There are several possibilities with this scenario.

1. These may be juvenile swans and are in their first year of nesting. Possibly, they have absolutely no idea what they are doing, but nature tells them to nest and the female to lay the eggs.

2. Female swans and other female birds, are programmed to lay a certain number of eggs throughout their lives. If something disturbs the nest (predators, flooding or other inclement weather problem), she will lay eggs to replace the ones lost, (they know how to count). More than likely, the eggs found in the new nests are the result of this "double clutching" to replace the eggs from the old nest. So, you are not seeing eggs from one nest being placed into another nest, but rather new eggs being laid.

3. If both of these swans are female, they may each be building nests and laying eggs. In nature, a male swan (without female mates) will mate with another male swan, build a nest, but obviously, no eggs will be laid. Conversely, a female swan will mate with another female swan (without male mates), build a nest and will lay eggs, obviously not fertile.

So, any number of possibilities could be relevant. The Regal Swan