Is this a Curlew Sandpiper or a Dunlin?

Recent update (14 Feb 2003): Martin Reid has pointed out that this bird was accepted as a Curlew Sandpiper by the Texas Bird Records Committee (TBRC). At least three independent observers submitted descriptions in which they clearly stated seeing an all-white rump. Martin goes on to say, "the wing structure is perfect and still hard to explain as a Dunlin. Assuming it is correctly aged as a 2CY bird, I can see the remains of a subterminal dark crescent on a central-rear wing covert - again hard to explain on any Dunlin. In fact, if you look at pics of fully-adult alternate CUSAs, some have residual subterminal crescents in the wing coverts; either there are first- alternate birds, or the adult basic wing coverts of CUSA can have a weaker version of the well-known juvenile pattern. This is an VERY instructive bird - but as a CUSA that could be passed-off as a Dunlin, not vice-versa!"

Brian E. Small sent me this delightful photograph of an interesting shorebird take on the upper Texas coast in April. The question is whether this a Curlew Sandpiper (Calidris ferruginea), a Dunlin (C. alpina) or something more complex. The long, gently curved black bill and long black-legs give the immediate impression of a Curlew Sandpiper, however, some who have examined the photo are puzzled by the amount of mottling on the breast and the pale supercilium seems less prononuced than on many Curlew Sandpipers. To see several more photos of this individual click here.

The fact that this bird shows no signs of alternate-plumage in April indicates that is is probably in its second calender year. It should be remembered that North American and Eastern Siberian races of Dunlin (pacifica, hudsonia and sakhalin) tend to be much longer-billed than those from Western Europe and central Asia (alpina, schinzii, arctica) and thus identification of basic-plumage Curlew Sandpiper presents more of a challenge in the Americas than in Europe or Africa (see summary of bill measurement below).

Figure 2. Richard Millington kindly provided several video-captures of a second-calender year Curlew Sandpiper taken at Cley in Norfolk, UK on 26 June 2000. Although this is a little later than the Texas bird, the images provide useful comparison with respect to structure. A series of additional photos of this bird can be viewed by clicking here. The images are a little dark having been taken in dull conditions through a telescope. Video grabs copyright of Richard Millington© 2000

Figure 3. For comparison, here are two Curlew Sandpipers photographed by Björn Johansson in Hong Kong on 10th April 1996. Except for a few freshly molted alternate-plumage feathers on the mantle, the left-hand bird is largely in basic plumage suggesting it is a first-summer. Photo copyright of Björn Johansson© 2000

Bill Length (exposed culmen) for males

Curlew Sandpiper (Europe), av. 36.2 mm (range = 33-39), ref. 1
Curlew Sandpiper (Indonesia), av. 36.0 mm (range = 32-39), ref. 1
Dunlin, sakhalin, av. 32.0 mm (range = 30-37), ref. 1
Dunlin, hudsonia, av. 36.1 mm (range = 34.5-39.3), ref. 2
Dunlin, pacifica, av. 36.8 mm (range = 32.5-40.5), ref. 2
Dunlin, arctica, av. 26.1 mm (range = 23-29), ref. 1
Dunlin, schinzii, av. 28.7 mm (range = 23-36), ref. 1
Dunlin, alpina, av. 31.1 mm (range = 27-35), ref. 1

Bill Length (exposed culmen) for females

Curlew Sandpiper (Europe), av. 40.1 mm (range = 38-42), ref. 1
Curlew Sandpiper (Indonesia), av. 40.8 mm (range = 39-43), ref. 1
Dunlin, sakhalin, av. 40.8 mm (range = 39-43), ref. 1
Dunlin, hudsonia, av. 39.0 mm (range = 37.5-40.9), ref. 2
Dunlin, pacifica, av. 39.1 mm (range = 35.8-41.4), ref. 2
Dunlin, arctica, av. 29.5 mm (range = 27-32), ref. 1
Dunlin, schinzii, av. 31.7 mm (range = 27-36), ref. 1
Dunlin, alpina, av. 32.9 mm (range = 29-36), ref. 1

Sources:
1-Cramp et al., (1983) Handbook of the Birds of Europe, the Middle East and North Africa. Vol 3. Oxford University Press.
2-Browning, M. R. (1977) Geographical Variation in Dunlins, Calidris alpina, of North America. Canadian Field-Natrualist 91(4): 391-393.

