The asteroid that caused the extinction event at the end of the Cretaceous period struck Earth with 60,000 times the energy of the world’s entire nuclear arsenal. The atmosphere would have glowed red hot for several hours and all the large dinosaurs that couldn’t burrow underground or hide underwater were immediately roasted.
When the smaller species came out of hiding they found a charred landscape and the air so thick with soot and sulphur dioxide clouds that sunlight was almost completely blocked out for the next year. It was too dark for photosynthesis, so the herbivores died, then the carnivores.
Birds are descended from the maniraptoran dinosaurs but they had two important adaptations that helped them survive. First, they had beaks instead of teeth, which allowed them to crack open seeds and nuts buried in the topsoil.
Second, their relatively large skull capacity suggests that they were more intelligent than the other reptiles. They may have lived in more complex social groups that could cooperate and adapt to find new food sources in the radically different post-apocalyptic landscape. This allowed them to eventually outcompete any other species of small dinosaur that might have survived the initial impact.