Facinating, I did not know this was possible!I don't know about Swans but turkeys can do parthenogenesis. Even under the best of conditions there is still only 1% that actually hatch. The vast majority of eggs that start developing because of parthenogenesis will quit developing before they reach full maturity.
It is possible to have an egg show a dark interior without development. When an egg is going rotten it can be very dark on the inside.