I read that codes of good practice advise against it, and know many guns (including myself) would walk from a shoot where if they thought it was happening - but is it illegal to release pheasants or partridge during a shooting season to 'top-up' bird numbers?
If not, why is it not (more) common? It seems likely to greatly increase relative return rates, and if the only objection is that it is unnatural, then it could equally be argued that expecting a 300+ bird day is equally unnatural. For corporate 'guns' who simply want to treat a driven day as a squawking simualted clay day, perhaps the ethical objections of mid season release could be outweighed by the economic and (from my perspective) the environmental benefits of releasing fewer birds but getting a similar return.
Obviously, for people who want to shoot wild(ish) birds, it's an anathema and fair enough, but these guns are often not the ones wanting the 300+ days.
Just some random thoughts stimulated by a weekend walking around Exmoor