How to find food, water, shelter and avoid predators are essential to birds' survival.
Though food is scarce in winter, dehydration can be the biggest threat to birds. While birds can eat snow, it takes much more energy for for them to warm it to body temperature than it does for them to drink unfrozen water.
Water is not only important for hydration, but it also helps birds preen their feathers. During those freezing nights, they fluff their feathers to trap heat like a down jacket. Without water to conduct a proper preening, birds' feathers won't stay positioned. When feathers are in alignment, cold air can't pierce through to the skin, and make them lose body heat and freeze to death.
You can use a heated bird bath or add a heater to your existing plastic, metal or stone bird bath to make some water available even on the coldest day and attract birds that may not visit feeders very often. Heated birdbaths do not create warm water, but just keep it from completely freezing.