Good news. The poop theory was a good thought, but it is a wive's tale (actually mother's who don't want their kids to pick up birds) about birds smelling human scents. Birds don't have that much of an olfactory sense but obviously, have other ways of telling which cygnets are theirs and if something is wrong. In any case, by introducing them to the other 3, he/she was accepted and that is great. The Regal Swan