Imo There are dozens of theories and potential reasons for the losses, magntic fields, clashing of birds, weather conditions, poorly trained, unfit, and birds carrying ailments, and our old friends BOP the list goes on & on, for me some birds just dont have it in them, birds that are well trained and fit n healthy are better equiped to make it, but even then theres no guarintee, also the fancier has to accept part resposibility as he/she is the one training feeding and responsible for the birds welfare John