More than likely they have migrated with their cygnets (young) to another area. Swans as with all birds have wintering, spring and summer migration areas. Swans will migrate from cold weather to a suitable area for nesting and raising cygnets. Once the cygnets are old enough to fly, the parents will take them back to their wintering grounds so that there is enough food and other natural resources during cold inclement weather. Since it is the summer, the swans, may have moved to an area which is less hot (since you folks have been receiving an enormous heat wave this summer) or has suitable food and other resources for the offspring.

The only other explanation is that the habitat was rendered unsuitable in the birds' minds because of lack of food, shelter or the presence of predators (domestic i.e., dogs, cats, humans or wild such as foxes, minks, coyotes, bobcats, etc.)The Regal Swan