Arobin tugging an earthworm from the ground is a symbol of spring. But that wormit’s eating hasn’t always been here.

Whenglaciers pushed south into what is now the U.S. around 20,000 years ago, theyscraped off the soil layer and spelled the end of native earthworms except inthe southern states. So that earthworm plucked by the robin is probably arelatively new arrival, most likely a species Europeans conveyed to theAmericas in plant soil or in the ballast of ships.

Soif not earthworms, what were robins feeding their chicks before Europeansarrived? Well. Probably some of the more than a hundred kinds of insects andother invertebrates, as well as berries, that robins are known to eat.

Robinsprefer to forage in short grass to avoid potential predators. But after thelast ice sheets melted back, where was the short grass they liked? Onespeculation is that prehistoric bison, horses and mammoths grazed heavily inplaces, creating robin-friendly landscapes.

Justas robins now share pastures with cows, perhaps 15,000 years ago they hoppedamong giant bison or woolly mammoths.