I know it hasn't been mentioned, but hot water freezes faster than cold water. Although I think offering tepid water is a benefit sometimes, my dog will drink a lot more tepid water in one go than water that's sitting with a rim of ice on it. I provided him with a bucket of tepid for a good drink, then emptied and filled with boiling water, thinking that the boiling water would be still be water in several hours time... but it actually freezes before cold water. Weird.
When it was cold last winter, my birds got to drink twice a day, morning and evening, they were lively, healthy, ate well and laid right through from November, so I think this regime was acceptable.