I’ve never seen this before. This morning I was looking at a group of pregnant females all sitting down in the rain and there were two crows or rooks, anyway large black birds, wandering amongst them. One of them started pulling fleece out of one of the alpaca’s tails, big beakfulls of it. She turned her head irritably but stayed sitting. When the bird had three big lumps in its beak, it flew off. That’s going to be a very warm, comfy nest.