Hi Sahara:
We’re not sure that we can ever come up with the correct answer for this scenario. No, swans will not steal eggs or cygnets and they will certainly not raise another swan’s cygnets. The plausible scenario would be that Pharoah and Sheba may have built in a prime location and another swan pair wanted the nesting area. This would be the reason that Pharoah went missing, because he was being harassed and chased by the Alpha male. If the eggs went missing, then it is likely that there was a predator in the area that ate the eggs (and chased the parents from the nest); there was flooding of the nest; or something was wrong with the eggs, i.e., not viable or rotten and the parent’s got rid of the eggs. Finally, the appearance of a new pair and cygnets would probably mean that they came from a nearby stream or other nearby water body in which the parents could walk the cygnets. If the area that Pharoah and Sheba inhabit has better water, less predators or other disturbance and good food and shelter resources, then it would make sense that the family accessed Pharoah’s and Sheba’s pond for a better habitat for their young. You may know the area very well, but swan parents can hide in areas that you may never even consider for having a nest and cygnets. In fact, we have an area that we consult in which we have lost cygnets overnight and had no idea where they went, yet they hid and reappeared the next day. So, swans are very good at hiding their young and nests from predators and humans. The Regal Swan