Why do humminbirds migrate north for the winter?

Because they eat nectar, they follow the flush of the bloom. Down in their wintering grounds they may have enough flowers to keep them alive through the winter, but the flowers there tend not to come in big flushes as they do in areas with a more pronounced seasonal cycle. When nesting time comes around and they have extra mouths to feed, they need to be in an area where they have easy access to vast fields of blooms.

To keep their tiny butts from freezing?

Perhaps all hummingbirds would prefer to live and stay in warmer climates. Ecuador has around 130 species of hummingbirds. So I’m thinking that all of the hummingbird niches in the south are filled and resources limited in those ideal areas so this forces hummingbirds to survive in less than ideal climates. Thus the migrations.

As a side note: I live in Tacoma Washington and I have humming birds visit my feeders all year around. Even through the cold snowy winter, Anna’s and Black-Chinned hummers constantly visit my feeders. I would surely die from cold exposure if I stayed outside all day everyday during the winter. Those hummers are hough hombres for sure.