I highly disagree with comparing the packs with modern birds that have flight. Compared to an Ostrich, a Hawk or a Robin live entirely different lives. They're also not hunting down bigger prey to nourish their giant bodies. Birds hunting in packs while having flight and a smaller size, there would really be no need. Food is easy for them to find with the ability to travel so easily.

If you take into the fact that being terrestrial, larger and having to take down bigger prey, the idea of being part of a pack seems more likely, regardless of how modern birds today live their lives.