Brian may have additional shots of the individual, but will be unable to look them out before mid-July. Anyway, he and I would appreciate your thoughts on this interesting bird. Comments will be added to this web page. Let me know if you wish to remain anonymous

Comments

This one looks perfect for Dunlin. I haven't seen a basic Curlew [Sandpiper] before, but the bird in the picture has the exact expression of Dunlin. beyond the simple look in the eye, the shape of the bill and angle of the head are right on for Dunlin. Basically, it looks exactly like the thousands of other Dunlin on the gulf coast in winter. Anon, USA (18 Jun 2000 09:10)

I lean towards Dunlin. Do your sources for bill measurements also have tarsus length? That might be a more interesting measurement to use with this picture. Noel Wamer, Florida (18 Jun 2000 10:08)

I think that this bird is a Curlew sandpiper. Dunlin can be excluded for a few raisons but the most obvious one is that the wings project clearly beyond the tail, a feature that is not seen in Dunlin (In the latter wings level or fall even a bit shorter).
Anon, Belgium (18 Jun 2000 21:00)

[Follow up message from 25 Jun 2000 13:13] I think that this bird is a Curlew Sandpiper, probably 2nd calender year). In my opinion points to look at are:

The fact that the wings project very clearly beyond the tail, a feature that is not seen in any race of Dunlin (In the latter wings level or fall even a bit shorter) and keep in mind the effects of plumage wear on the outer primaries (the picture was taken in April). The position of the bird in the picture might not be the most easy one to appreciate features like lenght of the legs and neck, but the legs are still looking very long, again favouring Curlew Sandpiper.

The supercilium and even the breast streaking are both within the possibily of Curlew Sandpiper. Looking more detailed to the supercilium it looks indeed not so bold, but nevertheless very prominent in front of the eye (emphasised by a darkish line across the lores) and continuing very far behind the eye. When you look very well you might even get a hind of the split supercilium effect (onfortunately the position of the head makes it difficult to see this featurs). So again the supercilium fits better Curlew Sandpiper than Dunlin. Some people have made remarks about the colour of the upperparts. I don't see any problem with that (besides the colouration can be affected by wear, being slightly changed in the development of the photo and by scanning the picture).

The only thing in the first impression of the bird that doesn't look like [a] typical Curlew Sandpiper is the bill. It does indeed look a bit shorter than the "typical" Curlew, but I don't see a problem in this. Besides there is a good deal of variation possible with the females average lager than males, particularly in bill length where there is little overlap in measurements (bill males 32-39mm /females 38-44mm).

Based on the elements above, this bird is to me a Curlew Sandpiper, probably 2nd c-year (the bill length could suggest that this bird might be a male?)

Doesn't look like a Curlew Sandpiper to me, but does look OK for a Dunlin - even though it's somewhat odd-shaped compared to what we get in England, but OK for a US Dunlin I would have thought. Dick Newell, UK (19 Jun 2000 10:49)

Judging from its extensive, well-defined dark streaking on its breast (even centrally), throat, hindneck and cheecks, combined with its rather dull supercilium and fairly darkish-grey or dirty grey upperparts and crown, I think this bird should be a Dunlin. Peter Adriaens, Belgium (19 Jun 2000 11:25)

I really don't know what this bird is. Its close to being a Dunlin, but I can't imagine that's the whole story! It has the feel of a Curlew Sandpiper, but it isn't one of them. Nor is it a 'Cox's' It also has a whisper of White-rumped Sand about it, so 'Cooper's Sandpiper' comes to mind. Did anyone see its rump? So, my guess has to be hybrid, much as I like to avoid that route! If its a hybrid, my (limited) understanding is that it needn't necessarily look like a halfway bird, or indeed very much like either parent, which leaves me a bit high and dry.

