Curlew Sandpiper or a Dunlin?

Continuing debate on the field separation of long-billed races of Dunlin and Curlew Sandpiper.

The original picture and first round of comments are here. A series of videograbs of a basic-plumaged Curlew Sandpiper taken at Cley, Norfolk, UK are here.

This photo looks as if the bird has opened its bill slightly AT THE TIP, changing the shape of the bill and making it appear more curved than it is. (snip) But the lower edge of the lower mandible is a clue. It looks to me to be too straight for a Curlew Sandpiper. Since the plumage looks fine for a Dunlin, I vote a long-billed Dunlin. Bill Principe, California, USA (27 Jun 2000 15:36). 
With correspondents from this list as divided as they are, I entirely agree that it is best left unidentified! I cannot add anything substantive to what has already been said, but I lean towards Dunlin. The few basic Curlew Sandpipers I have seen have never shown anything like that amount of streaking on the breast and by the time they are in pre-alternate moult and starting to show any colour on the underparts they also show new scapulars and coverts. Peter Wilkinson, England (29 Jun 2000 11:29). 
As much of the discussion of the mystery bird has dealt with structural details, I thought I would point out what I find to be the most interesting structural difference between the bird in Figure 1 and the bird on the left side of Figure 3. This is the apparent length of the middle toe. The bird in Figure 3 has a middle toe that is at least 20 percent longer, and probably 25 percent longer, than the bird in Figure 1. That is a fairly striking difference, but of course, I have no data on toe length in the two species. Noel Wamer, Florida, USA (04 Jul 2000 12:07). 
I've been somewhat puzzled by the response to the photo of the mystery sandpiper at Angus Wilson's site:

On first glance, the bird struck me as a Dunlin, odd only in its posture. This didn't seem extraordinary, as the bird was caught in motion, and seems to have its feathers tightly pressed to its body, possibly against the heat of a Texas beach in April. Otherwise, the expression and all plumage features didn't seem any different from the other thousands of basic Dunlin that spend a good part of the "colder" months along the Gulf Coast. It basically seemed like the face of a Dunlin frozen on a running body.

I suspected that the photographer was caught in a common situation, i.e., being sure at the time of what he was photographing, and not seeing anything remarkable about it until he looked at the photo afterwards and noticed that it looked reminiscent of another species. I don't know if this bird was ever put on a hotline, or if any other shots were taken of it. If this bird was submitted to us as a quiz bird, perhaps there are more diagnostic shots of it on the roll. If it was submitted just to solicit commentary, I'd have to guess that my suspicions are correct and that he thought as he was photographing it that it was a Dunlin.

I was surprised to find that others thought it might be a Curlew Sandpiper. Many of the responses are based on the bird's posture, which is misleading. Motion shots catch views that we rarely see.

Some suggest that the bird's legs are too long for Dunlin. Certainly, when Dunlin are preening or resting, they appear very squat, with short tibiae, but when feeding, especially in muddy fields or hot beaches, they can appear a bit stork-legged. If anything, this bird looks too short-legged for a Curlew, which seems contrary given that the bird is stretching out to move.

Others thought that the bird had too long of a neck for Dunlin. However, views can be misleading. The Curlew photos show a bird that seems have no neck at times, but nobody suggested that it might be a Dunlin. Nor did the plumpness of the Curlew have anyone calling it a Dunlin.

The fact that the wingtips seem to distance the tail attracted many comments. Maybe this bird is a Curlew, and maybe this is indisputable proof, but I have a feeling that it's just a side-effect of the bird being in motion. Its wings are pressed more tightly to the body. Anyone who's ever measured a wing knows that a flattened wing can be a great deal longer than a wing "at rest", and my guess is that this is what we're seeing here.

The supercilium seems perfect in shape for a Dunlin, especially a worn late-winter one. This droopy, wispy supercilium is a major contributor to the characteristic facial expression of winter Dunlin. It shows up more distinctly here because we're dealing with a great, well-focused photo.

My experience with basic Curlew comes strictly from photos, but I have yet to see a Curlew face in any photo that looks anything like this. I don't mean plumage features strictly--I mean the same type of individual personality that allows us to tell one human from another. Dwelling too intently on plumage in this regard could well mean missing the forest for the trees.

