They are looking for tasty leatherjackets (daddy long legs) and other grubs.

Either leatherjackets or chafer grubs, hopefully the former. Water a patch of grass and lay a sheet of plastic (preferably black) over it at dusk, then remove it first thing in the morning - if its leatherjackets, you should see the grubs on the top of the grass. There is a nematode treatment to kill them in grass (one of the Nemesys range) but the temperature has to be a minimum of 12 deg C (at night) for it to be effective, so by the look of the weather forecast, probably best to wait for things to warm up a bit.

This seems to happen to mine each year for a few days in August, will soon be over.