I don't know why the birds can't use the birdbath. It's safe.
But it seems instead, a lot have to try to use a bucket or one of the kiddie pools we have for the big animals to drink from. For the adults, it usually works out okay, but for the young ones, well, not so much. Every so often I find a drowned bird in one of the water buckets or pools.
This morning I actually found a little bird that was still alive in one of the pools. I scooped it out and sat it in the sunshine on top of the wellhouse, so it could dry out.
It sat there a little while, then flew down into the grass.
That's probably better since the wind isn't blowing so much on the bird. Maybe with the wings spread out like that it will dry quicker.
I guess I prefer to find a live bird in the buckets and pools instead of a dead one, but if I had my druthers, there would be NO birds there.