Without seeing or examining the cygnet, it is difficult to determine the significance of the lump. It could be anything from an abcess (infection from injury or illness), pine straw penetration, hook or fishing line, the list could on. The cygnet needs to be captured and taken to a wildlife rescue facility or veterinarian and returned to the parents within 2-3 hours, if possible so they will not abandon the young swan. If treatment cannot be rendered within this time period, the cygnet will need to treated and then be placed in a safe secure home and raised with other waterfowl. It cannot be returned to the parents as they will reject it once it has been out of their sight for any extended period of time. They will chase the cygnet to keep it away from their habitat causing great stress, possibly death to the cygnet. The cygnet cannot be placed back into the wild or on another pond at this age because it cannot fend for itself which is why a wildlife rescue facility may be the answer.
Additionally, some states like New Jersey and Michigan want to kill all Mute Swans and may require that the cygnet be killed. So, you need to cautiously inquire if the veterinarian or wildlife facility can help the cygnet without killing it. Lastly, if the cygnet is injured or sick, the parents will abandon the young bird unless it can be quickly treated and regains its health which may be what you are now observing. The solution is to get the cygnet to a veterinarian quickly. Please let us know how this situation progresses. The Regal Swan