Timing is Everything?

Scientists researching the dinosaur extinction have actually determined that the Chicxulub asteroid did indeed cause the extinction of most dinosaur species (We call the ones that survived "birds").

On top of that, if it had happened 5 million years sooner or later, i.e., not in the middle of a major period of volcanism, then we wouldn't be here. Or maybe we would be dinosaurs. The dinosaurs were successful and stable through several other mass extinctions. 160 million years is a long time...long enough that it's surprising they didn't evolve technological intelligence (birds are certainly capable tool users). Or maybe they did and it never got to the point to leave traces that survived what happened after the impact - a global firestorm from which earth's atmosphere still hasn't recovered.

Which links up to my personal theory on why they didn't make it. The firestorm was followed by decades of global winter, but there was a distinct drop in oxygen levels. The Earth of the dinosaurs had more oxygen in the air than the Earth of today.

No bird gives birth to live young. Even the ratites, who don't have to stay lightweight for flight (and besides, bats manage fine). It's entirely probable that no dinosaur did either. In Japan, there's a rather interesting species of lizard. It's a very ordinary lizard - except that the subpopulation that lives at sea level lays eggs and the subpopulation that lives at high altitude is a live bearer. Scientists studying the little creatures to work out why realized that it's harder to raise young in eggs at altitude - because eggs need a lot of oxygen.

Did the dinosaurs not survive the impact, ultimately, because the drop in oxygen levels prevented their young from coming to term inside such large eggs?

Maybe.

Or maybe it was the decades of winter, although growing evidence that dinosaurs were probably endotherms, like us, and grew feathers, the best natural insulator there is...