Evolution of birds

Our understanding of the early evolution of birds has advanced over the past 2 decades, thanks to an ever-improving fossil record. Extraordinary fossils have revealed new details about the evolution of the avian brain, respiratory system, digestive tract, and reproductive system. Many of the traits most strongly associated with birds first arose in nonavian theropod dinosaurs. Theropods evolved pennaceous feathers, incipient wings, and gliding flight long before modern birds appeared. Birds likewise inherited features such as an expanded forebrain, gizzard, dorsally immobile lung, pigmented eggs, and paternal brooding system from their theropod ancestors. Yet, the earliest birds also retained primitive traits such as teeth, clawed hands, long bony tails, partially buried nests, and slower growth. The evolution of birds was profoundly influence by the Cretaceous–Paleogene mass extinction, which wiped out the previously dominant Enantiornithines (“opposite birds”). This sets the stage for modern birds to radiate into the most diverse major clade of tetrapods.