It could also be pigeons, they love cabbagey things too. Use netting or other forms of bird scarers.

I now grow my PSB from seed and keep them in pots for some time so they get nice and sturdy and able to withstand the onslaught. Once they go out they need a stout stake as they get really tall and they need netting. Pigeons are a real pest and then, later in the year, the dreaded cabbage white butterfly. I net all my brassicas now as I am so fed up with the wildlife eating them all.

thanks