pond ... bird watering question

[rossb]

I'm not sure if anybody else maintains a water source for outdoor birds but if you do...I have a question. This year I just started putting a small heater in my pond for a watering hole for the birds (we feed them all winter). It leaves an area about 2 square feet open. I have noticed that the water seems to really evaporate or sublimate quickly. In fact the area dries up evey 3-4 days, which is about 10-15 gallons. Does this seem normal?