It would be a little peculiar for the swans to stay in a field and not fly or walk to an area with a pond or other suitable wetland. Apparently, there is plenty of food source if the swans have not left. You might contact the RSPCA or Wendy Hermon at Swan Support in Datchet to check-out the swans.

There may be an estuary that has dried (frozen) temporarily or some other issue in which the swans cannot leave the area. Usually, swans will seek grazing material during the migratory path and in the winter months when ponds and lakes are frozen. You might just be seeing them temporarily and only a few residual swans actually remaining in the same area. In any case, it would not hurt to have the swans and the area checked for any problems. The Regal Swan