Because of the late severe weather that we have had this spring, many birds were late in nesting. No bird or other animal is going to sit anywhere for any significant amount of time without a reason. Female swans know when the eggs are infertile or not viable. The swans also will get off the nest throughout the day and night to eat, so she is not going to starve. Leave the nest alone, but keep watching her. We think she will have cygnets within the next few weeks. If not, then she will leave and she will be fine. The Regal Swan