Cockchafers. Also called May bugs. Each one is a lovely big meal for a blackbird. Do not kill them, that would be a sad waste. When not eaten they emerge from the dirt with steam punk wings to bumble about in flight like heavy crazy things (after seven years underground, I am told).
In the larvae stage they look like freaky aliens. The adult cockchafer has just a few days to mate, poor thing. We hear them excitedly bouncing off the window glass of an evening in May.