Science of the Everyday: Why Earthworms Surface

The early bird may catch the worm—but what's it doing out and about anyway?

By Matt Ransford Posted 03.12.2008 at 12:50 pm 5 Comments

Robin and Worms Greg 7

Spring is close at hand in New England, which means we’re getting a lot of rain and the robins are out in force. You’ve certainly seen flocks of the orange-breasted birds out on a wet lawn running and stopping, running and stopping, then pouncing on a worm. It's a common springtime scene; but why exactly do the worms come out when it rains and expose themselves to hungry birds?

Robins are among the first each year to breed and lay eggs. Chances are, if you see robins during in the spring, they’re already making nests. And while they feed on a variety of invertebrates, fruits, and seeds, they prefer earthworms for their young. They gather together on lawns to feed because the group dynamic gives them protection from cats and hawks. Using their sharp eyesight, they spot worms in the grass (contrary to popular lore, they aren’t listening to the worms in the ground).

Generally, however, earthworms are loathe to come to the surface as they rapidly lose moisture when exposed to UV rays. Your driveway in the summertime is probably a graveyard of crusty worms, a good example of the dangers of leaving their natural surroundings.

When it rains heavily, though, the worms have little choice. Earthworms get their oxygen from the soil in which they live. When that environment becomes more water than air, they come to the surface to “breathe” just like we do when we’re swimming. And that exposes them to predators: A field day for the robins.

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5 Comments

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sublumjack

03/12/08 at 3:13 pm

wow....so thats why they come to the surface....i think everyone over the age of 4 already figured that out...thanks popsci!

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ggouge

03/12/08 at 7:52 pm

That was a great post and so so true. the should add that to the pile of why was that reasearched an how much money did we waste

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baggley88

03/13/08 at 5:56 am

Hi to other commenters, actually this was interesting and useful to my home schooled child!

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bartra

04/06/08 at 9:46 pm

Hi, Does anyone know why they surface every night when there is no rain. We have them on our dirt driveway every morning - getting killed by our wheels in droves.

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Danl

04/11/08 at 2:39 pm

It's not quite that "simple". The surface of an earthworm is coated with slime. This wet, slippery coating is necessary for the diffusion of oxygen from the environment to the "skin" of the worm. The oxygen does not have to come from the soil only, it can also come from water the worm may be in. Earthworms can live under water provided the water has oxygen in a concentration that will facilitate its diffusion to the "skin" surface of the worm. If earthworms surface because their burrows are flooded its not the water that's the problem it's the low concentration of oxygen in that water. My question is this, why so many earthworms surfacing and why the large numbers only in the spring? Sure they surface at other times like night time (higher humidity & less likely to be captured) and for other reasons (mating and faster movement to new locations without burrowing). But why so many in the spring?