We've had a couple of first-summer Curlew Sands this year, and they certainly didn't look like this bird...It has the facial 'character' of Dunlin, and the forepart streaking is surely not correct for Curlew Sandpiper. It does look rather too long-legged and long-winged for a Dunlin though.
Anon, UK (19 Jun 2000 11:27)

From the photograph I would suggest that the bird is a Dunlin based on the amount of streaking on the breast, face (cheeks) and neck. I would have expected the supercilium to be brighter if it were a Curlew Sand and I also think the upperparts are too dull grey for that species.
Eric Dempsey, Ireland (19 Jun 2000 11:54)

I've taken a look at the sandpiper on your web-page. My immediate reaction was Curlew Sandpiper. I enclose a shot (Figure 3.) of a similar plumaged bird that I took in Hong Kong on April 10th 1996. I'd say they're almost identical in plumage and proportions. The bird in my photo however has just starting to moult a few feathers on the back. For once a mystery bird in a good photo showing all characters!
Björn Johansson, Sweden (19 Jun 2000 19:06)

I think it is a Curlew Sandpiper. It has a long, narrow look about it and long looking legs. I guess it is lanky. The tertials and greater coverts seem to light for a basic Dunlin and the forehead is pale. In all of my photos of basic Dunlin they have a dark forehead. The fineness of the bill tip also leans me towards Curlew Sandpiper. The length of the primaries seems to favor Curlew Sandpiper also. A shot of the rump in flight would be nice. Or even sitting. One vote for CUSA. Bob Abrams, USA? (19 Jun 2000 19:12)

On the shorebird, it looked good for Dunlin to me. I have seen pretty good numbers of CuSa from living in the Middle East, spring trips in Asia, and winter trips down under. I think the plumage and bill structure are better for Dunlin. As you say, looking at pics can be misleading, but these pics do not say Curlew to me. Matt Heindel, USA (20 Jun 2000 21:55)

Since it will be shorebird season in a few weeks, I thought I'd exercise my brain on this one. I see nothing that is wrong for Dunlin on this bird, and nothing that would point to Curlew Sand. The wispy supercillium that more or less starts at the eye is typical of Dunlin. The mushy streaking across the breast is also a classic Dunlin feature. Though the bill is slightly open and thus somewhat more difficult to get a feel for shape, you can clearly see that the upper mandible is basically straight for about 2/3 or 3/4 of the way, and then droops towards the tip; Curlew Sand's bill should droop more or less evenly through its entire length. Also, the bill looks short for Curlew. The shape of the bird feels more like Dunlin; I always get the impression that Curlew has a fairly small head and long neck for its body size, evident in the second photo. This bird is neatly proportioned. The primaries do indeed seem to project beyond the tail, favoring Curlew, but I would guess its only half a centimeter at best, and not nearly as pronounced as on the Curlew on the left of the second picture. Dunlin's primaries tend to be even with the tail, though I suspect there is variation between individuals (and perhaps races?). Finally, the legs do not look nearly long enough for Curlew; the long-legged look is clearly shown in the second picture of the two Curlews. Evan Obercian, USA (21 Jun 2000 03:57)

The most instructive aspect of this photo for me has been in regards to the amount of streaking on the head and breast. I am not really sure if the bird is a Curlew Sandpiper (CUSA) or a Dunlin, but I am guessing Curlew Sandpiper based on structure and streaking. The bird appears to be an adult in basic plumage but as you point out it could be a 2nd winter bird. However the apparent lack of molt into alternate plumage could also be due to the health of the bird as much as to age, so I cannot say with any great confidence what the age is other than it is more than a year old.

I have looked through both our specimen collection and photo collection and am basing the following on 10 specimens of basic plumage CUSA and 13 photos (7Juv. and 6Ad.) and 172 specimens of basic Dunlin and 43 photos (both #s include some first winters) The majority of the Specimens (154) are of C. alpina pacifica collected in the last week of Dec. in Humboldt Co. CA. We also have 6 alpina, and 1 arctica. I am not sure about the subspecific breakdown of the photos but they come from both the West and East coast as well as Britain, Oman, and Africa.

From this sample it seems that both C. ferruginea and C. alpina in basic plumage are streaked on the breast, sides of neck, and face. The difference is in the pattern and density of the streaking. The description "murky streaking" best describes Dunlin. The difference, on the scale of the individual feather, is on Dunlin the dark streak along the feather shaft is shorter and broader, and set on a generally gray feather which pales towards the edge. On CUSA the streak is finer and the rest of the feather is paler - almost white. So on the scale of the whole bird the streaking on Dunlin is more blended with the rest of the feathers and in some individuals approaches spotting against a uniform grey back color. Dunlin also tend to be plainer on the face as well. On CUSA the streaking is finer, cleaner and more distinct. especially on the face and neck.

