BLACK SWAN EGGS FOR SALE NO FURTHER A MYSTERY

I am no professional on swans. I somewhat imagine that an evening-time predator filched the eggs. I've listened to persons say that their birds( chickens, geese, what ever) remove infertile or nonviable eggs from their nest. In 40 years I haven't experienced a chook get rid of their own personal egg/s within the nest other than accidentally. Ferti