An atypical American Golden-Plover (Pluvialis dominica)
on South Beach, Chatham, Massachusetts

1 & 3 May 2005

Blair Nikula

Postscript: This bird was still present on 14 May, by which time it had progressed farther into alternate plumage, showing more extensive black on the flanks and under the tail, as well as a white bulge on the sides of the neck, features that would seem to indicate the bird was in fact an American, not a Pacific as first suspected. The discussion below was written prior to this revelation.

These seven images were digiscoped with a Leica Televid 77 scope and Nikon Coolpix 995 camera, late in the day on 1 May under overcast conditions. These images have been cropped somewhat, sharpened, and the contrast adjusted, but are otherwise unaltered.
AMPLaltSBch050105a1.jpg (69488 bytes)
  AMPLaltSBch050105a2.jpg (65176 bytes) AMPLaltSBch050105a3.jpg (73602 bytes) AMPLaltSBch050105a4.jpg (71620 bytes) AMPLaltSBch050105a5.jpg (50305 bytes) AMPLaltSBch050105a6.jpg (57965 bytes) AMPLaltSBch050105a7.jpg (55267 bytes)

Two additional photos taken on 3 May 2005 with a Canon 20D and Canon 100-400 IS Lens (these are heavily cropped), late in the day in full sunlight.
AMPLaltSBch050305a1.jpg (73826 bytes)
AMPLaltSBch050305a2.jpg (75394 bytes)

All images taken by Blair Nikula.

Discussion:

I observed (and photographed) this bird for about half an hour on 1 May at distances ranging down to about 100 feet under overcast conditions, and for about 5 minutes on 3 May at distances ranging down to about 150 feet in bright sunshine. I saw the bird in flight several times, though never very well (i.e., flying directly away, and/or in poor light, or low to the ground making it difficult to see the underwings well). However, I saw it well enough to see that the underwings were dusky, not white, and that the upperwings and tail were uniformly golden-brown, with no wing stripe, thus eliminating the possibility of European Golden-Plover (P. apricaria). Despite seeing the bird fly several times, I never heard any sound from it, nor was I able to determine whether there was any toe extension beyond the tail.

Although I have extensive experience with American Golden-Plover (P. dominca), my experience with Pacific is very limited. I saw the April 2002 bird on Plum Island (Massachusetts), and saw both forms on the breeding grounds in Nome, Alaska in 1987 (before the two forms were split).

Following my discussion below, I have appended comments from several people who have viewed the photos and offered their impressions.

Identification of the South Beach plover:

Separation of Pacific and American golden-plovers in any plumage other than full alternate is difficult at best and in some individuals (such as this one?) probably is not possible. Virtually all of the characters distinguishing the two species are variable and most overlap to some degree. However, the salient features of this individual seem either to be correct for Pacific or to lie within the range of overlap with American. I can see no features that would eliminate Pacific, while some features appear to be outside the range of American (with the caveat that some of the important mensural characteristics, such as bill length, tarsus length, and projection of folded primaries beyond tail tip, cannot be determined with certainty in the field or from the photos). Some of the features of the South Beach plover discussed below (especially primary projection) were difficult to determine in the field and are based upon examination of the photos.

One of the most recent papers on the identification of these sibling species is that of Johnson & Johnson (hereafter J&J), published in the Wader Study Bulletin in April 2004 (“Morphometric features of Pacific and American Golden-Plovers with comments on field identification”). This paper is available online at: http://web.uct.ac.za/depts/stats/adu/wsg/pdf/wsgb-apr2004-golden-plovers.pdf. The authors have been studying Pacific Golden-Plovers for over two decades, capturing over 600 individuals since 1979. The following discussion draws heavily from the Johnson’s work as well as a variety of other sources, including Byrkjedal & Thompson (Tundra Plovers, 1998), the web site of Hugh McGuinness discussing the bird on Long Island in September 2003:
http://mail.ross.org/~hmcguinness/East%20End%20Birds/EEBphotos/PGP.htm
and the web site of Angus Wilson discussing the bird in New Jersey in September 2001: http://www.oceanwanderers.com/NJPGP.html

Features that have been used to distinguish these two similar species, in addition to the distinctive alternate plumages and calls, include: bill size (particularly length), tarsus length, primary length (relative to tertials and relative to tail tip), relative lengths of the ninth and tenth primaries, and tertial length (relative to tail length). However, in their paper, J&J conclude that many of these features, such as tarsus length, overlap to such a degree that they are not reliable for identification.

