Our Brocolli was attacked earlier this year just as it was about to crop by the huge (I mean huge!) Wood Pigeons that seem to hang out in our garden most of the time.
They started on the Brassica's this year just after they'd been planted and the damage was very different to slug damage - very angular, beak like chopping and no visible slime. Everything's been securely netted since and there's been no further damage. Good luck!