There are several reasons for this behavior. Yes, the swans can lose fertility as they age. But, there are also other possible causes:

1. The mother has a nest and is taking a break. Swan mothers will take a quick dip and return to the nest. Usually, the dad will stay in the area, but if they are still in the process of laying eggs, then they will leave the nest a little longer.

2. A predator got to the eggs and scared the parents from the nest.

3. Michigan DNR is notorious for killing the cygnets and the parents in the middle of the night so that no one knows what occurs. They refuse to listen to reason that the Mute Swans do no harm and are actually a Sentinel species which alerts to problems in the environment. If the Michigan DNR had not been so proud to exterminate as many of the swans a possible in lieu of the larger Trumpeter Swans for Trophy Waterfowl Hunting purposes (it's all about the money and not conservation), they might have discovered the lead and other problems in the Flint River prior to people being poisoned through drinking the water. This is the ultimate problem with destroying this early warning system (the Mute Swans) and until the public force politicians to stop this slaughter, it will continue and more habitats and people will suffer. The Regal Swan