In the Belcher Islands, way up north, the Inuit people one year saved their lives when the caribou did not migrate, by making coats out of tanned goose skins. I saw such a coat at the Yellowknife museum in the Northwest territories when I was up there researching on sabbatical leave one year. The skins were cut in a rectangle, and the feathers remained on the skin to make an incredibly beautiful coat which was obviously very warm. I'm not sure about this, but I believe the skins were simply removed, meat and fat scraped off, then stretched out to dry. They are thin skins and simple manipulation would soften them. Judging by the color of the inside of the coat I saw, I would say that the skins were also smoked in a similar way as other hides.
I've been following this thread with interest because I made a Grouse Hat which still works (though it is a bit ratty and missing quite a few feathers) after 10 years. I've also tried it with other birds. This is what I did. With the grouse, I used the hot rock trick to cook the bird. When the bird was finished the skin slid off as it is supposed to do. I ate the bird. I turned the bird skin inside out and stitched the neck, wings and legs closed with gut from the bird and a needle made from bird bone. Nothing fancy. I filled the inside out bird skin with sand, till it was stiff. This made it easy to scrape the hide with another piece of bird bone. The sand protected the feathers. After it was scraped I used a borax/salt solution to wash the skin. I'm going to drop the narrative at this point and just go to the steps.
Let the skin dry for two days. It will feel starchy and appear to be semi-transparent. The feathers will be visible. Bathe the dry skin with a 50/50 mixture of glycerin and alcohol (the stronger the better Tequila is no good, Everclear is OK and Isopropyl works) don't drink the mixture. Rub the liquid into the skin until it begins to soften (I've been using the Alcohol/glycerin soak mix on snake skins for years. It works great with no other processing/ Well, pull off the fat with your fingers first! If you want to, soak a rag in the mixture and wrap the skin. That will keep the mixture in contact with the skin for a longer period of time. When the skin feels fairly supple, open up your appendage holes and let the sand drain out. Carefully turn the skin back to the way it was when it was hatched. Now you can cut or wear the feather bag. It will remain fairly soft. If it stiffens just use the mixture again.
I've done this with quail, which is problematic since the skin is so thin, and even with a few sparrows (hacky sack sparrows).