Something digging up my cabbages?

I'm only on week 2 on my allotment, & I'm very much learning as I go.

I planted some Savoy cabbages a couple of days ago, last night when I watered they were fine, but this morning 2 of them have been dug up & dragged a few inches away.

Is it most likely birds?? How is best to protect them?? I found a nice long poly tunnel in one of my sheds, would this be ok to put over them?

Any advice much appreciated, thank you.

Do as aelf say and net them I do this as soon as they are planted, you could always cover with wire mesh for the birds but the cabbage whites will get in.