Rooks’ rations require true grit to digest

WE drove round a corner and the road was covered in rooks. They took to the air and the car was surrounded by large black birds. Miraculously they all managed to avoid us but the number of rook corpses you see on road verges shows that they aren’t always so lucky.

“Why do they do that?” my passenger asked. “Because they’ve no teeth” I answered which, judging from the look on my passenger’s face, was not an adequate response.

We still had some distance to go so, to pass the time, I started a short lecture on rook behaviour.

Rooks live in communities which usually consist of between 50 and 70 pairs. They roost together in tall trees but not in their nests, very few bird species use their nests outside the breeding season. At this time of year the nights are long and cold so by dawn the birds are very hungry.

They fan out from the rookery in search of food and if some of the birds find a particularly good source of it they will call to attract the attention of the rest of the community. This kind of altruistic behaviour is quite uncommon in the natural world, in fact it’s not that common in the human world, but honey bees do the same thing.

Rooks originally evolved as digging birds. They have very powerful beaks and adults have a bald patch at the base of the beak. This is to prevent their feathers getting dirty when they’re digging deeply.

They probe into the ground looking for leather jackets, worms, beetles and grubs living in the grass roots. This is very good for grassland as many of these creatures eat grass roots and reduce the productivity of a pasture or meadow. This means that farmers who shoot rooks, as many of them do, are doing the equivalent of shooting themselves in the foot.

The food is swallowed whole, because a rook’s beak is good for digging but not for chewing, and is stored in the bird’s crop. When the crop is full the bird stops feeding and goes to search for grit — tiny pieces of sharp stone.

The rooks you see on roads are not normally feeding — there isn’t a lot to eat on a road. But passing traffic slowly erodes the road surface producing just the sort of grit a rook needs. The tyres have a tendency to spray the grit towards the edge of the road and that’s where the birds concentrate their search.

When they have enough grit they go and sit on a tree branch, ESB wire or rooftop and for the best part of an hour they appear to be doing absolutely nothing apart from the odd bit of preening. But wild birds can’t afford to do nothing during the short days of winter.

A rook’s crop is basically a bag of very tough, powerful muscle.

During this time it’s “chewing” its food by squeezing and kneading it in the crop along with the sharp pieces of grit. When it has been reduced to a paste that’s easy to digest its swallowed, but the grit is swallowed as well.

The bird then flies off to look for more food, then more grit, and the cycle continues until dusk. If food is relatively easy and quick to find a rook may get five, possibly even six feeding cycles in during the course of a winter’s day. It’s only when the days get longer, that it will be able to spare the time to repair the nest ready for the eggs.

It’s a busy life being a rook.

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