You need to be extremely careful when reintroducing the cygnet. We would suggest placing the cygnet in a small dog carrier with a thin towel or non slippery mat so the cygnet does not incur a leg or foot injury. Place the front of the carrier facing the pond several feet from the bank (approximately 20-30 feet), so the parents have to come out of the water.
Once the cygnet sees the parents, it will start to cry and the parents should come out of the pond to check it out. If there is no interest, you will have your answer. This should continue on a daily basis for 30 minutes at a time, 3-4 times a day (3 days). If the parents come over each time to check out the cygnet, then you should open the carrier on the 3rd day and see if there is any aggressive behavior toward the cygnet. If the cygnet is allowed around the other cygnets and they do not peck at it or the parents do not try to chase it, and it is allowed to go into the water with the family, the reintroduction has a chance. However, you will need to keep a very close eye on the cygnet in the water to make sure there is no aggressive behavior and have a boat ready for a rescue. This close supervision make take approximately 2 weeks. Any chasing or aggression by any family member should require the immediate removal of the cygnet.
Now, having said all of this, we have had very limited success in re-introducing cygnets removed from parent swans for more than 24 hours. There is a bonding period that the parents will lose due to prolonged periods of seperation which makes reintroduction very difficult. Adult pairs are a different matter and reintroduction can be successful.
Also, understand that if the parents or other cygnets refuse the cygnet, it is going to be stressful on the cygnet for being abandoned and abolished from the group once again. The cygnet has already been seperated from the group for more than two weeks. At this point in time, he is not used to seeing the family and vice versa. It would be better to go ahead and provide him with a new home at a good safe sanctuary where he can make new friends with other waterfowl (ducks, geese, other cygnets or very young swans) and grow up to be a swan and learn from other waterfowl. This will also be much easier on you than the time required to closely supervise an introduction.
Many times people want to give animals human qualities such as mourning from the loss or seperation of a family member. Yes, there is grieving, but nature does not allow extended time periods to grieve as this would be detrimental to the health and survival of the parents and other offspring. Nature forces the family members to get over separation and loss quickly so they can move on with survival. Once the cygnet was removed past 24 hours, the grieving from both parents and cygnets was over. There is really no reason to subject them to the same stress again. The Regal Swan