There may be several reasons for the swan not sitting on the eggs. If she has not completely finished laying eggs, she will intermittently sit and leave the nest. This is why when humans count incubation they are usually wrong in their calculations. Incubation (continuous sitting on the nest) does not occur until the last egg is laid. So, she may not have completed the egg laying process and will return in the next few days to complete the incubation process.

If there is something wrong with the eggs, i.e., non-viable due to extreme temperature changes (from what seems to be the end of winter and beginning of spring only to have the cold temperatures return), infertile or the presence of a predator or some other disturbance to the nest. Disturbances to the nest area can include human presence or activity and/or the rising of water levels which may not be conducive to the raising of young. If a nest was built in an area that appears dry, but later becomes water laden due to the time of year, etc., the swan will abandon the nest. The Regal Swan