This is bird note.

A robin tugging an earthworm from the ground is a symbol of spring.

But that worm it's eating hasn't always been here.

When glaciers pushed south into what is now the U.S., around 20,000 years ago,

they scraped off the soil layer and spelled the end of native earthworms,

except in the southern states. So that earthworm plucked by the robin is probably a relatively

new arrival. Most likely, a species Europeans conveyed to the Americas in plant soil or in the

ballast of ships. So if not earthworms, what were robins feeding their chicks before the Europeans arrived?

Well, probably some of the more than a hundred kinds of insects and other invertebrates,

as well as berries, that robins are known to eat.

Robins prefer to forage in short grass to avoid potential predators, but after the last

ice sheets melted back, where was the short grass they liked? One speculation is that prehistoric

bison, horses, and mammoths grazed heavily in places, creating robin-friendly landscapes.

Just as robins now share pastures with cows, perhaps 15,000 years ago, they hopped among giant

bison or woolly mammoths.

It's fun to picture, at least. For Bird Note, I'm Mary McCann.