Gastornis
The majority view is that Gastornis was a predator. However, some doubt the bird was agile enough to catch fast-moving prey, and have doubts about the beak being right for carnivory. It may be that Gastornis was an ambush hunter, or used pack hunting techniques to pursue or ambush prey. If Gastornis was a predator, it would need some way of hunting prey through the dense forest.