This is bird note.

Around this time of year, common mergansers are flying south from Canada to wintering grounds in the

lower 48. What dictates when they decide to go? These ducks, like most birds, are well insulated against frigid winter temperatures, but they do have to find food, and they can only find their fishy prey by diving below the surface in open bodies of water.

So they're doing just fine, resting and feeding in southern Canada until they wake up one

November morning and a thin veneer of ice is forming on their lake.

That ice is a sign of the times and enough to let the ducks know they should be heading south. This makes common mergansers and some other diving ducks among the last of the migrants

to appear in the U.S. in late fall. In years to come, if they start to arrive later, it may be a sign that changes in climate are shaping their migratory behavior.

Read the latest reports of the Intergovernmental Panel on Climate Change to see why those Merganzers may not be coming south in the future.

For Bird Note, I'm Mary McCann.

Bird Note's newsletter delivers the wonder and joy of birds directly to your inbox.

Sign up at birdnote.org to get a weekly preview of our shows, stories, photos, and more.