good point Ron.About those bubblers...
Moving water will not freeze, but also does not add heat the water.
So.... are you now not giving your birds supercool unfrozen water?
Seems to me that if it is 15 degrees ambient temperature, so will be the water, if not frozen.
I'm no expert, but I don't think this will be good for your birds.
another good pointConsidering they will gladly peck through thin layers of ice on puddles to get to the water below or drink out of the non frozen portions of various dishes, etc that have ice in them, the birds don't seem to have any objection - and these examples are for birds with warmer water readily available.