Regurgitated beetles and cherry stones

We found this regurgitated pellet, full of cherry stones, near some wild cherry trees in a field beside the river Tees near Egglestone, at the weekend. I'm pretty sure it was produced by a rook, as there is quite a large rookery nearby and I've often seen them feeding in this field.

Regurgitated pellets of indigestible food held in the gizzard are usually associated with raptors and owls but many other birds produce them, including rooks, crows, magpies, gulls, herons and even oystercatchers, which expel pellets of sand grains swallowed with their food onto the seashore.

When I took this pellet home and soaked it in soapy water it fell apart almost immediately, revealing eleven cherry stones, plus ....



..... the remains of at least ten ground beetles. 

Strangely, each of the beetle heads was perfectly intact and snipped off from the rest of the body at exactly the same point. A ground beetle's jaws are quite powerful and I imagine that they would grip the bird's throat as it tried to swallow its prey, so I wonder whether this rook had leaned to detach the heads and incapacitate its victim before swallowing it.

The other debris included fragments of wing case, plus ...

..... lots of legs.

So how did the bird come to be eating cherries and beetles in the same meal? It might be because the ground under the trees was covered in rotting cherries, which might have attracted soft bodied invertebrates like slugs and worms, which might in turn have proved a fatal attraction for the beetles.