Blackbirds, like other birds, notice details that escape the human eye. They are able, for example, to follow the movements of earthworms just below the surface of the soil, or to identify traces of their passage: some grains of loose soil, or a streak.
The body of the earthworm is very elastic, and it contracts quickly if it is caught. When we look at a blackbird struggling with the prey then, we have the impression that it has captured a specimen larger than the real ones. The earthworm, on the other hand, stretches out to get out of the ground and becomes thin (it has a diameter of just a few millimeters), and the moist earth closes immediately after its passage.