Anatomy of A Wing Trim
By trimming only the primary feathers needed to prevent upward flight you promote a more confident and secure companion bird. Leaving as many remaining feathers as possible allows for better balance and coordination and a safer situation for your bird. An OVER TRIMMED bird can be unstable, insecure and prone to falling and injury. When nails are clipped also, over clipping is even worse since the bird not only looses it's ability stabilize and balance with it's wings, but it's ability to grip it's perch.
Strong fliers such as cockatiels, macaws and cockatoos will tend to need more feathers trimmed from the P1 to P5 range while others such as African Greys, and Jardines should avoid trimming any feathers in this range.
Never trim a growing feather through the protective covering (Blood Feather) as it will cause the feather to bleed and can easily bleed out the bird and cause death. By not trimming back farther than the primary coverts you will be less likely to have a blood feather issue when you trim or latter when a new feather grows in, but, you still must be aware of any that may be present when clipping. If a feather bleeds, the bleeding must be stopped. DO NOT use septic powder (Quick Stop) on blood feathers, as it can cause blood poisoning when applied to feathers. Corn Starch or Flour may be used instead. Pull the feather as a last resort only as the follicle may be permanently damaged as a result if not done properly. If Pulling the feather is required to stop the bleeding please get help from someone with experience, preferably and avian vet.