The "trimming" he did is extreme. It exposed the birds to cold, which can cause a lot of issues, leading to the death of the birds. Also, if you want to stop birds from flying away, you don't need to trim both sides. One side is enough, so that they can at least have some feathers for warmth.Also, you have to be very selective. Only trim the flight feathers, not the blood feathers.Blood feathers are developing, immature feathers in the process of growing. They have an active artery and vein runn