This is interesting Anne, similar hereabouts and I see many magpies, do have thrush and blackbirds. As Dazzerelli says, the populations of leatherjackets is off the scale in some places. The mild autumn and winter must have helped and I am wondering where I can plant lettuce and spinach.
After about the middle of June they should pupate and the problem disappears until next spring… I have bought nematodes but they are not delivered until July.