Who first noticed the terrible things? How did he know they were there?

The story begins with an unnamed narrator describes a beautiful clearing, shared by various animals. The animals lived in peace for a long time until one day, when the Terrible Things came. The Terrible Things claimed they came after the creatures with feathers on their back and one by one, every animal that did not had feathers excused themselves. The Terrible Things saw the birds on top of the tree branches and caught them with the nets they brought with them and took the birds away.