The larger siblings can and will pick on the runt of the family if it cannot keep up with the rest of the family. But, usually this harrassment by the siblings or parents means that the member being picked on has something wrong with it such as being sick or injured.
If this is the case, the sick or injured cygnet will be harrassed or abandoned by the family which causes the young bird to hide by itself. This will also cause great stress to the cygnet and its health may suddenly disintegrate.
Nature will sacrifice a sick or injured family member so as not to jeopardize the health of the entire family or flock. A sick member can spread disease to the family or cause a predator to be alerted and attack not only the sick or injured cygnet, but also the entire family.
Is there any wildlife rescue facility in your area that might be able to check on the cygnet? Having asked this question, you must also be aware that many states, but specifically New Jersey and Michigan will actively kill the parents and entire family if you seek help.
State and federal wildlife officials have perpetuated one of the greatest hoaxes on the U.S. taxpayers by labeling the Mute Swan as non-native and invasive (detrimental to the habitat). Nothing could be farther from the truth, but are non-scientific justifications for getting the public to accept the killing of the Mute Swans across America. The end goal of wildlife officials is to remove Mute Swans from their habitat and introduce the larger Trumpeter Swans for Trophy Waterfowl hunting. Trophy permits cost more and are a means to raise money for shrinking wildlife budgets.
Unfortunately, the taxpayers are paying billions of dollars for the entire eradication of one species for the introduction of a more huntable species. Additionnally, the National Audubon Society and Cornell University, both of which proclaim to be bird advocates promote this killing program. So, please check out your state’s stance on Mute Swans before seeking help as if you live in one of the killing states, the swan family can be at grave risk. The Regal Swan