all of the 10,000 species of birds that are alive today descended from only a handful of species of birds that survived the kpg extinction that caused the extinction of the dinosaurs. However, unlike mammals, birds were not just common but also very diverse before the extinction, and along with the extinction of the non avian dinosaurs thousands of species of ancient birds died as well. Birds evolved from dinosaurs in the Jurassic period about 150 million years ago but by the early cretaceous 140100 million years ago they had adapted to fill many niches across the world. Including some that had already evolved to become flightless, their wings reducing down to being small and instead like penguins they made adaptations to become strong swimmers. They were called hesperornis.

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Sources:
https://royalsocietypublishing.org/do...
https://www.sciencedirect.com/science...
https://www.researchgate.net/publicat...
https://onlinelibrary.wiley.com/doi/1...
https://www.semanticscholar.org/paper...