Clevenger, who has kept ornamental mute swans since 1985, was notified by state officials a few weeks ago that he needed permits for his two adult swans and that he would need to surrender the babies. But that's before they went missing.
State officials say they don't know where the five-week old baby birds are. Clevenger thinks they fell prey to a natural predator or were nabbed by someone else.
Mute swans are a non-native species that feed on environmentally sensitive aquatic plants.
State Fish and Wildlife Director Patrick Emory said Delaware is revising its state policy on mute swans, aiming to limit reproduction of the birds.