to modern raptors. Its respiratory, circulatory, and digestive systems were similar to those of birds as well. The location of fossilized eggs near Deinonychus fossils suggests that it may have brooded its eggs. This would mean that it was endothermic. (Can you explain why?) On the other hand, Deinonychus retained a number of reptile-like traits, such as jaws with teeth, and hands with claws at the tips of its wings.

Evolution of Flight

Scientists have long speculated about the evolution of flight in birds. They wonder how and why birds evolved wings from a pair of front limbs. Several hypotheses have been suggested. Here are just two:

  1. Wings evolved in a bird ancestor that leapt into the air to avoid predators or to capture prey. Therefore, wings are modified arms that helped the animal leap higher.
  2. Wings evolved in a bird ancestor that lived in trees. Thus, wings are modified arms that helped the animal glide from branch to branch.

Scientists still don’t know how or why wings and flight evolved, but they continue to search for answers. In addition to fossils, they are studying living vertebrates such as bats that also evolved adaptations for flight.


  • Birds are thought to have evolved from theropod dinosaurs around 150 million years ago.
  • Bird ancestors may have been similar to the extinct theropod Deinonychus, whose fossils convinced most scientists that birds evolved from dinosaurs.
  • Scientists still don’t know how or why wings and flight evolved, but they continue to search for answers.


  1. What was the bird ancestor?
  2. What was Deinonychus? What bird-like traits were evident in Deinonychus?
  3. What are the two ideas associated with the evolution of flight?

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endothermic The ability to regulate body temperature from the inside, through metabolic or other physical changes.
theropods Bipedal dinosaurs; had feathers and wings; though to be the predecessor of birds.

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