This may be a spring thing as mating season is upon us and the 2 year olds will be old enough this year to mate. Territorialism is part of the mating season and someone is going to land up being beaten. If the younger pair prevail, this may be a permanent problem.

The young birds may not leave the lake that they were hatched as this area is seen as a safe breeding environment and the other lake may not have the safe habitat with food resources. You may need to place your parental pair in a pen (1/2 in water and 1/2 on land with feeder in pen) for the spring session for safety. You will need to cover the top of the pen to keep predators from climbing into the pen and will need to use poultry fencing under the pen so that predators cannot dig under the pen to get to the swans.

If you can capture the young birds, it would be better to place them in the pen as they will understand that this may not be a good place to return if they can't get on the lake at will.

Also, as an aside, you really need to pinion future young as most states will shoot what they deem as feral (non-native) out of the sky and charge for the bullets and man-power to remove future feral populations. At least pinioned, the young will be easier to capture, send to another pond and not worry about having them fly back to the original pond. The Regal Swan