In a new study in the journal PNAS, scientists examined hundreds of birds in museum collections and discovered a suite of feather characteristics that all flying birds have in common. These "rules" provide clues as to how the dinosaur ancestors of modern birds first evolved the ability to fly, and which dinosaurs were capable of flight.

Not all dinosaurs evolved into birds, but all living birds are dinosaurs. Birds are members of the group of dinosaurs that survived when an asteroid hit the Earth 66 million years ago. Long before the asteroid hit, some of the members of a group of dinosaurs called Penneraptorans began to evolve feathers and the ability to fly.

Members of the Penneraptoran group began to develop feathers before they were able to fly; the original purpose of feathers might have been for insulation or to attract mates. For instance, Velociraptor had feathers, but it couldn't fly.

Of course, scientists can't hop in a time machine to the Cretaceous Period to see whether Velociraptors could fly. Instead, paleontologists rely on clues in the animals' fossilized skeletons, like the size and shape of arm/wing bones and wishbones, along with the shape of any preserved feathers, to determine which species were capable of true, powered flight. For instance, the long primary feathers along the tips of birds' wings are asymmetrical in birds that can fly, but symmetrical in birds that can't.