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There were a few lineages of avian creatures before the K–Pg event. Only 1 survived: Ornithurae.
Ornithurae specifies the common ancestor to all modern birds. By the time the dinosaurs went down, Ornithurae had already diversified, including the birds that became chickens, ostriches, and ducks.
The Chicxulub impact caused global deforestation. Bereft of trees, arboreal birds could not survive. Only ground-dwelling birds made it through the extinction event.
The forest canopies collapsed. Perching birds went extinct because there were no more perches. ~ American paleoecologist Regan Dunn
The regrowth of forests meant the return of opportunities among the trees, including food and safety. Birds re-evolved the ability to fly.
Modern birds have short tails, as contrasted to Archaeopteryx and other bird-like theropods.
