ajohn

Oriental Plover would typically have longer and paler legs, a long slim bill and would probably appear whiter on the head. The dark mask and lack of a white band around the rear of the neck confirm these as sand plovers.

The bird on the right is a Lesser Sand Plover (subspecies either mongolus or stegmanni) - note that the bill is rather short and blunt, the head is rounded and there are dark markings along the flanks. There is very little breeding plumage so it is probably a first-summer bird (possibly a female, but I think even too dull for that).

The bird on the left is a Greater Sand Plover - in comparison the bill is longer and heavier, and the head is squarer. Again, breeding plumage is not far advanced and this is probably a first-summer bird.

[ Last edited by ajohn at 3/06/2013 08:16 ]

ajohn

I think the bird on the left is slightly larger than that on the right. The bird on the right is more alert, with the neck stretched slgihtly, making it look bigger.
But it is true that size should not be relied on entirely in these species. Although Lesser averages smaller than Greater, measurements show that there is overlap in size between Greater sand Plover and mongolus Lesser Sand Plovers. atrifrons Lesser Sand Plovers are slightly smaller and probably do not overlap in size with Greater Sand Plover. In both species males are larger than females.
Size can be a difficult feature to judge in the field, especially if you do not have other birds for direct comparison. It is better not to rely only on size, but to also look at structural and plumage features.