Swans behave aggressively toward their recently matured young also. Swans are highly territorial and one bonded pair per pond (or stretch of river they somehow define) is about all you will ever see unless they are raising cygnets.

At the pond down the street for me, we have a pair in residence, and have for years. Every autumn about this time, the three to five young swans leave, and I have actually watched the adults encouraging them to, to the point of looking like they were driving them away.