Yandangornis longicaudus is, as the name ("ornis") implies, a bird. We think. It's actually very hard to say. Like birds, it is fully-feathered and has a beak with no teeth. However, it also has a long bony tail, a short fourth toe, and other skeletal features that make it more similar to dinosaurs. Since birds are dinosaurs, the distinction is purely one of interest to taxonomists, who work to figure out which species are more closely related to which other species. From a taxonomic perspective, Yandangornis is a dinosaur, as are all birds - but whether or not it can be called a bird for sure is still up for debate. Discovered in Zhejiang Province, China in 1999 from a largely intact skeleton, we do know that it was more adapted for ground-dwelling than aerial or arboreal life, and was similar in size and habit to today's roadrunners, likely chasing down insects, small reptiles, and primitive mammals on foot.
As a heads up, updates will be spotty over the next couple weeks as I finish my thesis and prepare my defense. I'll try not to go full hiatus on you guys!