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 the earthworm plucked by the robin in the park or on your lawn is probably a relatively new arrival, most likely a species Europeans conveyed to the Americas in plant soil or the ballast of ships.

More info and transcript at BirdNote.org. 

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