Wood Pigeons are a major problem in our allotment but I have found netting effective provided there is enough space between the plant and the netting. What the beggars can do is sit on top of the netting which bows it down and then peck away at the leaves or they can lean into the sides and do the same. Of course pea netting is no good as the holes are too big and they can poke their necks through. The textbook says that they only eat brassicas in winter and early Spring when there is less food about but, unfortunately, ours haven't read the text book and I have to keep the netting on all the year round.
A good idea is to by a few hoops of semi-rigid plastic tubing from, the DIY store(like they use for electric cables) and join two of them up and stick them in the ground and create a taunt "polytunnel" like thing with netting instead of plastic (you can replace the netting with plastic in winter if you want and create a polytunnel!). This prevents the birds from getting at the plants.
We get them nesting in our garden at home and I go round knocking the nests out as they build them. Eventually they give up. Alternatively you can by one of those super dooper long range water pistols and give them a good soaking. It discourages them and you can revert to being a 9 year old again!
However it doesn't stop them landing on out TV aerial . The weight of them taking off eventually pulled our aerial down and took a chunk of the chimney stack out with it!

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George the Pigman