Strange black fibers crawling out of wild bird egg after killing the embryo

Some wild birds were sitting on a clutch of eggs to hatch; the eggs were fertile and were developing. But shortly before they were to hatch, the parents suddenly abandoned the eggs. Some black, plastic-like fibers were moving and crawling out of the eggs. They are not worms, and there was no breeze etc to cause the fibers to be moved. It was moving independantly.