What if anything is a bird

Clearly, the old equation [feathers = bird] won't fly; there are now many examples of feathered, non-flying dinosaurs below Avialae on the cladogram. Likewise, the equation (warmblooded = bird) also doesn't work; these feathered dinosaurs were surely warm-blooded. Should Aves - traditionally birds - be restricted to all those organisms bearing the distinctive suite of characters of living birds? That would, of course, exclude Archaeopteryx, which certainly has a plausible claim on the designation "bird." Should the equation be bird = flight?

Cursorial Hypothesis

Figure 10.13. The arboreal and cursorial hypotheses for the origin of bird flight. (a) The arboreal hypothesis, which suggests that bird flight evolved by "birds" gliding down from trees. (b) The cursorial hypothesis, which suggests that bird flight evolved by "birds" running along the ground until the animals became airborne.

Figure 10.13. The arboreal and cursorial hypotheses for the origin of bird flight. (a) The arboreal hypothesis, which suggests that bird flight evolved by "birds" gliding down from trees. (b) The cursorial hypothesis, which suggests that bird flight evolved by "birds" running along the ground until the animals became airborne.

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