Washington State University

Ask Dr. Universe

Listening for worms

January 6th, 2012

Dear Dr. Universe,
We have a lot of robins that come in our yard to look for worms, which made me wonder: how do birds find worms underground?

Peter Cruickshank/Flickr

Peter Cruickshank/Flickr

Thank you, Charleen, for asking one of THE big questions that everyone wonders about. Of course, in spite of everyone’s wondering, this is another one of those questions whose answer no one is absolutely sure about.

I called ornithologist Richard Johnson here at WSU. He didn’t know the answer, but he dug out an article from 1965 in which a scientist from California named Frank Heppner reported the results of his worm-finding experiments with robins. After a series of experiments, Professor Heppner decided that robins find earthworms by sight.

That makes sense. Have you noticed how robins cock their heads from side to side? Their eyes are on the sides of their heads, so they have to turn their heads to see straight ahead.

However, says Professor Johnson, their EARS are also on the sides of their heads. You can’t see them, but they’re there, covered up by feathers.

So for 30 years, scientists tended to believe that robins find their food by sight.

Then we found an article that came out just last year, in a magazine called “Animal Behaviour.” Two Canadian scientists, Robert Montgomerie and Patrick Weatherhead, came up with a different conclusion.

They’d watched robins catch worms in fairly long grass, and it seemed to them that maybe they used another sense to find them. So they decided to do their own experiments.

For each experiment, they buried a bunch of mealworms in a tray of soil and let the robins go at it—but under different conditions. First, they buried two live mealworms and two that were frozen to death. The robins found the live ones, but not the dead ones. Since all the worms seemed to smell the same, the scientists concluded the robins probably don’t use their sense of smell to find them.

Another possibility was that robins sense worm vibrations in the soil. So the scientists rigged up the tray so the robins could not feel the vibrations through their feet. The robins had no problem finding worms. So vibrations are at least NOT NECESSARY for finding worms.

Next, the scientists buried the mealworms not quite an inch deep, laid a sheet of thin cardboard over the tray and put more soil on top of it. This would eliminate any visual cues, such as particles of soil moving around above the mealworm.

The result? No problem. They pecked right through the cardboard!

So where does this leave us? Besides taste, the only sense left is hearing. Do they HEAR the worms?

Professors Montgomerie and Weatherhead buried a little speaker in the soil with the worms and played “white noise” through it to block out any possible worm noise. White noise sounds like static on your radio.

Although the results were not completely clear-cut, the noise did cut down on the robins’ success rate. In fact, they didn’t even strike the ground as many times as they did in the other experiments.

So it seems that they DO hear the worms. Professors Montgomerie and Weatherhead figure the robins could hear some worms even through the white noise. Robins must have pretty sharp hearing!

By the way, what do worms moving in soil sound like? The scientists say that, when amplified, they sound like a person walking on gravel.

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