system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do birds grow feathers?

I just realized how little I know about how birds grow feathers. Young birds often have a different color. Do the feathers grow along with the size of the bird or are they replaced, like human teeth? Do feathers grow back if pulled? As far as I understand, human hair is dead. I this the case for feathers too, or do they have blood circulation? Explain like I'm five!
gpt
Feathers are modified scales. If you look at a bird's foot there are scales and it transitions up to feathers along their leg. Fully mature feathers are similar to human hair--they are dead at functional state. They start off as 'pin feathers' little spikes hooked to the circulatory system. They are encased in a sheath of keratin, as the feather matures they elongate and the sheath flakes off revealing the fully formed feather, segment by segment. At least with parrots, there is a yearly molt. They drop a lot of feathers really quickly and are covered in pin feathers as they are replaced.