The level of focus for the mystery photo allows a great look at the feathers, as well. The shaft streaking appears fine for Dunlin, especially the streaking on the median coverts, a few of which are peeking out. Compare these median coverts with those on the wing-shot at:

{while you're at it, view the other photos. unfortunately the basic dunlin is a bit blurry, but a good comparison shot.}

I don't know what plumage produced these coverts with their broad dark shaft streak tapering down to the tip, and I don't know if Curlew shows a similar pattern there, but it would be worthwhile for someone with specimens at hand to check it out.

All in all, I know that some heavier hitters than me have weighed in on this topic, many with an opposing view. I can only say that I'm going to be very surprised if additional photos prove that this is a Curlew. I've never seen a basic Curlew, but I have seen tens of thousands of Dunlin in the same geographic region that this photo was taken [and where a Curlew is a rarity that would stand out], and I'd be floored to find out that Curlew could look exactly like Dunlin. Paul Conover, Louisiana/Florida, USA (6 Jul 2000 10:41) 

When viewing other birders' reactions to this photograph over the last few weeks, I must say I have had very much the same reaction as Paul expresses here. There was one person who said to have a photograph of a basic Curlew Sandpiper that looked exactly like the 'mystery' bird -- I would definitely be interested in seeing it, and I wonder why it has not been passed on to someone with a scanner yet, as it would be very relevant in this discussion. Peter Adriaens, Belgium (6 Jul 2000 17:12). 
Ooops. Just now, while browsing the excellent book America's 100 Most Wanted Birds (Mlodinow and O'brien, 1996), I happened to come across the picture on p. 472 (credited to B. Chudleigh/VIREO) labelled "Curlew Sandpiper." Hmmm. IMHO, my comparison of this photo with the Small bird shows that its legs appear proportionally shorter than Small's, the bill is less evenly curved (especially the lower mandible), the breast area is just as dusky, and the supercilium is as poorly defined if not more so.

Having only seen one alternate pl. female Curlew, I don't have any experience with basic pl. to draw on. However, these two pictures appear to me to be nearly identical. Are they both Dunlin?

Maybe one of the authors or the photographer would give permission to scan this photo so that a direct comparison could be made for those who do not have the book. Steve McConnell, Alabama, USA. (6 Jul 2000 22:30) 

Hopefully the continuing debate on separation of basic-plumage Dunlin from Curlew Sandpiper has made it clear that this can be a tricky ID. We have heard conflicting, but seemingly reasonable, opinions from birders in both North America and Europe and have reached no strong consensus.

Steve McConnell's comments on the photo in "America's 100 Most Wanted Birds" reinforces this fact. I suspect the photo by B. Chudleigh he refers to, was taken in New Zealand where Curlew Sandpiper occurs widely in small numbers and Dunlin is a significant rarity. In the photo, the left wing is drooped forward slightly revealing predominantly white uppertail coverts, however, we cannot see the very center of the white patch to confirm or refute the presence of a dark central bar.

The somewhat out of focus shorebird in the background looks like a Wrybill (Anarhynchus frontalis) to me, which of course is a New Zealand endemic (fodder for another thread?). Perhaps Steve Mlodinow or Mike O'Brien can confirm or refute the location of the picture?

I will be very happy to post any additional photos of basic-plumaged Dunlin or Curlew Sandpiper for our collective enjoyment and scrutiny. Thanks again to everyone who has chipped in so far. Angus Wilson, New York, USA (Fri, 7 Jul 2000 00:19)

Pictures of birds taken from a certain angle or showing a bird in an odd position can indeed cause confusion. In most cases when you actually see the bird identification is mostly straightforward. This certainly is true in this specific case. Nevertheless are discussion like the one we are having now interesting because it forces everyone who is taking part to stand still by things you normally taken for granted.

I don't think you can explain such a substantial wing projection by what Paul is suggesting. The picture shows an active feeding bird, so there is no reason to believe that we are dealing with a flattened wing (besides both wings are showing the same clearly projection. I would even suspect that in the posture the bird is standing in, a flattened wing would not enlarge but if there would be any changing at all rather diminish the wing projection). The information I have tells me that in Dunlin the wings are shorter or level the tail (Others wrote that the wings can project a little bit). So because of this substantial wingprojection beyond the tail this bird can't be a Dunlin. Maybe the discussion should concentrate a bit more on this, in my opinion, diagnostic(?) feature.

