I'm doing something wrong here, and I'm not sure what. I'm in Blender 2.9, and I'm using a different feather texture than the one in the video. I've checked "from intancer" (thank you Duerer!) on the texture coordinate, and plugged my body color texture into the color slot of Principle BSDF. But what happens is that instead of the instance feathers taking on the colors of the mesh underneath, all the feather instances take on the color of the feather object's location in the UV map. I'm attaching a pic of my node set up. Any thoughts?
I think that From Instancer only works in Cycles and you are in Material Preview Mode, which uses Eevee.
Cycles:
And Eevee:
Might be more going on, but at least switch to Cycles.
I have another question about this. I've got the feather and hair systems taking on the body texture, but both of them are rendering with black "strings" attached to every instance of the feathers and hair particles. You can see it particularly in the hair system and at the edges of the feathers. Any idea what this is from and how to solve it? The images show the materials I'm using for the hair system and feather system.
I have no idea what is happening in your case, we might need the advice of 
duerer here.
What I can tell you is that you put a grey socket (single value) into a purple socket (vector: X, Y, Z-axes values), which puts that single value into all 3 axes:
instead you should use a Vector > Displacement Node:
Now, in the other screenshot:
you take the color output of your (to me unknown) texture and plug it into the Base and Subsurface Color. And drive the amount of Subsurface Scattering with the Alpha Channel of that mysterious texture. But you also plug the color info (RGB) into the Factor of the Mix Shader! NEVER do that! NEVER plug a color output into a grey socket.
EDIT: If the Texture is B&W, then the Color output is B&W, so then it doesn't matter.
What happens is that the Color (RGB) is being turned into a single value by this formula (in Blender, there are other, similar formulae for the Luminance, what is what this is called):
which is implemented per the ITU-R standard as 0.2126R + 0.7152G + 0.0722B. Completely unpredictable behavior!
No matter what your FeatherAlpha texture looks like, this setup seems to me as being 'pretty f*ed up'.
Maybe you want to use the Alpha output of the texture as a Factor in the Mix Shader?
Yes, that's why I tagged him (duerer) 🙂.
Btw. switching to non-color data in a Texture Node doesn't make it a black&white texture! It just means that the colors are interpreted differently....