If they are returning to the nest, then there must still be viable eggs. The birds will continue mating until she completes the egg laying process. After the last egg has been layed, then the female will begin sitting and incubating the eggs. So, incubation does not really start until then. If the birds keep going back to the nest, then it is a good indicator that something is still viable. And, yes, they do cover the eggs so that predators cannot find the eggs and maintain some semblance of heat until the female returns.

If the birds go away for hours at a time, it may mean that the eggs are not any good, and they may try to re-clutch (lay more eggs). If they leave and do not return, then nesting may be over for the season. We hope this information is of benefit. The Regal Swan