PETER THEOBALD says:
I would not say that pigeons PREFER spring rape over autumn sown varieties, it’s simply that it is available at a time of year when the birds have few other choices of food.
They tend to decoy better on the spring stuff for two reasons, firstly, the big winter flocks have broken up meaning that smaller bunches come out to feed, and we all know these are invariably easier to deal with.
Secondly, farmers grow fewer fields of spring rape which means the pigeons have less fields to clear off to, and will normally keep returning to, maybe, the only field in the district.
The reason farmers do not grow too much of it, is quite simple, it’s not as profitable as the autumn varieties.