Water is the most important thing you can give birds in winter.
This is being written at noon on Jan. 2, outdoor temp -8°F. Water, at least in our western suburban neighborhood, has to be next to impossible to find.
Except in our heated birdbath, seasonally misnamed because it's mostly a watering hole in winter. All of our feeder birds except woodpeckers use it.

Humans can live for two weeks or longer without food. No water for three days and you're a goner.
I can't find comparable numbers for birds. Certainly the time periods are much, much shorter, particularly for smaller birds.
I have Cornell's "Handbook of Bird Biology," Cornell's "Manual of Ornithology (Avian Structure and Function)," and the resources of Google Scholar, where you can find research papers for almost any subject you choose.
They offer much information on birds and hot weather, not much on winter water needs.
I have learned that when a bird exhales on a cold day it first removes and uses the water vapor included in that breath. It does the same for its waste, removing water before the void.