Plumage: The South Beach plover is in at least a partial, and perhaps nearly complete alternate plumage. The dorsal plumage is a mix of gold-spangled alternate feathers and plain brownish basic feathers. Likewise, the underparts are a mix of black and white feathering. It appears to me that the bird may still be molting, as some of the wing coverts look new and not fully grown. J&J state that the pre-alternate molt in both species is generally completed by mid April. If the South Beach bird is near the end of its molt, then it is almost certainly either a female or a first-year bird. The absence of any black in the undertail coverts in early May seems highly unlikely in an adult male American, and is probably atypical for a female as well. The black on the flanks, while more typical of American, does apparently (based upon a number of photos published in print and online) occur in Pacific as well and thus could be consistent with either species, depending upon the sex or molt state. The width of the white neck stripe, which still has some brownish basic-type feathers mixed in, is difficult to determine in this case, though to my eye it looks somewhat more consistent with Pacific (i.e., the white being narrower than on American). It has been suggested that in alternate plumage, the gold spots on the dorsal feathers of Pacifics are larger and the white markings smaller; the alternate feathers on the dorsum of the South Beach bird show very little white. In sum, though not conclusive due to uncertainty about the sex and molt state of the bird, the plumage seems a better match for Pacific, most likely an adult female.

Bill Structure: Pacific Golden-Plover has a longer bill, on average, than American, though there is extensive overlap (and females average slightly shorter than males). J&J found that the length of the bill was greater than the distance from the base of the bill to the back of the eye on 91% of Pacifics and 62% of Americans. The average difference in these two measurements was 2.6mm on Pacifics (n=40) and 1.6mm on Americans (n=18). On the South Beach plover, the bill, if folded back from the base, extends well behind the eye (see enlargement below). Using published bill lengths in J&J, I estimate the difference between bill length and base of bill to back of eye distance of the South Beach bird to be at least 3.5mm and perhaps as much as 5.5mm, strongly suggestive of Pacific.

Tarsus Length: One feature of the South Beach plover that initially troubled me was that it seemed to lack (both in the field and in most of the photos taken on 1 May) the “long-legged” look that has frequently been cited as an important feature of Pacific versus American. Though often touted as a key feature in separating these two forms, Pacific purportedly being consistently longer in this measurement, J&J found that, although Pacifics do average somewhat longer in tarsi (again, females average slightly shorter than males), there is extensive overlap. In fact, the shortest Pacific in their sample was shorter than the shortest American. Samples from other locations in the Arctic found similar variation and one sample of 38 Americans from Churchill, Manitoba had an average tarsus length that exceeded the average of any of the Pacific samples published! Thus, they conclude that tarsus length cannot be used to separate these two species.

Tibia Length: J&J found a significant difference in the length of the unfeathered tibia, Pacific averaging 4mm longer than American. However, there again is considerable overlap, and this feature is very difficult to determine in the field or from photos, the degree to which the tibia are visible varying with the posture of the bird. Although J&J do not correlate tibia length with tarsus length, a positive correlation seems likely, which would enhance the variability in total leg length. Although the South Beach bird does not look long-legged in most of the photos taken on 1 May, in the last photo of that series (right-most above) and the two photos from 3 May, it looks distinctly "leggier."

Primary Length: One of the key features separating the two species is the relative length of the primaries, American’s being longer; there is almost no overlap in this character, though it is a difficult feature to determine in the field. On Pacific, two or three primaries project beyond the tertials, while in American four or five primaries project beyond the tertials (J&J confirm this oft-cited character). In the field, I could be sure of only two primaries projecting, though I suspected there might be a third that I wasn’t seeing well. The photos (see blow-up below) clearly show three primaries projecting, with P9 being almost equal in length to P10 and thus being difficult to see. The fact that P9 is almost equal to P10 is another feature typical of Pacific, though J&J conclude that variation in P9 length results in considerable overlap, making this feature unreliable. They found an average difference of 2.7mm in Pacific and 5.0mm in American; the difference in the South Beach bird is almost certainly <2mm, and thus much more Pacific-like.
   Another way of examining primary length is to compare the extension of the primaries beyond the tail, again American showing more projection. J&J found that on Americans, the primary extension beyond the tail ranged from 12-22mm (n=34), while in Pacific the range was from 0-9mm (n=50). However, these figures are estimates made from observations in the field with binoculars or telescopes, not actual measurements in the hand. Obviously, wear in the primaries would also affect these measurements.
   
