Robins have made their way back from the woods and are running across lawns again. If robins have returned then earthworms are active or soon will be. Earthworms are the American Robins main food spring through fall. Three things coincide this month, robins, worms, and rain. Let’s explore how these three things intertwine and interact in the ecosystem.
The Worm Moon
March is the month of the Worm Moon. Here in the east, Algonquin peoples call from the current new Moon to the next new Moon, the Worm Moon. It’s the time that worm-like animals are first seen after the cold of winter. See Algonquin Moon Names
Earthworms, Indigenous and Introduced
The large earthworms we often see aren’t indigenous to the Mid-Atlantic and Northeast above Maryland. In the Mid-Atlantic other soil organisms were the main soil builders and conditions. Decomposers such as snails, termites, sowbug, and fungi are the indigenous animals which eat and break down organic matter to release nutrients for plants and animals to digest.
Native indigenous earthworms are smaller than the introduced larger Eurasian worms. The large familiar earthworms of farms and gardens are introduced members of the family Lumbricidae. The native earthworms include only a few members of the Lumbricide but are classified in a different genus (Bismastos) than the Eurasian earthworms. Other indigenous worms are classified as members of a few other families. (Nardi, p.78)
Robins and Earthworms
American Robins (Turdus migratorius)have a distinct behavior of scurrying, stopping, cocking their heads from side to side and pointing their eyes at the ground. Then they may quickly stab at the soft ground and pull up a worm. If they are feeding chicks, they fly off with the worm dangling from their beaks or they eat the worm themselves.
During the summer, worms make up 20% of a robins diet. Sometimes as many as 20 worms are captured in the lawn topsoil. In the ecosystem, robins serve as a predator of earthworms.
Since Eurasian earthworms have been introduced, I wonder how it has affected the American Robin population. I do know that the species is more common that they were in the past. The removal of forests and the presence of lawns surely must contribute to it.
I suppose robins of the past ate the native indigenous worms? Or maybe their diet has completely changed. The larger Eurasian earthworms has become a mainstay of the birds’ diet. This may have affected chick health and species population growth.
Once the earthworms are no longer easily hunted, the birds switch from a spring and summer diet of mostly bugs and worms to fruit and berries. Robins feed on the ground for most of the year. I guess since they eat fruit and berries in the fall and winter they feed in the trees.
Earthworms, Rain, and Robins
Earthworms are active near the surface early and late in the day. And there is always a large number of earthworms on the surface of the ground after a rain. You often see dead worms floating in puddles. I don’t know what killed them but worms don’t drown. Worms breathe oxygen through their moist skin.
We don’t know why worms emerge above ground during and after a rain. Science has three ideas as to why they surface. The first idea is they can move and disperse more readily on the wet surface of the ground better than they can while underground.
The second idea focuses on the worms finding bits of organic matter they eat as food, on the surface in greater abundance. But isn’t that true all the time?
The third idea is the worms can find potential mates easier above ground than underground. The short answer is we really don’t know why worms surface especially after a rain.
However, it could be as simple as this: as worms surface on lawns and pavements they are more easily seen than in meadows or fields. Robins use this situation. This is why we see the birds hunting among the grass of fields and lawns.
Robins, Earthworms, and Rain
A robin eating an earthworm after a rainstorm connects to history, colonization, ecology, and protecting the health of the Good Earth. Our actions are no less connected and important to the the making, creating and workings of our world.
Williams, Ernest H. The Nature Handbook: A Guide to Observing the Great Outdoors. Oxford ; New York: Oxford University Press, 2005.
“How Robins Find Worms”, Gill, Frank B. Ornithology. 3rd ed. New York: W.H. Freeman, 2007.
Nardi, James B. Life in the Soil: A Guide for Naturalists and Gardeners. Chicago: University of Chicago Press, 2007.
Earthworm Invaders (Smithsonian Environmental Research Center) – EcosystemontheEdge.org