Good point. A problem that is mitigated if you are herbivorous, and chomp all day a lot of grass and other things that contain calcium, like goose do. Aquatic birds can deal or at least understand the concept of extreme conditions in water. You will not see a flamingo dissolving, and they live in extreme alkaline waters. They know were is time to go.

In any case there are several solutions that would put an end to the problem or alleviate a lot it at least. Is a question of money, but also of will.

If there are a lot of birds dying in your pool, maybe you should admit that the water is not only acidic but also poisonous and that they are desperate for the lack of suitable habitats. Solution, just build a second damned pool close and keep it clean and full of shrubs, reeds and plants. As long as birds can choose to stand or fly and feel safe, they would favor the correct pond.

Or even better, fix your water in the main pond.