Of the 3 subspecies of Dunlin in our specimen collection the subspecies arctica, and alpina more closely resembled CUSA in terms of face, neck, and breast streaking. Of the CUSA specimens I could not see any difference in the streaking b/twn birds collected in Africa and those collected in Australia. The collection dates for the CUSA specimens are Sept, Oct, Nov, Dec, and Feb. However all of our CUSA specimens are very old (late 1800, early 1900) and some of the very fine details may be obscured.

Another noticeable difference b/twn CUSA and the pacifia series of Dunlin in the collection is the amount of streaking along the flanks; none in CUSA, variable in Dunlin but rarely absent altogether. On some specimens the streaking is obvious and can extend down to the base of the vent. On others the streaking was much reduced along the flanks and on maybe 5 individuals appeared absent entirely. However on these specimens the apparent lack of flank streaks may have been due to displacement of feathers during the preparation of the specimen. On some apparent white flanked birds a little preening revealed dark streaks. To what extent the streaking is related to age was not immeadiately obvious in the pacifica series but there is good bursa data on all the labels so it would be possible to investigate that question a little further. Matt Sharp, USA (26 Jun 2000 10:43)

I'm an Italian birder with a very good experience in both species. I've seen your mystery photo and I'm strongly convinced it [is] a Curlew Sandpiper. These are the reasons why I pointed on C.S.:

1) the mystery-wader is structurally wrong for a Dunlin, with too long legs (especially tibias) and sligthly but clearly longer wings (the primary projection is undoubtely longer than tail)!!!
2) the color of upperparts fits well with C.S. and not with Dunlin: medium light and uniform grey on mantle, very slightly darker but still uniform scapulars and coverts with limited amount of darker grey on center of each feather (more like a thin darkish shaft streak than a cloudy shade or faint spot as in most Dunlin)!!!
3) presence of a long and sinuous neck, head shape and proportions (gently rounded and small compared to body) and colouration, apart from strangely faint supercilium (paleness of auricular coverts, paler than lores, and paleness of neck, clearly paler than crown and back, creating an obvious capped appearance) are very good clues for a typical C.S.!!! Igor Festari, Italy (26 Jun 2000 20:32)

I believe that the mystery bird is a Curlew Sandpiper. I base this conclusion on a comparison of the mystery bird with my photo collection, which includes Curlew Sandpiper in basic/non-breeding plumage in Australia in April (where Dunlins are virtually unknown), and of Dunlins, particularly hudsonica (the likely confusion race), also in basic/non-breeding plumage, in Texas in April. I'm sorry that I am unable to post scans of these, particularly since one of the Aussie Curlew Sands is virtually identical to the mystery bird.

The initial impressions from the photograph are of a bird "structurally correct" for Curlew Sandpiper, including elongate, elegant shape, smallish head and longish neck, and longish legs. The slightly open bill makes it appear less slender, inviting confusion with Dunlin. The supercilium is not particularly contrasty in the photograph, but is still more obvious than in almost all Dunlins. The extent of throat/upper breast streaking is quite similar in both species, and is not, in my opinion, a very helpful character.

However, the two diagnostic features are: one, the wing projection beyond the tail, as expected for Curlew Sandpiper, whereas in hudsonica Dunlin (and also pacfica) the folded wing-tips typically fall level with the tail. The European races of Dunlin tend to be slightly longer winged, but only exceptionally are they as long as Curlew Sandpiper. Two, hudsonica Dunlin in the corresponding plumage shows fine but clear dark flank streaking, which is lacking in the mystery bird (but also in most other races of Dunlin). These two features, together with the its general structure, in my view rule out any of the likely Dunlin races, and confirm Curlew Sandpiper. R. J. Chandler, UK (27 Jun 2000 13:53)

For additional photos of the Texas sandpiper click here

For a lot more commentary on this bird, as well as additional reference photos of Curlew Sandpiper click here

For a series of pictures of a presumed 2nd calender year Curlew Sandpiper taken by Richard Millington at Cley, England click here

Acknowledgements

Thanks to Brian Small for allowing me to post this picture, Björn Johansson and Richard Millington for generously donating reference photos and to Ron Pittaway for access to Ralph Browning's article. I'd also like to acknowledge all those correspondents who provided useful feedback.