Earthworms are a concentrated source of protein and fat for many bird species, especially those that forage on the ground. While most birds consume insects, a specialized group actively hunts for earthworms, making them a significant part of their diet seasonally. This prey provides a substantial nutritional boost distinct from the smaller invertebrates birds often consume. The pursuit of these burrowing creatures has led to the evolution of specific hunting behaviors and sensory adaptations in avian predators.
Common Worm-Eating Bird Species
The most familiar worm-eaters are ground-foraging birds with generalized diets, often seen on lawns and in fields. The American Robin is the most iconic example, commonly observed hopping across grass before stopping to pull a worm from the soil. Other members of the thrush family also actively seek out earthworms.
European Starlings are another common species, often probing the ground with their long, pointed bills in a technique called “open-bill probing” or “gaping.” New World blackbirds and crows frequently walk across disturbed soil or short grass to locate a meal. These species thrive where earthworms are abundant near the soil surface, especially after rain or irrigation.
Shorebirds and certain waterfowl also consume worms, but their prey often includes aquatic or mud-dwelling types. For instance, the American Woodcock is a specialized worm-eater that uses its long, sensitive bill to probe soft ground in wooded areas.
Hunting Techniques for Locating and Extracting Worms
Birds employ a sophisticated combination of senses and behaviors to find worms hidden beneath the surface. For the American Robin, the characteristic stop-and-start movement followed by a head-tilt is a common hunting sequence. Research suggests robins primarily rely on keen eyesight to detect subtle visual cues, such as a worm poking its head out or a change in soil texture.
The act of “worm charming,” or “worm grunting,” is a distinct foraging technique used by species like gulls and lapwings. This involves the bird vibrating the ground, often by rapidly tapping its feet, which mimics the sound of a burrowing mole. Earthworms, sensing this disturbance, often come to the surface where they are easily captured. Other ground birds may use foot-vibrations to sense or encourage worm movement.
Physical adaptations also aid in successful extraction once a worm is located. Birds like the Killdeer and various shorebirds possess sensory cells, known as Herbst corpuscles, concentrated in their bill tips. These receptors allow the bird to feel minute vibrations and pressure changes in the soil, helping to pinpoint a worm’s location even when buried several inches deep. Sharp, pointed beaks are then used to quickly thrust into the soil and secure the prey before it retreats.
The Nutritional Importance of Worms in Avian Diets
Earthworms provide a rich source of protein and fat beneficial for a bird’s energy demands. On a dry matter basis, earthworms contain between 60% and 70% crude protein, comparable to high-quality sources like fish meal. This protein is rich in essential amino acids, such as lysine and methionine, necessary for growth and development.
This high-protein content makes worms important during the breeding season, particularly for nestlings. Growing chicks require a diet densely packed with protein for feather and muscle development, and worms are an ideal food source. Worm availability often peaks in the spring and after rainfall, coinciding with the period of highest demand from nesting parents.
Earthworms are distinct from insect larvae, or grubs, which birds also consume. While grubs are nutritious, earthworms offer a higher concentration of certain minerals and long-chain fatty acids. Because availability is reduced during dry periods or deep winter freezes, birds must adapt their foraging strategies to take advantage of this temporary dietary resource.