Thanks John, I did not notice the bill of the left one and the size of it is similar to the right hand side, so I think both of them are Lesser Sand Plover (as Greater Sand Plover is larger than Lesser Sand Plover). But after you mention the bill structure, it is clear that the left one is Greater Sand Plover. In that case, size of Greater Sand Plover may not bigger than Lesser Sand Plover? Of course, I know that this is not the most important element to distinguish the two, but I think it is a quite easy method to distinguish the two in the field.