Getting really anal, I measured the primary projection in the first photo of the South Beach bird as a percentage of the bill length, then calculated the possible range of the projection length based upon the range of bill lengths (both species) published in J&J, I come up with a range of about 11-16mm for the primary projection of the South Beach bird. In other words, the South Beach bird has a bill length/primary projection ratio of about 5/3, so if its bill is extremely short (20mm), then the primary extension is about 11mm, but if the bill is at the long extreme (26.6mm), then the primary extension is about 16mm, which places it beyond the range of Pacific and at the short end of the range of primary extension estimated for American by J&J. However, I also applied this technique to the photo of a female Pacific in Byrkjedal & Thompson (pg. 70) and came up with a primary extension range of 14-19mm, again well beyond the supposed range of Pacific. Likewise, the Pacific illustrated at this web page shows a similar projection beyond the tail (note also the very short tertials on this bird). So, either my reasoning is flawed, J&J's estimates are flawed, the photos in Byrkjedal & Thompson and on the Asian web site are misidentified, or primary projection beyond the tail is much more variable than J&J suggest.  J&J also suggest that the ratio of primary projection to bill length is <½ in Pacific and = or >½ in American; the South Beach bird matches American on that basis.

Tertial Length: Pacific has, on average, longer tertials than American, the tertials typically extending to at least ⅔ of the tail length in the former and generally less than ⅔ in the latter. This is another variable and unreliable character, made all the more so because it is difficult on a sitting bird to determine exactly where the tail begins (anterioraly). The J&J paper includes two photos of alternate-plumaged male Pacifics on which the tertials extend only about halfway out the tail. I judge the tertials on the South Beach plover to extend about ⅔ the length of the tail.

In summary, most of the salient features on the South Beach plover are consistent with Pacific; one feature, the primary extension beyond the tail, is suggestive of American, though it is unclear if that character is reliable. Although I have very little experience with Pacific Golden-Plover, from the time I first saw this bird in the distance there was something unsettling to me about it’s overall appearance. It just did not fit my conception of an American Golden-Plover. I was also struck by how difficult it was at times to pick out of the numerous Black-bellied Plovers with which it associated, unlike typical American Golden-Plovers in my experience. However, it was only after examination of the photos and a review of the literature that I became convinced that it was Pacific Golden-Plover. Nonetheless, there is enough ambiguity in many of the characters that I remain open to contrary opinions.

Several people generously (and courageously!) offered comments, which I have reproduced (with minor editing) below. Opinions about the identity of the South Beach bird are quite divergent, to say the least!

Steve Mlodinow (5/6/05): The primary extension would be exceptionally long for PGP. Also, the black on the lower flanks argues against PGP (I just saw a photo-essay in a back-issue of Western Birds -- sorry don't have volume number available -- that had a similar bird ID'd as a AGP). Given the presence of black in a location atypical for PGP and the quite lengthy primary extension, I think this is an AGP, perhaps a first year bird.

Steve Mlodinow (5/9/06): My comments regarding the bird's underpart plumage were not meant to imply that they were diagnostic for AGP but merely that they were not inconsistent with this conclusion. Certainly, a PGP could appear this way beneath.
    Molting golden-plovers, or birds in incomplete alternate plumage, are often unidentifiable.
    And though I've heard of, and seen apparent photos of, long-winged PGPs, this bird is very long-winged, and I'd be surprised if it were within the range of PGP.
    RE: Jizz. Our comments about apparent shape. I'm always rather wary of such from photos; at least there was more than one and they are of good quality. To me, the jizz of this bird is not that of a PGP. Its legs don't look quite long enough. The bill seems intermediate. But these are weak marks.
    Impossible that this is a PGP? Perhaps not. But combine the very long wings with the ambivalent underpart markings and the bird's location, and the liklihood of the bird being a PGP becomes quite small.

Curtis Marantz (5/6/05): When I looked at your photos and read your commentary, I was struck by how good the bird looks for Pacific structurally, but I'm not sure the plumage characters are as good. To me, the white on the sides of the breast seems broad and rounded, more like American, and the blotches of black on the belly and flanks seem to come right up to the wings, both of which I'd say favor American. The bill size looks okay for Pacific and the wings do look short to me. I must admit, though that I really hate these transitional-plumaged golden-plovers because they really are a pain and one can never really tell how it will look when the full alternate plumage comes in.
  
I remember back to the PI [Massachusetts] bird that many argued that the early stage of molt (basically in almost full, alternate plumage in April) was good for Pacific, which molts earlier (at least in part). By contrast, the later stage of molt in your bird seems more like an American, which often remain in mostly basic plumage well into migration. That said, a first-alternate Pacific is probably going to look about like what you had through the summer, as you noted.
    Additional things that complicate matters are the fact that worn tertials may make the primary projection look longer, but missing or incompletely molted primaries will result in projection looking shorter. Given that date, I doubt that the bird is molting the longest primaries (especially both of them at once). Taken together, I'd probably go with the structure of the bird supporting Pacific over the plumage which may be equivocal or tending slightly towards American. Given the extreme rarity of Pacific on the east coast, I'm not sure I'd accept a record as such, but my inclination is that the bird is a Pacific, and it is probably at least as well documented as the bird on PI (which I myself looked at for three hours, though never as a distance as close as you observed your bird).
    P.S. The bill measurements that you mentioned seem awfully large, suggesting to me that they are taken from the base of the bill at the skull. If so, much of the measured part may be hidden by feathers, which in part may explain the problem that you had with your comparisons.

