This week’s question was asked by the optical technician at the Gundersen eye clinic.
QUESTION: Why don’t robins eat worms off the sidewalk or pavement after it rains?
ANSWER: I’ve often wondered about that. Reports from people seem to vary on the subject. Yes, we’ve all seen it; a whole smorgasbord of tasty delights slithering over a wet roadway, parking lot or sidewalk. And what do we see Mrs. Robin doing? She’s not feasting at the banquet. She’s over on the lawn searching for worms to feed the young ‘uns. There are several theories on why the worm buffet is passed over by Mother Robin. One theory says that the robin finds half the fun is pulling on the worm until it comes out of the ground. Picking up a worm laying on the sidewalk is simply no challenge at all. This theory doesn’t hold much water.
Some people report seeing robins gorging at the earthworm buffet with never a worm left behind. When birds find a large source of food lying around, it seems they tell other birds, and suddenly there will be a whole flock of them gobbling it up. “It starts with one, then it flies away and returns with more, then more,” reports one lady.
Earthworms are unable to drown like a human would, and they can even survive several days fully submerged in water. Earthworms don’t have lungs. They breathe through their skin and they need a balanced level of moisture in order to survive.
After a rain, they come up out of the ground to get away from too much water. When the soil is wet, they move closer to the surface. When the soil is too dry, they burrow deeper to find moisture. It is essential that worms live in a moist environment, but during drought conditions, life is certainly more difficult for a worm.
When there is adequate surface moisture or humidity, they venture out above ground and look for new places to burrow. The wet weather of spring and fall seems to bring them out in droves.
Soil experts now think earthworms surface during rain storms for migration purposes. Dr. Chris Lowe, University of Central Lancashire in Preston, England, says, “It gives them an opportunity to move greater distances across the soil surface than they could tunneling through soil,” said Dr. Lowe. “They cannot do this when it is dry because of their moisture requirements,” he adds.
Earthworms are actually a good news story. Geologists estimate that one acre of good black soil can have up to one million worms. Makes your skin crawl! Earthworms supply the ground with air and cause that air to circulate, enrich the soil, move nutrients around, digest and break down organic matter and improve drainage and soil texture. Earthworms do a good job of aerating the soil.
Earthworms are considered such beneficial creatures that most pest-control products have been designed not to harm them. There are currently no products specifically for controlling earthworms, and it’s really not recommended to try to kill them at all.
Another explanation involves rain drop vibrations on the soil surface sounding like predator vibrations, like that of moles. Earthworms often come to the surface to escape moles.
“Rain can set up vibrations on top of the soil like mole vibrations,” said Professor Josef Gorres of the University of Vermont’s Department of Plant and Soil Science. “It is similar to how earthworms move upwards and out of the way when predator vibrations are felt. They could move in a similar way for rain vibrations.”
Humans create vibrations when “grunting” for bait earthworms. To coax worms from their burrows, fishermen run a piece of steel or a hand saw across the top of a stake, which causes a rubbing sound to occur as the stake vibrates. Earthworms are then moved to the surface, much to the fisherman’s delight.
Earthworms use touch to communicate and interact, according to scientists who performed experiments on earthworm swarms. If you come right down to it, they’re touchy-feely creatures.
Send questions and comments to: email@example.com.
Larry Scheckel is a retired Tomah High School physics teacher.