Could be mine aren't feathered well enough, but I have read that the cold can cause the urates to crystalise and this can in turn cause serious internal damage. If they are in the hut and it is -10 outside, they sh 1 t no bother, but ???? I honestly don't know enough. I go by what I read, am told by fanciers whom I respect and then test it for myself to give me a full conviction. I accept I could be well wrong, but for lots of fanciers, something went wrong. Obviously it was ok at the lib end and flight path, but some pigeons hit bother. It may be they were not fit enough or something else, but this is part of the enjoyment. The mystery makes us think, or it should.