Did she roll it around the nest and it accidently fell out of the nest? Or did she intentionally move it to the edge of the nest and push it over? If there is something wrong with the egg, swans will push it out of the nest so that when it explodes after rotting, it will not soil the nest. If she accidently pushed it out of the nest, unless you or someone immediately retrieved it, the egg would no longer be a viable environment for a developing cygnet as the cygnet's air supply would be cut off. The Regal Swan