This is not usual behavior. A parent swan will practice "infanticide" if there is something wrong with the cygnet, i.e., sick or severely injured, not because of a threat. The parents do not want to have a predator follow the rest of the flock or have an illness/disease spread throughout the flock. If the fox hurt any of the cygnets, to the point that they were not going to survive or so severely injured that they could be slowed and attacked by other predators, then the adults will try to kill the cygnets and put them out of their misery. The Regal Swan
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