How do little birds stay warm in winter?

I live in Ottawa, Canada and it gets well below freezing here, with snow and ice. Yet I see little birds hopping and flitting around, apparently not caring that it’s cold. They’re scarcely bigger than a golf ball – how do they stay warm?
No, I mean little birds, not baby birds. Like little finches.

Canadian Snow Birds Flock to Santa Barbara Vacation Rentals

When Canadian Anne Murray sang her hit song “Snowbirds,” she wasn’t singing about Canadian Geese. She was singing to the thousands of snow-bound Canadians called ‘snowbirds’ who make the yearly pilgrimage from wintery Canada and the Pacific Northwest to warmer climes in places like Southern California. There, snowbirds nest for the winter months, basking [...]