Gastornis
The mid-Eocene saw the rise of large creodont and mesonychid predators in Eurasia and North America. The appearance of these new predators coincides with the decline of Gastornis and its relatives. This was possibly due to an increased tendency of mammalian predators to hunt together in packs (especially in the hyaena-like creodonts). We know of no bird which weighed more than half a metric ton. Perhaps they could not evolve to larger sizes. If so, they could not compete by sheer bulk as some mammals can do.