Birds are the most species-rich dinosaurs known, and despite the cataclysmic events at the end of the Cretaceous period that wiped out all non-avian dinosaurs, they still outnumber their mammal neighbors even today (there are 5,000+ known extant mammals, while there are 10,000+ known extant birds). Birds encompass every land-based ecosystem across the planet, with some species even traversing incredible distances by flight as they make annual migrations. Birds achieve flight by use of their keeled sternums (which anchor flight muscles), lightweight and air-filled bones, and feathers attached to adapted hands and arms. Feathers also play other roles, such as insulation, camouflage, and displaying and signaling to other birds. Though feathered dinosaurs are known as far back as the Jurassic period, the first true birds probably evolved in the later Cretaceous period, as evidenced by fossils.
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