Birds evolved from a group of dinosaurs known as theropods. These dinosaurs, including the Tyrannosaurus Rex, walked on two legs and many also had flexible tubes protruding from their skin as well, known as "dino fuzz" Over time, these tubes became more and more structured and eventually developed into the feathers we see on modern birds today. Theropods also got smaller and smaller, which made them quicker and more agile, as well as hollow bones that would make flying and other movement easier. These changes allowed for a type of organism that would appear to be a reptile, but also be capable of flight. In 1861, just 2 years after Darwin published his work On the Origin of Species, a fossil later named Archaeopteryx was found in a limestone quarry in Germany, dating back 150 million years to the end of the Jurassic Period. This creature harbored bird and reptile traits, being that it had wings and feathers likely capable of sustained flight, but also had sharp teeth and claws aligning closely with dinosaur and reptile fossils. This discovery was one of the first transitional species that Darwin predicted in his work, and gave much evidence to the hypothesis that birds evolved from dinosaurs. Since then, other ancient species similar to Archaeopteryx have been discovered that also support the hypothesis that birds evolved from dinosaurs, but Archaeopteryx is still widely regarded as the "first" bird. Modern birds, or Neoaves, came into existence around the time of the Cretaceous Extinction, where the lack of competition and natural selectors coming from other dinosaurs allowed them to diverge into a great number of orders, families, and species.
Further Watching: How Did Dinosaurs Evolve Into Birds?
Further Reading: An Ode to the Archaeopteryx
Further Watching: How Did Dinosaurs Evolve Into Birds?
Further Reading: An Ode to the Archaeopteryx