If water sources are frozen, it can be very difficult for birds to find a drink. They may have to travel a long way to an open source or resort to eating snow (if there is any).
Birds need drinking water to maintain a healthy metabolism to stay warm and hydrated.
They need it for bathing to keep their feathers in top insulating condition and keep them waterproof.
Do you have nights with hard frosts or any freezing weather? Use a heated bird bath or add a heater to your existing plastic, metal or stone bird bath to keep open water available for the birds.