Why don’t birds freeze when they sleep outside in winter?

From Futura

On winter nights, when temperatures plunge well below freezing, birds often sleep exposed on branches, rooftops, or open fields, and yet, by morning, they are still alive. How do creatures so small maintain an internal temperature of around 104°F while the air around them freezes? The answer lies in a fascinating combination of physiology, behavior and clever biological engineering. Birds survive not by luck, but through highly efficient energy management, specialized insulation and even internal heating systems that continue to inspire engineers today.