Get a black shower bag for camping. Set it out in the sun while you hunt and it will be luke warm. Feels good for washing hands on a cold day. I just use surgical gloves to field dress and finish at home.
Thanks for the suggestions, I am not looking to boil water, just have warm water to rise birds off and wash hands on those really cold days. Many of my trips are overnight trips so need to get the birds pretty clean. I really hate cleaning birds in cold water on the really cold and windy days.