Random thoughts......
You need insulation around the water and under the heater.
A lid that reduces the exposed surface of the water might help.
An aquarium pump bubbling air through the water might help and would use far less power (if the birds don't mind the bubbles).
A hole in the ground directly under the water container will allow the cold air to fall to the bottom, be heated by the ground, rise to the top, heat the water, and cycle back. The water container
should block the top of the hole. The hole needs to be deeper than the deepest possible ground freeze to work.
Cattle ranchers use this in remote places, called a "heat well".
More as I think of them...