It is one of the most common of all sights in spring and summer across most of the U.S.: the familiar American robin (Turdus migratorius) bobbing across a grass lawn or garden, cocking its head and pausing frequently, then pulling a long earthworm right out of the soil. While robins also devour beetles, caterpillars, fruits, and berries, it is the sight of that robin tugging at a worm that is iconic.
The Biology of the American Robin
The common American robin is a migratory songbird in the thrush family. For most of the year, it can be seen over large areas of North America, with large numbers migrating north into Canada and the northern U.S. in the summer, and heavy numbers migrating south into Central America for the winter. The robin's diet consists of worms, snails, spiders, and other invertebrates, but fruits and berries comprise 60 percent of the typical diet. The bird has very sharp eyesight and hearing; the familiar back and forth cocking of its head as the bird hops along the ground is the robin's effort to see and hear the movement of worms or beetles beneath the ground.
Worms are the ideal food for omnivorous birds such as robins and other thrushes. While birds will eat just about any type of worm they can catch, earthworms and insect larvae are the most common food for young robins. An American robin, in fact, may eat up to 14 feet of earthworms in a single day, and worms make up about 15 percent of its diet.
How Robins Locate Worms
Robins and other birds use most all their senses when seeking worms, although different species—and perhaps even different birds within the same species—may lean more heavily on some senses than others. Several research studies have been conducted with different bird species to determine their ability to locate worms with different sensory detection methods.
- Vision: Birds, including robins, find worms mostly through sight. Birds have exceptional vision, and their keen eyes can spot the tiny end of a worm as it pokes out of the soil. They can also see small changes in soil and grass as worms move about just below the surface, movements that indicate where a worm is located. Songbirds have monocular vision in which each eye can operate independently, so when they cock their heads to the side, they are actually turning one eye to look more closely at the soil. When they see a worm, they strike.
- Hearing: As worms move about, they disrupt the soil and small particles of dirt rub together, making noises too faint for humans to hear. Birds have a very acute hearing, however, and this sensory data is one aspect of how robins pinpoint the location of worms and other prey in the soil. This is only one stimulus, however, and detailed studies have not proven that any birds can find worms by hearing alone.
- Touch: Another possibility is that robins detect the subtle movements of worms in the soil beneath their feet. The upheaval of the soil or the gentle movements of grass blades disturbed by worm activity may alert a nearby bird, leading to a successful worm hunt. Like hearing, however, it is unlikely that touch alone is the key to a robin's successful hunt for worms.
The remaining senses of smell and taste are not useful for robins hunting worms. Smell and taste are generally rather weak in most birds, including robins, and these senses probably play no part in finding worms.
Tips for Helping Robins Find Worms
It is fairly easy for backyard birders to help robins, thrushes, starlings, and other worm-eating birds find their next mouthful:
- Keep the lawn evenly trimmed with shorter grass. This makes it easier for birds to see the soil and spot movement. It also makes it easier for them to reach the next bite.
- Water lawns and gardens in the early morning. This will bring worms closer to the surface or even encourage them to emerge from the lawn in search of air, making these tidbits more accessible to hungry robins.
- Keep the lawn in good condition with yearly aeration and thatch removal. This makes it easier for birds to reach the soil and remove worms from soil that is looser and less compacted.
- Avoid using weed fabric or other coverings in garden areas or flower beds where robins may be hunting. Birds cannot easily poke through these covers, even when they do find worms.
- Leave fallen leaves intact for birds to forage through. The damp, organically-rich soil under leaves and wood mulch is an ideal habitat for worms, making it easier for birds to find tasty morsels.
- Offer mealworms in low tray feeders or small dishes at a bird-feeding station. Robins, catbirds, bluebirds, and other worm-loving birds will appreciate the easy treat.
Robins are very adept at finding worms, and birders who understand just how they do it can take steps to help birds find bigger, juicier, more nutritious worms in the backyard.
Heppner, Frank. Sensory Mechanisms and Environmental Clues Used by the American Robin in Locating Earthworms. The Condor, vol. 67, no. 3, American Ornithological Society, 1965, pp. 247–56, doi.org/10.2307/1365403