The meteorite impact that coincided with the disappearance of the dinosaurs 65 million years ago, also saw a rapid decline in primitive bird species.
Only a few bird groups survived through the mass extinction, from which all modern birds are descended.

There has been a long standing debate over the fate of the earliest "archaic" birds, which first evolved around 200 million years ago.
Whether their populations declined slowly towards the end of the Cretaceous period, or whether they suffered a sudden mass extinction at the Cretaceous-Tertiary (K-T) boundary is unresolved, owing to conflicting evidence.
DNA studies have attempted to date the origin of modern birds; some suggest that they appeared before the extinction of dinosaurs, with large numbers of them surviving through the extinction event.
But the molecular clock suffers from "method issues", explains Dr Longrich of Yale University, and well-dated fossils are needed for "stratigraphic constraint" of the extinctions.
There are problems with the fossil record however. It is incomplete, owing to the extreme rarity of bird fossils.
Bird bones are very difficult to preserve as fossils as they are small and light, and easily damaged or swept away in rivers.
But the new research, headed by Dr Longrich, has made use of fragmentary bird fossils collected up to 100 years ago, from locations across North America.