I don't have a bird bath but this weather is kind of making me wish I did! Poor babes!
It doesn't have to be an actual bird bath.
Anything fairly wide and shallow that holds water will do. A baking tray for example.
I have a baby seagull in the garden at the moment and he's big but he just hasn't figured things out yet. I put a washing up bowl out there for him. He's having great fun with it!