I have the same problem, not all the time but this happens. The split, funny my split always happens to the wing near me. I am sure this is do to wing attachment to the hook. Setting of this style wing, is sometimes not easy. It might be possible i am pulling the thread toward me and not straight down.
Many times the split occurs while wrapping the hackle. Hmm?
I tie fast, so to tie one every 6 or seven minutes, i would rather double or triple that.
Back in the old days they sold "wing formers". Never tried one, seemed it took time or i did not have the money.
I feel the lack of practice has some effect also. When young at tying all flies had duck quill wings. Then i started with side feathers, woodchuck and deer hair wings. Seems for a while all my flies had deer hair wings. I really liked them.
Funny, when you hold a fly above your head and look up, like a trout, you do not see the duck wing tied on the fly.
Good luck, i am sure you will do just fine!