10 July 2012

Greater Sand Plover or Lesser Sand Plover? – Dammam – Al Khobar Wader Roost South

I went to the Dammam – Al Khobar Wader Roost South on 4th July and photographed this bird at high tide, where it was under a bank between the sea and the area where the new housing complex is going to be built. As it was high tide most of the birds had already been pushed off and were on the land behind the roost and all the mud flats were covered with water. As a result no feeding action was, seen just very close views from the car and then brief flight views as the rising tide made the area it was standing waterlogged. Separating Greater Charadrius leschenaultii and Lesser Sand Plovers Charadrius atrifrons is far from simple, with this bird being a case in point, is it a Greater Sand Plover (GSP) or Lesser Sand Plover (LSP) as it appears to have features of both species?
The photographs of the bird on the ground shows well the plumage, bill size and leg length.

Typically, GSP is a bulky bird, while LSP is more delicate and usually looks smaller. The race columbinus of GSP, however, somewhat approaches LSP in size and can cause confusion, even among experienced observers. The sub-species of Lesser Sand Plover Charadrius atrifrons that occurs is C. a. pamirensis (C. a. atrifrons may possibly occur particularly on Red Sea coast but this bird was seen on the Arabian Gulf coast). The sub-species of Greater Sand Plover Charadrius leschenaultii that occurs is C. l. crassirostris (C. l. columbinus occurs on the Red Sea coast and possibly northern Arabian Gulf). Unfortunately this bird was on its own and very close so size was extremely difficult to judge.
Leg colour
Leg colour is usually greyish-black or black on LSP and yellow-green or greenish-grey on GSP although LSP sometimes shows greenish-grey legs and GSP sometimes having greyer legs. The toe joints of LSP are concolorous with the rest of the legs, while GSP often (but not always) has darker toe joints. The leg colour of this bird is greenish-grey rather than greyish-black indicating GSP
Bill shape
Both species have a black bill, that of GSP (of the two eastern subspecies) being the heavier and longer. On GSP, the mandibles gently taper to form a long point whereas, on LSP, they taper more steeply and produce a blunt tip. A major pitfall is race columbinus of GSP which has a short and sometimes 'weak' bill, although it is never as blunt ended as on LSP. The bill shape of LSP varies with the Arabian Gulf subspecies from the western 'atrifrons group' having a more slender and pointed bill than the other subspecies, and in this respect their bill approaches that of race columbinus of Greater. This bird has a gently tapering bill without the blunt ended appearance of many LSP therefore indicating GSP but note that the subspecies of LSP that occurs in the Arabian Gulf is the more slender billed ‘altifrons group’.
Bill length
A character widely described as useful in the field is bill length in relation to the eye. Birds with bill length equal to or shorter than the distance between the base of bill and the rear of the eye are considered to be LSP, while those with a bill longer than this distance are thought to be GSP. This rule is not always regarded as a good one with Hirschfeld et al 2000 stated this rule may not always be reliable, as columbinus GSP can have a very short bill (approaching average length of LSP). Having said that birds with long bills are always GSP, but shorter-billed individuals can be either LSP or columbinus GSP. This is a short-billed bird and the bill appears to be equal to the distance from bill base to rear of the eye indicating LSP but this is marginal.
Length of nail on bill
In relation to total bill length, the nail (the 'hump' at the tip) is longer on GSP and shorter on LSP. On GSP, the distance from beginning of nail to bill point is longer than the visible bill between forehead and nail; it is shorter on LSP. On this bird the bill tip nail appears to be about equal to the forehead to nail.

From the flight shot (sorry for the poor quality) a number of features can be seen.
Length of feet beyond tail (in flight)
In flight, the feet always project beyond the tail on GSP, while they are invisible or just barely visible on LSP, but this can be difficult to assess subjectively. The legs of this bird do extend beyond the tail but only slightly, not as much as in many GSP I have seen but I am not sure the legs are being held in a level plane? Is this a LSP pointer?
There are also differences in the wing-bars of the two species. On the primaries, the rear edge of GSP wing-bar bulges, while it is straight and of even width on LSP. On this bird the wing-bar looks fairly even although it may bulge slightly? Again closer to LSP
Sub-terminal tail-bar
GSP tail has a dark sub-terminal bar which contrasts well with the paler base and (in fresh plumage) a white terminal band; the tail of LSP looks more evenly sandy-brown, and the slightly darker (narrower and more ill-defined) sub-terminal bar is generally obscure or virtually impossible to see in the field (fig. 8). The tail may often be important when identifying single vagrants. GSP also has more white on the tail sides than LSP. This bird does not appear to have a dark sub-terminal bar on the tail and the tail sides, although difficult to see exactly do not appear to have white on the tail sides? Features suggesting LSP although it does have a white terminal band suggesting GSP.
Tarsus/bill-length ratio
Millington (1988) proposed a formula involving tarsus length (from knee to foot) divided by bill length (from tip to where feathering meets culmen) for identifying the two species from photographs, the ratios being 1.59:1 for GSP and 1.85-1.99:1 for LSP. Although this formula seems to be fairly accurate, the ratio is difficult to determine, as the angle from which the photograph is taken needs to be considered. From my measurements it appears to have a ratio of 1.57 making it a GSP.

Weighing things up more things point to Greater Sand Plover than Lesser Sand Plover but I am not certain on its identification. If anyone has any thoughts on this bird please feel free to comment as maybe I am missing something obvious?

Erik Hirschfeld, C. S. (Kees) Roselaar and Hadoram Shirihai (2000): Identification, taxonomy and distribution of Greater and Lesser Sand Plovers. British Birds 93:162-189
Millington, R. (1988). Greater Sand Plover in Cumbria. Birding World 1: 250-252.