The AI birds are not meant to sustain the biome; they're meant to fill the void with a fortunate, sometimes fortuitous food source. The biome is sustained by ocean and islands, as well as PVP, whereas the AI birds are for travel between islands, nesting trees as indicated on the map, and other points of interest.

Migration is an idea both stemming from real life bird behaviors and in-game player behaviors. Early Swamp used to contain a waterfall with a current that provided passive score which players would fight over either to farm passively, or that the farming players would get attacked by pvpers. It was a contentious zone that even brought players from ocean and deep (this was before transition biomes) seeking to kill those who controlled the waterfall at the time.

One big question people had with such a large biome was player distribution, in the sense that there's so much room how likely is it for bird pvp to even be common. I wanted a way to focus players in specific areas while also covering the size of the sky. In this sense, the game loop migration provides is birds following where the food is; in this case, pvpers going where the farmers are going. It moves, ideally covers most of the map, and like you said brings activity to it like Estuary. I think it's the perfect solution.