Ask an expert: blackbirds and worms

Andy asked: could you tell me how blackbirds see or hear worms under the surface in my garden. Especially after rainfall.

Blackbirds have a surprisingly varied diet, ranging from seasonal berries, fallen fruit and kitchen scraps to caterpillars, beetles, snails and of course earthworms, of which they will eat all year round as long as the ground is damp enough. Recent studies have shown that Blackbirds, during the wintery months of December and January, can catch one to two earthworms per minute! With this is in mind, it is not unusual for Blackbirds to catch more than they can actually eat and these unfortunate worms are often left discarded on the surface. So determined are Blackbirds to catch worms, that a 1958 study noted a Blackbird using an 8cm long stick in its bill to clear snow so as to forage for earthworms in the soil beneath it.

It has been speculated that earthworms appear on the surface of the soil after rainfall for migratory purposes; earthworms may be able to move greater distances above soil than through it in such conditions. It has also been suggested that vibrations caused by rain drops may emulate the sound of predators, such as moles that worms will want to evade.

Blackbirds find earthworms in one of two ways, either by spotting the tail or head end of the worm sticking out of a burrow entrance, or by sound. Blackbirds have remarkably acute hearing and are able to detect invertebrates moving under the surface of the ground. When Blackbirds forage for earthworms they will run and hop along the ground in a series of straight lines and may pause briefly, before changing direction. Safe in the knowledge that earthworm distribution tends to be close knit, Blackbirds, even though they may have successfully caught a worm, will then search the surrounding area as if they had been unsuccessful.

Though it is not unusual for Blackbirds to wash their food before eating it, adult Blackbirds are able to swallow most worms whole. As regards feeding young, earthworms are often dragged over soil to remove slime or add grit.