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. The Vegaviidae ia a newly named group of diving waterbirds that somehow lived through the mass extinction. It's possible that modern-day birds evolved from these and other groups.
[…] vegaviids represent the first avian lineage to have definitely crossed the K–Pg boundary, supporting the idea that some avian clades were not affected by the end Mesozoic mass extinction event, countering previous interpretations.
Source The Science of Nature volume 104, Article number: 87 (2017)
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.