Laying eggs on the ground is a good way to get them eaten by a predator. Some reptiles bury their eggs, that’s one option. But then you can’t really keep an eye on them or take care of the babies once they hatch. So maybe you try to find a nice safe place to keep them off the ground instead. Critters who were better at keeping their eggs from falling out of the tree or off the cliff had more babies, so nest-building behaviors get reinforced, even if it’s just laying a few twigs in the crook of a tree. Stick-loving birds get rewarded!