Often when I’m out walking I pick up a feather and sometimes put it in my hatband, this made me think about the meaning of something being “a feather in your cap”. I haven’t heard the expression for a while but to me it indicates an achievement. I wondered if in days long ago it might have indicated a skilled hunter and in a way it is apparently connected to that. In different cultures across the world not only hunters but also warriors used feathers to advertise their proficiency at killing and many Armed Forces today still incorporate feathers in their headgear.
Even the song, “Yankee Doodle Dandy” refers to a feather in the hat, you can read the story behind that here:
https://www.atlasobscura.com/articles/the-macaroni-in-yankee-doodle-is-not-what-you-think
I was trying to think what I’d consider a “feather in my cap” and decided it would have to be my University qualifications because the Guidance Officer who came to my secondary School left me in no doubt I wasn’t capable of being successful there!
Musings of a Retiring Person 
very interesting history