I enclosed a picture of juv. moulting to 1winter plumage Curlew Sandpiper that I took in Belgium. A fine example of an non-confusing picture. Verbanck Koen, Belgium (7 Jul 2000 15:40).

In case you might want to post this for the discussion, here is a jpeg of the Curlew Sandpiper from CT. Quality poor but the wing extension and the leg length is readily obvious. Mark S. Szantyr, Connecticut, USA (7 Jul 2000 11:15)
On examining the photos and on reading the very interesting discussion of Small's Dunlin/Curlew SP, I came to realize that I had based my initial opinion of "Dunlin" primarily on the size, shape and proportions of the head and bill. In particular, the latter appeared to be characteristic of Dunlin in that the upper mandible drooped at the tip instead of curving downward quite evenly throughout its length.

My personal experience with CUSA is too limited to state that the presence of a "typical Dunlin bill" argues convincingly against CUSA, absent other evidence. Subscribers with sufficient experience with CUSA might care to comment. But I can say that the absence of a "typical Curlew Sandpiper bill" cannot be used to reject the possibility of that species. Other evidence pro and con should continue to be sought.

. Curlew SP is rare but regular in Massachusetts. Since I see only one or two a year, I take the opportunity to study the birds carefully. I have found our spring and summer birds to vary considerably both in plumage and in bill shape. The upper mandible does not always curve downward evenly; indeed, on North Monomoy Island in the mid 1980s, I encountered an individual in nearly full alternate plumage which presented very nearly a straight bill. It wasn't a Dunlin bill, but it sure didn't look anything like the CUSA bill in the field guides. As a result, several participants in the field trip I was leading refused flatly and loudly to consider Curlew SP despite the evidence of all the other field marks one could want.

Recently, in fall, I had the opportunity to work on a bird in basic plumage at Plum Island, MA. It was feeding in a shallow pool close by, and was clearly visible in silhouette. Its overall structure suggested CUSA very strongly, but in the glare of the sun, very little could be seen of its plumage, and the upper mandible, though evenly curved, was only very slightly curved. I acknowledged that I might be engaging in wishful thinking. Then the bird flew, showing its rump and calling about five times, giving several people with me the convincing evidence they needed for a life bird. Jim Barton Massachusetts, USA (7 Jul 2000 12:57). 

Open up to page 160 in Photographic Guide to the Shorebirds of the World. Take a look at pictures A, C, D, E, G and K. It looks to me that in each of these cases the wingtips are longer than the tail. Is this from the angle of the pictures, or is this reality? Look at the photo in question - I do not think the wing projection in the photo in question is that much longer than some of these photos to warrant a definative "not a Dunlin". Love the discussion, very informative, but I can not comment too much further on the bird since I have zero experience with CUSA. Dave Lauten Bandon, Oregon, USA (7 Jul 2000 18:55). 
In my last post regarding the mystery sandpiper, I questioned whether the pattern evident on the median coverts [dark central wedge or stripe, basically] of this bird was of use in identifying it to species.

Unfortunately, it seems that we'll need to break this bird down on a feather-by-feather basis, which is at the same time one of the most informative and most tedious exercises of this forum.

I was hoping that someone with a collection at hand could provide descriptions of median coverts for the different plumages of both species, as it'll be a few weeks before I'll be able to access specimens. If anyone can, it would be appreciated.

Meanwhile, in browsing through the standard references and various magazines, I've pored over photos of both species trying to get the hang of the median covert patterns. From what I can see of exposed sections of this tract, it seems that Dunlin have median coverts with dark wedges in at least some basic plumages [it's hard to age some of the birds from pictures]. This marking is present possibly as early as, or only in, the first basic.

If the photo of the spread wing that I referred to at

is that of a first winter bird, [which I gather might be the case, if the presence of buff margins to the marginal coverts is indicative of retained juv feathers], then Dunlin may have these feathers as early as first basic, or maybe only in first basic. Of course, they have them earlier, in juvenile, and later, in first alternate, but I'm referring to basic plumages at the moment.

