One of the most common arguments you'll hear against evolution (or, at least, one of the most common arguments I heard growing up amongst creationists) had to do with transitional forms. An eye is a valuable thing, this argument goes. But half an eye? That's just a disability.
Like many of the really common arguments against evolution, this one crumbles the minute you start to apply the slightest bit of fridge logic. Sure, half an eye is less useful than a full eye. (Or, more accurately, a clustering of light-sensitive cells don't have all the functionality of a modern eyeball and optic nerve system.) But, if most of the other creatures have no eyes, and you have a few light-sensitive cells, you've got an advantage. And an advantage is all it takes.
Now apply that to the evolution of birds. One of the cool things about this process is that it appears that feathers evolved before flight. In fact, feathers seems to have evolved rather independently of flight.
You might ask: What's the point of that? How are feathers an advantage if they can't help you fly? Is this just about looking pretty? Maybe. But on his blog, The Loom, Carl Zimmer presents another hypothesis. Feathers and wings, even without flight, might have given their owners a physical advantage over bare-skinned cousins. The birds in this video aren't flying. You can see that their feet don't leave the ground. But the act of flapping those feathers around helps them to walk up inclines that would otherwise be impassable walls. That's enough to escape a predator and live to breed another day. And it's also pretty damn astounding to watch. You'll find more footage at The Loom.