Water is especially important for birds to stay warm in the winter.  Because water helps birds keep their feathers preened.  Preened feathers are important to birds because it keeps feathers properly positioned and aligned.  If they are not in alignment, the cold can seep in the gaps of their insulation.  Which causes them to lose their body heat faster.  When there is no water to bathe in the birds will take snow baths.  This helps preening and fluff up their feathers to make a insulating layer.  Each feather is controlled by a small group of muscles.  When birds fluff their feathers, they create many tiny air pockets that drastically reduce heat loss.  This is why down coats are so cozy in the cold.  On cold nights birds bury their beaks and feet into their feathers, which helps reduce heat loss further.  

Winter is lovely, but can be a trying time for the animals who live out in it.  You can help alleviate their hardships by providing a reliable water source.  And don't be fooled with the old myth that if you start feeding and providing water for the birds they will starve if you stop.  Nonsense!  They will no more die of thirst or hunger than you will if you favorite restaurant goes out of town!  You may be miffed but you certainly wouldn't die!  Providing a haven for your feathered friends with bird feeders and heated bird baths will not only help the them out, but in return, birds are so fun to watch.  And will offer you many hours of amusement and entertainment.