Size and Weight:
Archaeopteryx gets all the press, but there's a convincing case to be made that Epidendrosaurus was the first reptile to be closer to a bird than to a dinosaur. This pint-sized theropod was less than half the size of its more famous cousin, and it's a sure bet that it was covered with feathers. Most notably, Epidendrosaurus appears to have been adapted to an arboreal (tree-dwelling) lifestyle--its small size would have made it a simple matter to hop from branch to branch, and its long, curved claws were likely used to pry insects from tree bark.
So was Epidendrosaurus really a bird rather than a dinosaur? As with all of the feathered "dino-birds," as these reptiles are called, it's impossible to say. It's better to think of the categories of "bird" and "dinosaur" as lying along a continuum, with some genera closer to either extreme and some smack in the middle.