http://www.newsweek.com/only-handful-birds-survived-dinosaur-killing-asteroid-now-scientists-have-944381

Sixty-six million ago an asteroid crashed into the Earth, with a force one million times greater than the most powerful atomic bomb. The impact and its after-effects ushered in a great extinction event that wiped out nearly three quarters of all plant and animal life. Among the victims were the dinosaurs, but birds were also badly affected.

It has long been debated how many birds—which evolved from dinosaurs—survived the extinction. Now, a new study published in the journal Current Biology, has found that most avians were wiped out as well, save for a handful of small, specialized ground-dwelling species, which are the ancestors of all birds today.