During these cold months of winter, access to unfrozen water is key to the survival of birds. Birds typically need to drink at least twice daily to maintain metabolism, even in winter. When necessary, as a “last-resort, birds can and do eat snow, but it is not ideal because it takes more calories to melt it in their bodies than to heat that water to body temperature. Thus, more energy is needed to do this process and by losing critical calories in winter birds become more susceptible to freezing temperatures. Some research has shown that relieving only on eating snow for hydration can cause significant calorie loss in birds, up to 70-75% of fat reserves overnight in some cases.
Additionally, having access to unfrozen water is also essential for birds to bath in as clean, well-oiled, and preened feathers can hold more air to keep birds more insulated and warmer. 
If possible, consider using a heated bird bath, an aerator in pond or water features (to keep water moving), or manually replace water daily in a shallow, 3-inch-deep dish for easy access for birds for their health and energy conservation in winter. Also try to position bird baths in sunny areas and clean them regularly to prevent ice buildup.
Here are just some of the birds we have been getting at our water feature at the MCR Visitor Center over the last few weeks (Yellow-bellied Sapsucker, Dark Eyed Junco, Mourning Dove, Hermit Thrush):