Many of the photos or portrayals of alternate plumage birds show this pattern also. Again, it's hard to age them, but if these birds are older than first summer, then the pattern is apparently present in definitive basic plumage as well, as the median coverts are holdovers from the previous basic plumage.

If this bird is a Dunlin, the presence of this pattern on the median coverts supports the ID.

Curlew Sandpiper in juvenile plumage of course has the strong subterminal markings which are the diagnostic point that birders searching for this species often look for. This pattern is present on the median coverts, which are retained throughout the first year. In all of the pictures of juvenile Curlew Sandpiper which I have at hand [I have to add that the majority of the pictures that I can find of this species are of juveniles], there's no dark stripe or wedge present. The dark in this area of the feather is limited to the shaft. Cramp and Simmons echoes this description.

Cramp and Simmons describe the median coverts of adult plumages of Curlew Sandpiper as having dark shafts only, as well. All of the photos that I have at hand of definitive plumaged Curlew Sandpipers show a dark shaft only there, giving the feathers a very even appearance.

If the mystery bird is a Curlew Sandpiper, it can't be a first basic bird. If it were, we might see some of the subterminal markings, which may or may not have worn off by April. What we wouldn't see is anywhere near the amount of dark which is, in fact, present in the center of these median coverts. It also seems that it can't be an adult Curlew Sandpiper, as the median covert pattern fits no adult plumage of that species.

I've felt from the outset that this bird is a Dunlin, and that confusion may stem from some observers' inexperience with American subspecies of Dunlin. The bill length, leg length, and the lack of flank streaking is all well within the range of American dunlins.

Also, the chances of any given variant of Dunlin in an area where there are possibly millions of dunlins to choose from is certainly greater than that of Curlew Sandpiper in an area where they are very rare, and certainly far greater than of finding what would have been such a hard to ID Curlew Sandpiper [i.e., a dead ringer for a Dunlin].

I have only ideas as to why the wings seem longer than the tail, but it's possible that this field mark is an indicator at best. Perhaps more attention needs to be paid to this feature among American dunlins. I would think that it's a pretty minor field mark to use to try to prove the identification of what would be a very good rarity, especially on the basis of one photograph. I also think that we need more and better pictures of these species on the internet! Paul Conover, Louisiana/Florida, USA (8 Jul 2000 10:45). 

I haven't had time to give the sandpiper much thought, but overall I strongly agree with the comments by Paul Conover. I have seen this problem from the other side, having strained like hell, but without success, to find a Dunlin in Australia. In doing so, I have seen many Curlews with Dunlin-like characteristics, such as clouded head patterns or unusually streaky breasts. When I have looked at skins, I have been impressed at how few characters there are which would be of any real use in Basic plumage. Yet Dunlins do look different when you see real ones. I doubt if the differences would necessarily show up reliably in a photo - that is, almost any pattern on one could be duplicated by a few of the other, providing of course you have no access to the tail pattern.

I strongly feel that a bird which magically pops out in a photo, but wasn't noticed in the field, is nearly always going to be the common species. Is that what happened in this case? Why did anyone suspect Curlew Sand in the first place?

I have little faith in features like wing projection, which can be extremely deceptive in photos, and not usually reliable in the field anyway, except in extremes (like Bairds or White-rumped). To me it looks unusually skinny for either Dunlin or Curlew Sand, which commonly happens in photos. I doubt if the bird could be reliably identified from just that photo, and therefore, unless there are compelling reasons to think it might be a Curlew Sand, then it's a Dunlin by default!

I certainly don't think any of the contour feathers visible in this photo are retained juvenal feathers. A bird in April which hasn't started its second molt (to 1st Alternate) could be expected to retain juvenal contour feathers only on the most proximal medians, but they won't be visible in this shot anyway. The feathers which are visible are quite fresh, Basic-like feathers, though it is also possible they could be 1st Alternate feathers that have come in grey. Chris Corben, California, USA (08 Jul 2000 18:58). 

The original picture and first round of comments are here. A series of videograbs of a basic-plumaged Curlew Sandpiper taken at Cley, Norfolk, UK are here.

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