Rick Heil (5/6/05): I'm not at this time convinced that this is fulva. Structurally it appears more dominica to me. Legs don't look excessively long to me in most of the photos. The bill does look long like fulva but probably doesn't eliminate dominica, I think. Some black feathering coming in on the flanks is a dominica feature as is the apparent extensive white "blob" out onto the breast sides. I would expect SY+ fulva to be in more advanced state of molt by now, nearly in alternate. Dominica in May, I think, are often in a similar state of transitional plumage to this bird. State of molt more typical for dominica for this date. Despite the fact that I can only count 3 primaries beyond the tertials, and P10 and P9 are very close together (both features are excellent fulva characters) the tertials still seem short. But maybe some tertials are broken or worn? Fresh tertials in fulva often extend nearly to, or to, tail tip. Note that all three primary are at (apparently P8) or extend well beyond the tail (P9 and 10) on this bird, not typical for fulva. However, if this is dominica, why aren't there the typical 4-5 primaries showing beyond the tertials? Was it missing any primaries (like P7) when seen in flight? So in sum this is a confusing bird and therefore I am not yet ready to say it is definitely fulva or dominica, but currently lean a bit towards the latter.
    I haven't yet read the J&J paper so may revise some comments (and opinion) when I have.

Richard Crossley (5/6/05): I don't know if your bird is still present but I would encourage people strongly to go and see it. I prefer to see birds in the field than photos, particularly just one.
    Bills and legs are of no importance in my opinion (contrary to literature). Tertial length, wing point, primary projections and spacing show variation, sometimes extreme but this pattern is very much Pacific.
    There is a lot of white towards the upper breast but the shape is not classic American and I would prefer not to have black protruding through the white flank line for Pacific but all in all I think fine for Pacific. Extensive white undertail strongly Pacific.
    Supercilium on Pacific tends to curve down quicker with a broader dark nape line - this can be striking at long range in direct comparison - appears to show this.
    The most striking feature in the field is structure. Although they can look remarkably tall and slim, typically they look fat and big headed (and big eyed) - at times remarkably Black-bellied like - probably enhanced by superc. shape.
    Adult assumedly female, probably close to the end of its molt. I hope its still there and you got to see it.

Julian Hough (5/8/05): Blair's bird is interesting and I found it a tough call, since none of the features jump out as good for either species. Structurally it looks good for fulva as do the unmarked vent, and if anything, the upper flank area seems to have one or two "chevron-shaped" dark bars sitting quietly among all the solid black area. If this were a bird further into breeding dress, I feel that these bars would be in an area on the upper flanks consistent with fulva.
    I initially thought it was a funny dominica, but on closer looks and Blair's description it could well be fulva. The flank bar and white undertail coverts sway me toward fulva. Fulva can show black undertail coverts like dominica, so any bird showing colour here could be either. Since the Monomoy bird is pale-vented, I'm unsure how diagnostic this is for fulva. Can dominica's show pale-vents like this? The width of the dark-neckstripe (broader on fulva) is hard to judge and the wingpoint and bill is sort of inbetween and doesn't help me much.
    Having seen both species, both as vagrants (in the Uk) and on breeding and wintering grounds, I found that many of the photos I have seen of vagrant fulvas seemed to be relatively straightforward. I always thought I'd be confident in calling one here in CT, but I'm glad I didn't find this one....!
    One to talk about rather than panhandle an opinion.
    If I saw this in CT, I'd be hesitant to put a name to it straight away, but I'd be calling people to look at it.
    Interesting that it hasn't generated more discussion.

Alvaro Jaramillo (5/9/05): I would go for American on your bird although a real short-winged one. The short wings are the main issue of confusion for me, the rest of the structure (part short legs) is typically American. The white undertail covs are not a big issue as female Americans are often white here. However the black feathers on flanks are fine for American, and argue against Pacific. I have been to Hawaii 3 times in the last couple of yrs and have been able to study hundreds and photo a lot of basic Pacifics. The overall golden colour of upperparts is pervasive, particularly on scaps but even on coverts. All of the basic type feathers on your bird are dull, some with nice whitish fringes, this is typical of basic American but not Pacific. If a Pacific, many of the old scaps would show golden fringes. The fresh primaries also point to American as someone else suggested. Sorry I can't go into more detail and that this note is so disjointed, when I have my other hand back I can e-mail you some photos showing typical basic Pac. upperpart colouration.
    The question is- why is this bird so short winged? Is it still moulting for some odd reason?