Astronaut Piero:
This is the second layer of "Body Feathers" covering the head controled by a "Hair Density Texture". A second texture is needed to control the length of the feathers.
Final Cycles rendering for this lesson with the second layer of "Body Feathers" in place at Piero's head:
Render Time: 6 Minutes 4.31 Seconds
Looking good duerer! I've only got 2 notes:
Thanks @theluthier! The "Alpha Texture" definitively needs to be adjusted in order to avoid the card-like look. And the feathers are still too glossy what makes them distinguishable from the underlying body texture. Getting this all right is really tricky since there're many settings and therefore many occasions to do it wrong 😉.
I'm also not sure whether the "Anisotropic Shader" in Blender 2.90 is working the same way as the one in Blender 2.66.1 since your Piero's fur feathers are looking much better than my Piero's fur feathers despite the same settings. Another point is the "Displacement Input Socket" of the "Material Output Node" (grey in Blender 2.661, violet in Blender 2.90; thanks to 
spikeyxxx for the hint!) for the modeled feathers which now needs a "Displacement Node" where I plug the "Bump Texture" into the "Height Input". I'm wondering whether I should use the "Principled Hair Node" for the fur feathers.
Hey duerer I have a question for you. I've noticed you asking a lot of questions throughout your journey through the Piero course. Good questions!
But if makes me wonder if this many questions means it's time to retire the course from our library. As painful as that would be for me to admit 😢
Objectively speaking and based on your experience with it , do you think this course is too difficult to learn from in 2020+?
@theluthier This course is the reason why I came to CG Cookie! There's no other course out there that shows you the creation of an animated bird from scratch! Of course, some things are outdated since we don't need "Blender Internal" anymore for the texture painting and thanks to the "Collections" introduced with Blender 2.8, we no longer need a "Scene Layers Management Addon". I will proceed with the course to completion in Blender 2.90 and notice what needs to be changed. I've also written down my steps for the feather creation in GIMP and made some notes on what has changed from Blender 2.66.1 to 2.90. One question which I haven't yet answered is whether I need this "Bone Layer Management Addon" which you installed at the beginning of this course or whether Blender 2.90 makes this obsolete, too.
I've already seen some Piero's and relatives here on CG Cookie that have been modeled following your course, Kent!
For example here. And what about modeling this bird here by Alyssa Veysey or Piero's little brother here (modeled by 
pieriko (concept by Svetlana Bukanova) in Blender 2.9? An eagle or an owl (as e. g. here) would be worth being modeled and animated, too 😉.
Wow did not expect this. Some of your questions have seemed to be explanatory in nature, as if you were bridging the gap from 2013 Blender to modern Blender, in case others wanted to learn from Piero as you are.
It's incredibly rewarding to see Piero "relatives" posted by members 🤩 I'd only seen about half of those you posted.
So do I take it that, despite the difficulty of the Blender version being super old, the Piero course is still worth learning from?
This course should stay in CG Cookie's library at least until there's a modern successor to it. And beside this, I think, it should stay there even after that as a "Classic" and "Milestone" in the "Blender Teaching History" 😀👍✨💎🏆🥇!
There would be no Piero or any other bird from me without your course 👍👏!!! You've paved the way. My Piero is almost complete for a single image rendering 🐦. And the animation will follow 😉💪!
Wow, that response means a lot 😊 I suppose we won't retire it, at least not yet. Thanks for the feedback duerer. Made my day if I'm honest.