It's now widely accepted that the Cretaceous–Tertiary (K–T) extinction event (also known as the The Cretaceous–Paleogene (K–Pg) extinction event) played a large part in the extinction of the non-avian dinosaurs. (see : Dinosaur extinctionplugin-autotooltip__plain plugin-autotooltip_bigDinosaur extinction

"The question why non-avian dinosaurs went extinct 66 million years ago (Ma) remains unresolved because of the coarseness of the fossil record. A sudden extinction caused by an asteroid is the most accepted hypothesis but it is debated whether dino… )

The branch of the dinosaurs which 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)

A 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.

One hypothesis as to how modern birds survived the Cretaceous–Paleogene mass extinction when other dinosaur species did not could be related to their ability to adaptively radiate. Due to the fact that the avian ancestors of modern birds did not take up all of the niche space where other species did fill up their niche space, birds could have been able to produce a higher level of ecological diversity and innovation that helped them to faster adapt to different environments. These rates of evolution could in part be due to their small body sizes.

Source : Wikipedia

A subgroup - the Neoaves - which led to the existence of around 95% of current-day birds (10,000 + species), have been extensively investigated both at a molecular level and through fossils.

Patterns of diversification and timing of evolution within Neoaves, which includes almost 95% of all bird species, are virtually unknown.

Source : Evolutionary Biology, Volume 2 Issue 4.

Also see Avian flight evolutionplugin-autotooltip__plain plugin-autotooltip_bigAvian flight evolution

It's now widely agreed that modern-day birds evolved, during the Mesozoic Era, from dinosaur species which survived the Cretaceous / Tertiary extinction event. (See: Avian K-T survival )

But the question of how and why birds acquired the ability…

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