Intestines of the Soil

Aristotle called them, “the intestines of the soil.”  For Darwin, they were the most ancient, most important plough in the history of the world.

For Robins, they are breakfast.

The lowly earthworm, 80% water, 14% protein, 6% miscellaneous nutrients, chugs along just beneath the surface of the earth; eating plant matter, protozoans, plankton, nematodes, bacteria, fungi, and the decomposing remains of other animals — including, one supposes, Polonius and others of his species — creating the good soil from which all the plants that sustain other life grow.  Without them, all life on the planet would be gone.

Motoring around just beneath the surface, they are easy prey for robins.  Cocking their heads first one way, then the other, following with a bill-pounce into the earth and coming out with a worm, robins give people the impression that they actually hear the worms.  That is unlikely.  Robins’ eyes are fixed in their skulls, unlike ours, so they must turn their heads in order to see and locate prey, not to hear it. Most likely, the robin sees motion, locates the worm visually by head-tilting, and then snatches the worm.

A dubious statistic holds that a young robin will eat 14 feet of worms a day.  Frequently cited, that statistic comes from a marvelous website dedicated to earthworms and on which there is much good information. But that particular statistic comes without a reference.

But scientists have plowed through plenty of robins’ stomachs. They find that worms constitute about 15% to 20% of the summer diet for robins. The rest consists of fruit and insects. One study of 1,236 robin stomachs contained 42% animal matter and 58% vegetable matter.

Their diet of earthworms is seasonal.  Many more are eaten in the spring and summer, which makes sense, because the soil is warmer then, the worms more active and easier for the robins to find. In the fall the robins are busy eating seeds and fruit, including that of the juniper, a plant species almost entirely dependent on robins and other birds for dispersal.  Robins adore honeysuckle too.  Some actually get intoxicated from eating too much of it.

Robin Wine

Getting drunk on honeysuckle doesn’t hurt robins.  DDT does though.  In a futile effort to eradicate Dutch Elm disease, Americans sprayed ailing elm trees with DDT to kill the elm bark beetles which transmit the fungus that kills the trees.  Because the molecules of DDT are persistent and non-dissolvable, they continue to kill insects for as long as the leaves are on the trees.  But when autumn comes, the leaves fall, the earthworms eat the leaves, the robins eat the earthworms, and the robins die.  Before DDT was banned from the United States, it wasn’t just the high-end predators like Peregrine Falcons and eagles that were killed by DDT. Songbirds too died by the millions. On the campus of Michigan State almost every robin on the campus died  after DDT was sprayed on the campus elms, as did migrant robins who were merely passing through.  The elms died anyway.

The robins died because the earthworms, doing their job, were eating the fallen leaves and depositing the DDT into their bodies which in turn killed the robins that ate the worms. A movie could be made about that: “The Revenge of the Earthworms.”

Now that DDT is banned from the United States, the American Robins who live here go unmolested about their business of hunting earthworms; for it is true, as the mystery writer P.D. James has written, “God gives every bird his worm, but he does not throw it in the nest.” Robins do most of their earthworm hunting early in the day.  We said earlier that they think of earthworms as breakfast.  The old saw that the early bird gets the worm appears to be scientifically accurate.  Robins spend more of their time looking for worms early in the day, shifting to seeds later.

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The science in this post comes from Sallabanks, Rex and Frances C. James. 1999. American Robin (Turdus migratorius), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online: http://bna.birds.cornell.edu/bna/species/462 (subscription required), last visited August 22, 2009 and Worm Digest,  http://www.wormdigest.org/, last visited August 21, 2009.

The earthworm photo was made by Michael Linnenbach.