These are impressive and beautiful.

I'm still wondering how the cut out sections hold together.

If you have looked at feathers closely, you will have noticed that there is the main stem, then the lateral ones, then on those are two rows of hooked hair that hold on each other.

They do hold together pretty well (they must withstand the air pressure of flight, after all).

He probably still picks carefully what parts to cut off.

Looking at it I am pretty certain that he is putting a clear backing on the feather and probably using a cutting machine to get those exquisite and detailed cuts. Then each tiny element is mounted to the back

on what looks like a fine wire or stiff filament. Just gobsmacking.