Assertion: parrot's feather | /r/MadeOf | tough fibrous protein keratin
Facets:
Source sentences:
text: Birds have feathers which are made from tough fibrous protein keratin, a protein which is also used to make horn and hair by different animals and also beaks of birds.
text: Birds have feathers which are made from tough fibrous protein keratin, a protein which is also used to make horn and hair by different animals and also beaks of birds.