Yesterday above Buckinghamshire, England we saw about 20 large white birds circling high overhead during the afternoon, as if to gain height. On a sunny day here their bodies appeared very white and their wings black (although possibly they were just in shadow).

We've never seen this before - I'd estimate the birds to be at about 1,000 feet, too far overhead to have a clear view. They seemed much larger than the gulls we see frequently.

Could these have been migrating swans?