Not all the dinosaurs died out during an event such as a massive meteor strike.
The larger dinosaurs died out because:
1) they were more specialized in the range of food they could eat- they either tended to be herbivorous or carnivorous. I don't think I have heard of a large omnivore.
2) a large meteor strike would cause a sort of "nuclear winter", blocking out a signifigant portion of sunlight, thereby killing off a lot of foliage the herbivores eat.
3) the herbivores starve to death because there is not enough plant material to eat.
4) as the large herbivores die off, the food supply for the large carnivores diminishes rapidly and the large carnivores die off en masse.
5) the smaller dinsoaurs tended to be more opportunistic and omnivores.
6) birds descended from smaller arboreal dinosaurs that glided from tree to tree.
7) as the musculature for flight developed, these same tree gliders could transition from gliding to actual flight.
8) it would only take one such glider that had a habit of flapping their wings while gliding to go from gliding to powered flight.
9) if the offspring of this "flapper" were to have a more developed musculature in the chest that allowed them to flap and glide farther than their parents, within just a few generations, the genes for more developed chest musculature become the dominant genes and their progeny would pass them on to their children, and so on...
