Leatherjackets probably - there are treatments you can use, just not sure what's available now, so much has been withdrawn, and not sure when they can be used either. Ask at your garden centre. Leatherjackets damage your lawn anyway, without the birds interfering - they eat the roots of your turf. Placing a sheet of polythene over the grass in the evening and then removing it first thing in the morning should reveal quite a few on the surface, which you can then pick off and destroy. The other thing you can do is lawn maintenance - aerating particularly lessens the risk of habitation by leatherjackets - they prefer poorly maintained, compacted grass.

Three-fourths of pest and disease prevention on lawns is good soil and turf maintenance.

Apparently nematodes are effective against leather jackets

i've sent you a private message with a link

And, this weather the magpies need food too. When the warmer weather comes, they'll move on, and your lawn, well aerated, will recover very quickly. Live and let live. Worthy