Birds have evolved to survive in winter. They don't use just one strategy. Some birds, like little chickadees, can let their body temperature drop a lot overnight. When they wake up early mornings they will immediately fly to the nearest bird feeder and refuel, so that they can raise their temperatures again. Much activity at bird feeders peaks very early and very late in winter.
Other birds will cram into a bird house and share body heat, staying nice and fluffy and warm. Bird houses should have openings facing away from prevailing winter winds. Birds outdoors, like quail or even penguins, for instance, will tightly group together to share that body heat, with each bird taking turns on the cold outer edge. For quail, their survival depends on having a large enough group to take those turns.
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