[bryanthetrumpet]

I've recently (In the last 8 months) Taken over my Stepfather's 2 allotments.

Our Cauliflowers are being attacked by pigeons or rooks, according to the locals, but our broccoli, cabbage, sprouts are ok. any ideas why?

[8doubles]

Pigeons will eat most brassicas , it might be that the caulis are the tastiest at the moment.
Rooks are insect eaters but will pull plants up to get to grubs in the roots.
A flock of rooks can do a lot of damage  to a field of maize in a short time but to the allotmenteer they are rarely a problem.
Another bird that has a taste for seedlings is the house sparrow so they may be responsible.

[mumofstig]

On our allotments -if you don't net it - you don't get it 

If the birds don't get them, the caterpillars will