This had me going, 'ha, that's funny'.
So, there I was, feeding the chickens, and one of the former dinosaurs leaps up and flies, vertically up to the level of my face. So, I figure, time to clip their wings. Last time I clipped them was in november. I fetch the scissors, grab the in-my-face-flying chicken and ... hang on, these wings have barely grown since november. This chicken quite literally still only had half a wing. And with that half a wing, it flew, vertically 1m70-ish high (5.5 feet).
And why did it do that? To get at the food in my hand. Theoretically, it would have had an advantage over it's less flight capable sisters.
Yes, yes, I know this doesn't actually prove anything. The chicken still had all the other requirements for flight, the muscles, the length of the bones making up the wing, the hollow bones. Still, the damn velociraptor-wannabe flew, higher than its competitors.
It's not hard to imagine a scenario
where a branch of dinosaurs begins growing feathers. For the insulation they give, for the protection agains rain and hail and sandstorms, for the difficulty they provide for predators and parasites to gain purchase. And one of these dinosaurs just happens to have slightly longer/stronger arms. Suddenly a predator shows up. In panic, our long/strong armed friend frantically waves about its arms and, purly by coincidence, flies a few feet. Its buddy also frantically flaps its arms and ... becomes lunch. The long/strong armed dinos keep having more itty-bitty long/strong dinos. Few million years later, voila, birds!
Now, I have a vivid imagination. I write for fun; I make stuff up and I'm rather good at it. But, I can't, for the life of me, figure out what's so difficult about the above scenario!