How dinosaurs became birds
Feathers, hollow bones and the wishbone all appeared long before flight.
Birds are not descendants of dinosaurs in some figurative sense — they are literally living theropod dinosaurs, the closest relatives of tyrannosaurs and velociraptors.
The traits that look distinctly avian arose gradually and separately. Hollow bones filled with air sacs were already present in large predatory dinosaurs, lightening the skeleton and improving breathing. The wishbone, a pair of fused clavicles, is known in species that never flew.
Feathers were not originally connected with flight. The simplest filament-like structures served as insulation, then grew more complex and were used for display — in some species melanosomes are preserved, and their shape lets researchers reconstruct the original colouring, including iridescence.
Archaeopteryx, from the Jurassic deposits of Germany, combines teeth, a long bony tail and claws on its wings with fully formed flight feathers. Flight turned out to be the last item in the set, not the first.