The Weylan are born infertile. They can’t have children the natural way. The Albion designed them that way on purpose, after losing too many promising Weylan to the rigors of childbirth. The problem was the wings. Birds grow up inside an egg and then break out. Their wings are fully formed and ready to work at that time. But humans give birth by pushing their child through a thin tunnel and out in to the air. The birth process almost always broke the wings, and sometimes tore them apart inside the mother. So the Albion made their best creation infertile to keep them from dying out, and then built a massive collection of cloning chambers to grow new ones. Most of those died when the Ennead destroyed Arkadia, but the Weylan saved a relative few. They were the only way the Weylan could procreate for the next two thousand years, and they were too few to generate anything that might be termed a population boom.