Pigeon shooting expert
There is no foolproof way of achieving this, but there are a few things you can do to help.
Firstly, when it is hot, pick up quite regularly and lay the birds separately, in the shade.
If possible, take a large sheet of hessian and place the birds under it.
The best thing I have seen, was a bag made of curtain netting with a drawstring at the top.
A friend made several of these, each of which would accommodate about 20 birds, keeping them as fly-free as was possible.
If worst comes to worst, you just have to remove the eggs by hand, before storing or freezing the birds.
Make sure they are properly cooled before freezing, as I have known fly eggs to hatch into maggots inside the freezer, only to be discovered six months later when you empty it – not a pretty sight!