Giant meat-eating dinosaurs made a unique evolutionary voyage to become flying birds; however it didn't happen overnight, researchers say; instead it took a 50-million-year-long "shrink cycle."

During that time carnivorous, ground-inhabiting dinosaurs known as theropods underwent continuous bouts of downsizing to acquire the distinctive traits that would lead to the appearance of the world's earliest birds, they say.

Using techniques taken from molecular biology, the researchers analyzed anatomical qualities of 120 different dinosaur species in the theropod group, which included giant examples like Giganotosaurus and Tyrannosaurus rex in addition to the line that eventually produced birds.

"Our study measured the rate of evolution of different groups of theropod dinosaurs," says lead researcher and study author Michael Lee, a paleontologist at the University of Adelaide in Australia.

"The fastest-evolving group also happened to be ancestral to birds," he says. "So, ultimately, the most adaptable dinosaurs proved to be the best long-term survivors, and surround us today in their feathered splendor."

Starting around 225 million years ago, a number of species in that group dropped from an average weight of 400 pounds to less than two pounds by around 160 million years ago, the time when the first true bird, Archaeopteryx, appeared, Lee says.

While it still had primitive traits including teeth, and an extended bony tail, Archaeopteryx may have shared attributes with modern birds.

"What was impressive was the consistency of the size change along the dinosaur-to-bird transition -- every descendent was smaller than its ancestor," Lee says.

"They just kept on shrinking and shrinking and shrinking for about 50 million years."

Modern birds are linked to the ancestral theropod species through characteristics such as feathers, hollow bones and wishbones.

In becoming reduced in size in a process of "sustained miniaturization," the dinosaurs that would eventually evolve into modern birds were able to move into an evolutionary niche unavailable to their larger dinosaur relatives, Lee says.

That would have provided the evolutionary opportunity to experiment with new and different body configurations as well, he says.

"Being smaller and lighter in the land of giants, with rapidly evolving anatomical adaptations, provided these bird ancestors with new ecological opportunities, such as the ability to climb trees, glide and fly," he says.

That evolutionary voyage to new capabilities and shapes was likely a factor in the their ability survive the "killer" asteroid impact 66 million years ago that completely killed off the larger dinosaurs, the researchers suggest.