Seems like a bug.
Steps to reproduce:
- Find an area with no birds
- Wait for you to approach
Actual behavior:
Birds appear suddenly. Not like they're following you, just like... one frame there's no birds, next frame there's birds.
Expected behavior:
Birds obey object permanence, same as any other physical matter.
Locations of birds is not correlated with your presence, unless you're engaging in an obviously pro-bird activity like scattering seeds or something.