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Traditionally, birds are classified as a class of dinosaurs and the term non-avian dinosaurs is introduced to refer to the rest.
As recent discoveries suggest that some dinosaurs may have been warm-blooded and that some dinosaurs had feathers, the distinction between birds and dinosaurs seems blurred.
I have often heard statements like "Birds are the only dinosaurs that survived the Cretaceous-Paleogene extinction event.". With recent discoveries, isn't this how the clade of birds is defined rather than a fact about the extinction?
// not sure if this question belongs in biology, sorry
The placement of the bird clade within the dinosaur clade is based on much more than just "having feathers and being warm-blooded"; paleontologists have discovered many, many ancient bird and theropod fossils, and as a result their understanding of what the theropod clade was like overall and how the bird clade developed within it is precise enough that discovering another group of dinosaurs was warm-blooded or even had feathers won't result in the conclusion "wow that dinosaur was closer to birds than we thought", but "wow feathers were a much more common feature among dinosaurs than we thought".
If you look at the Wikipedia page you can see there are many "bird-related" clade names that cover different levels of similarity to modern birds. From "Neornithes", which is defined as the clade containing all modern birds, to "Paraves", defined as the group containing all organisms closer to modern birds than to Oviraptors, which are all within Maniraptorans, which is itself pretty deeply nested within the Theropod clade…
So basically what I am trying to say is, our understanding of where birds fit within Dinosauria is precise enough at this point that no discovery about a single trait is going to "blur the distinction between bird and dinosaur"; it might change our idea of dinosaurs in general to be more bird-like, meaning that traits we thought were bird innovations might have been standard-issue in dinosaurs for a long time before, but it will not necessarily change our whole understanding of how birds and dinosaurs are related.
As for this :
With recent discoveries, isn't this the definition of what a bird is rather than a fact about the extinction?
It isn't; the paleontological definition of what a bird is is not at this point an ad-hoc definition related to "whatever survived the K/Pg extinction". Even Neornithes, the crown group of all living birds, originates to before the K/Pg extinction. For example, the splits between ostriches and other birds and within that latter group, of ducks and geese from other birds are thought to have happened during the Cretaceous. And there are some researchers who argue that a few fossils indicate that some non-avian dinosaurs survived the extinction; the consensus view is that those fossils are dated wrong, but that this position exists shows that "non-avian dinosaurs went extinct at the K/Pg boundary" is a fact-based question, not a definitional one.
One modern definition of birds, due to Gauthier, is the smallest clade containing all extant dinosaurs. However, this is not equivalent to the dinosaurs that survived the Cretaceous-Paleogene event, as these are not a clade. The bird clade dates back about 150 million years, and the Cretaceous-Paleogene event was also a mass extinction for birds, although after that they became much more diverse.