Since the first feathered reptiles appeared 200 million years ago, birds have undergone phenomenal changes to survive in rapidly changing environments. Birds are far from through evolving. They will continue to adapt through whatever changes the future has in store for them.
The earliest ancestors of modern birds are thought to be dinosaurs, more specifically a branch of dinosaurs called theropods. They were most likely earthbound, but walked on two bird-like legs with claws and even had beaks. One of the most numerous of dinosaur species, the Hadrosaur, is thought to have had a beak and feet resembling an ostrich. They also laid eggs, but so do fish, reptiles and insects and they aren't birds. But it is thought Hadrosuars were warm blooded, like birds.
Birds are known for developing their signature feathers. Feathers are thought to have evolved from scales that became thinner and thinner over time. Flying lizards of today give us some clue as to how feathers may have evolved, although today's flying lizards glide using extended flaps of skin.
However, the first feathered animal that we know of didn't much resemble a bird. He was a small lizard that lived in Central Asia over 200 million years ago. He had a name about as long as he was Longisquama insignis. He was only discovered at the turn of the millennium, so it is still not sure where he fits in the evolution of today's birds. It's not even sure if he glided from high spot to high spot like a flying squirrel as opposed to flying like one of today's birds.
Perhaps little Longisquama was a forefather to Archaeopterix, who you might remember from the dinosaur books you had as a kid. He was the funny looking dino partially covered in feathers and seemed to have primitive wings. He lived about 145 million years ago and most probably glided as opposed to flying.
From then on, things get a little blurred. Science isn't entirely sure that the two dinosaurs already mentioned were responsible for today's birds, or were flukes of nature. One scientist, Professor Alan Fedduccia, thinks that one of the first forefather of birds, Liaoningornis, only appeared 50 or 60 million years ago. This animal clearly had a breastbone that looks like a bird's, as well as powerful flight muscles.
During the Ice Age, many species were incredibly huge in size, such as the well-known Mammoth, Aurochs and Saber Tooth Tiger. Also in that time were huge "terror birds" that had beaks and feathers, but were flightless. They most likely resembled the bizarre shoebill stork, but on a much larger and swifter scale.
Nature constantly experiments to see what will work and what won't. She still is doing is today.