I dislike using soft plastic or nylon mesh nets as birds can easily get caught in them, in fact I rescued a blackbird from underneath someone else's the other week. However, more sturdy metal mesh can be used to protect against birds or cabbage white butterflies as slow below:
This photo shows it protecting our lettuces, which we found are a tasty treat for House Sparrows, in fact they tell their friends and all have quite a party nibbling them down to a stump if unprotected. Interestingly, this behaviour has only been noticed in the past couple of years, maybe they just got away with it before then, or maybe their tastes have changed.....
Now, it is over the blueberries as Blackbirds and thrushes will gobble the lot if left unprotected!
Some of our allotment neighbours have a problem with pigeons going after cabbages, however this only seems to be in the more "open" allotments, ours has hedges around and I think that it is the enclosed nature of the allotment that pigeons do not like - in fact I have seen pigeons sitting on the fence at the back, having a nosy and then ignoring our cabbages in favour of ones in a more open allotment!
As is said in this earlier post disguising crops by planting flowers in amongst them is a good idea, and I have also found that garlic and onions in with carrots will help reduce carrot flies (the best defence is height!), some people use very small knit mesh or fleece. Garlic barrier spray works well on peas and beans and damsons to some extent.