Some birds/animals just have a nuturing quality which is why there are stories of a female dog/cat nursing other animal's young. In this case there is even a more simple answer. A baby swan (cygnet) is known as "the ugly duckling" because of their duck like appearance before they grow into the elegant long necked white swans. The swan may associate the young ducks as her own cygnets if she cannot have cygnets (no mate) or she is lonely and has adopted the ducks as young family members since she is older and larger. It will be interesting if the ducks, and swan still share this relationship as the ducks age. The Regal Swan
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