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First, Trumpeter Swans are a protected species because U.S. and state wildlife officials are trying to reintroduce the species for Trophy Waterfowl hunting purposes. This is why these same wildlife officials (since the late 80’s and continuing today) kill Mute Swans.
This Mute Swan killing program and reintroduction of Trumpeter Swans has been rejected by swan specialists and the general public for the exact reason you are citing. Trumpeter swans are much larger than other swan species and much more defensive of their territory.
It is a known fact that a male swan may or may not take a mate. So, the first thing to do is not try introducing any other female Trumpeters on the lake.
Trumpeter Swans and basically, all male swan species will try to remove another family’s cygnets from their immediate territory. If a cygnet enters another territory or gets in the middle of a fight between two adult male swans, they can get injured or kilked just from the sheer size of the adult swans.
It is highly unlikely that any adult swan is going to go out of its way to drown cygnets. Unless someone is physically watching 24 hours and sees this behavior, it is highly unlikely. If the cygnets are ill or injured, they may be abandoned by the parents, and this will get them into trouble. Even a cygnet’ own parents will attempt to drown them if there is something wrong. This is nature’s way to prevent illness from spreading to the flock.
If there are many cygnets being lost, you need to look at illness or predation as the cause. Egrets, Herons, hawks, turtles, fish, raccoons, etc., will feast on cygnets.
Back to your question of what can be done to quarrantine the Trumpeter, you cannot. He is a protective species and your park had to get a federal license to have him or other Trumpeters. If they do not have the license, they are in violation of federal wildlife laws. Since this swan seems to have been at the lake for years, then they definitely would have had to receive a license. If anything happens to the swan, the park is mandated to alert wildlife officials that the bird was harmed, killed or died which can cause an investigation into the park’s policies. If the swan is placed in a pen or removed from the habitat for any purpose, it can be deemed as inhumane care. Therefore, since the swan is exhibiting normal behavior, he needs to be left alone. The Regal Swan