So as i was shooting at the bird outside of of the sanctuary, the one the seems to phaze through arrows, i notice random egg nest randomly spawn in certain areas on the cliffs below.
The nests have always been there, they do not randomly spawn, they are always in exactly the same places. There are at least 19 nests spread around the area 17 around the cliffs, one on a broken pillar and one in the hut with the mammoth skull just after the last nest. No idea what the egg hunt is for, but you can have fun collecting them. I have never been able to shoot that damn bird! The bird is seen at two locations that I know of in the Forgotten Vale, I never found anything at the alternate location (bottom entrance to Glacial Crevice), still could not kill the bird.