Hi Ruth
If there was something wrong with the cygnet, the adult male will kill it to put it out of its misery, prevent illness to the family or attract predators to the family. Not all swans will do this, some may just abandon the cygnet, chasing it from the family, not allowing it to eat etc. Then nature will take its course.
Many wildlife officials promote the Trumpeter Swans over the Mute Swans. They misrepresent the Mute Swans so they can be killed and the larger Trumpeter Swans introduced for Trophy Waterfowl hunting. What officials fail to tell the public is how much more aggressive Trumpeter Swans are than a Mute Swan which only protects its family and territory. Trumpeter Swans intentionally or accidently kill many wildlife species just because of tbeir large size. A Trumpeter Swan cygnet also eats twice the amount of sub-aquatic vegetation SAV than an adult Mute Swan. All of these facts have been misrepresented to the U.S. taxpayer at the cost of billions of dollars to introduce Trumpeter Swans into areas that they never inhabited (East of the Mississippi, yet they knowingly prefer the larger swan stating that the.Mute Swan is non- native and invasive. More misinformation to get the taxpayer on board to kill the Mute Swans. This is all to say that the Trumpeter behavior you have witnessed is just one of many attributes that have been ignored so that Trumpeter Swans will be accepted. The Regal Swan