Dinosaurs aren’t extinct. There are about 10,000 species alive today in the form of birds. ~ English vertebrate paleontologist Robert Benson
Only 4 groups of terrestrial vertebrates evolved: amphibians, reptiles, mammals, and birds. The outlier of these is the one with wings and feathers, as well as a brain unlike any other. Birds are a highly distinctive animal.
Like mammals, birds are endothermic. Bats can fly and humans are bipedal, but there is little commonality otherwise between mammals and the class called Aves.
Many traits associated with flight evolved before the origin of Aves. ~ English evolutionary biologist Mark Puttick et al
Birds originated from maniraptoran theropod dinosaurs at least 160 MYA. Some basal avian traits appear a case of convergent evolution with earlier reptilian forms, as well as the endothermy found in mammals.
Though birds evolved at a quicker clip than other theropods, the avian body plan gradually came together, with no single leap to point to for the birth of birds.