It's now widely accepted that the Cretaceous–Tertiary (K–T) extinction event (also known as the The Cretaceous–Paleogene (K–Pg) extinction event) forced the extinction of the non-avian dinosaurs. The branch of the dinosaurs that did survive included the highly-feathered types. Vegaviidae, is a newly named group of waterbirds that somehow lived through the mass extinction. Modern-day birds probably evolved from these and other groups.
The question arises as to why the bird-like dinosaurs survived whilst all other dinosaurs were eliminated. This is especially puzzling as it's assumed that the Vegaviidae and other similar animals probably had a high metabolic rate, meaning that they had to feed regularly to survive. There are no currently accepted theories to explain this event.
Note: This item is flagged for expansion and further references