Sandwich or Cabot’s?

by Greg Neise on September 25, 2011

A year ago, Chicago birders were literally in a frenzy over Illinois’ second record of a Sandwich Tern. The bird remained in the area, although it was difficult to find, for nearly a month. During that time, several people including yours truly, were able to get good photos of the bird.

The first thing (besides the fact that it was a flippin’ Sandwich Tern!) that the many observers noted was how long and thin the bird’s bill was. On the phone with Dr. Fraker the night after I photographed the bird, he said, “Geez! what about the bill on that thing?? It’s like a hypodermic needle!”

Indeed it was. In fact, it was longer and thinner than any Sandwich Tern I’d ever seen…or ever seen a photo of. At least, until I started looking at photos of the nominate, Old World subspecies, Thalasseus sandvicensis sandvicensis. The subspecies that we see in the United States is Thalasseus sandvicensis acuflavida, also known as “Cabot’s” Tern (there is a third subspecies, “Cayenne” Tern, Thalasseus sandvicensis eurygnatha).

The British Ornithologists’ Union Taxonomy subcommittee recently released their annual recommendations report for 2011, and in it they recommend splitting sandvicensis and acuflavida:

Molecular phylogenetic analyses based on concatenated mitochondrial and nuclear DNA sequences indicate that Sterna sandvicensis acuflavida and Sterna sandvicensis eurygnatha are more closely allied to Elegant Tern Sterna elegans than to the European Sterna sandvicensis sandvicensis (Efe et al. 2009). An analysis based on CO1 sequences, with more individuals of each taxon, also recovered this pattern and identified several diagnostic substitutions for each clade. Garner et al. (2007) reported several differences between S. s. sandvicensis and S. s. acuflavida in juvenile plumage, first winter plumage, first summer plumage, adult winter plumage and adult summer plumage, and some of these may be diagnostic. Cabot’s Tern is best recognized as a separate species:
• Cabot’s Tern Sterna acuflavida (polytypic, with subspecies acuflavida and eurygnatha)
• Sandwich Tern Sterna sandvicensis (monotypic)

If the AOU and the ABA should follow suit, and split the European and American subspecies, the Chicago bird becomes even more interesting.

I believe that there is a strong body of evidence that the Chicago 2010 Sandwich Tern was indeed the European T. s. sandvicensis. 

Here is one of the first photos of the bird, taken by Rick Remington on September 11, 2010:

The bird is an adult, transitioning from summer to winter plumage, and in addition to the molt taking place on the bird’s head, we can see that the primaries are worn. A few things immediately jump out: the bird’s bill is quite long and thin, with almost no gonydeal angle, and the culmen is curved, giving it a slightly “droopy” tip. But also notice how much of the black cap this bird has retained.

Here’s a comparison of the Chicago bird with acuflavida:

Chicago tern at left (09/11/2010), T. s. acuflavida, photographed in late August in TX.

Note the thinner bill, with a distinctly curved culmen, of the Chicago bird. The bird on the right has a “spikey-looking” bill typical of acuflavida. Also note the dramatically different amounts of white on the forehead and crown.

Starting at the top, let’s look at a comparison of heads:

First, the bills:
the birds on the left (with the exception of the one on the bottom) are all acuflavida, photographed August-October. On the right, all are sandvicensis, photographed between June-February, except for the top and bottom birds.

As we can see, the bill of the Chicago tern is longer and thinner than any of the acuflavida. It is even longer and thinner than most of the sandvicensis sampled! The combination of the curved culmen (upper ridge of the bill), the almost non-existent gonydeal angle give the bill a curved and “drooped at the tip” appearance that is quite different from any of the acuflavida.

The feathering on the head is notably different also. In researching this I pored over literally hundreds of photos of acuflavida taken in summer and fall. Not one had anywhere close to the amount of black or gray feathers on the forehead and crown seen on the Chicago bird. As you can see above, acuflavida in late summer/fall has a bright white forehead and crown. Not only did I work over hundreds of photos, I also spent time with specimens at the Field Museum in Chicago. Same story: all acuflavida specimens from late summer/fall had white foreheads and crowns. No exception.

As you can see above, sandvicensis retains more grey feathering from the forehead to the nape, giving the top of the head a grizzled or salt-and-pepper effect. The images of the Chicago bird at the top and bottom of the right column, fit in perfectly with the sandvicensis, but are noticeably different than the acuflavida to the left.

Another other thing to note is the eye ring. In acuflavida there is usually a noticeable eye-ring that is wider at the back, that is greatly reduced or lacking in sandvicensis. The Chicago bird shows no white in front of, behind or above the eye.

Lastly, concerning the head, in acuflavida the nape feathers are longer and “greasy black” with little or none of the broad pale edging of sandvicensis (see the Dec. and Feb. birds in the right column above). The Chicago bird has not progressed enough to show this (or a lack of it), however, in the last photos of the bird, taken on Sept. 30, the molt on the head has progressed significantly and there does appear to be one or two nape feathers with the broad pale edging characteristic of sandvicensis:

The most exciting aspect of researching and documenting this bird was the outer primaries. When I saw the bird on September 15, there were about a dozen people soaking it in. At one point I remarked about shape of the tips of the outer primaries: they had “hooks”, quite unlike anything I had noticed in any bird before. This was before there was any discussion of the bird’s possible origin… and I filed it away.

What I’ve learned since then is that the hooks are very important. Garner et al compare (Identification of American Sandwich Tern, Dutch Birding Vol:29:5) the tips of the outer primaries of (Adult winter):

A major difference acuflavida and sandvicensis is the width of the white fringe to the inner web of the outer primaries. On acuflavida, the white fringe is very narrow (1-1.5 mm) whereas, on sandvicensis, the fringe is wider (2-4 mm), each primary also having 3-5 mm of white at the feather-tip (lacking in acuflavida) (Olsen & Lasson 1995).

To illustrate this, let’s look at the wingtips of each with fresh primaries:

T. s. sandvicensis, December 19, Portugal

T. s. acuflavida, December 31, Florida.

When the feathers are very fresh, as in the two above, the difference is quite easy to see. Note the  “3-5 mm of white at the feather-tip (lacking inacuflavida)”, that gives the darker gray part of the primary tips a hooked shape. In my research I found that this shape is particularly pronounced on P 6-8. As mentioned, in fresh feathers with nice solid fringes, the difference is rather easy to see. But what happens as the feathers wear?

T. s. sandvicensis, April 14, Portugal.

T. s. acuflavida, April 14, Texas.

The broad white fringe of the sandvicensis above is 50% worn in the example above, yet the hook shape of the primary tips is easy to see, especially on P6-8. The moderately worn primaries of acuflavida show only a very slight white edge to the inner primaries.

T. s. sandvicensis, August 29, Scotland.

As the feathers wear even further, the white fringe all but disappears …but they retain a visible white edge that defines the hook-shaped tips of the individual primaries (again, particularly noticeable in P6-8):

T. s. sandvicensis, August 29, Scotland. (Blow-up of the image above)

Garner et al again:

This exact difference has been noted on all specimens examined in the Natural History Museum (NHM), Tring, and has been put to the test on published images of acuflavida and sandvicensis and appears to hold true. Therefore, the value placed on this difference to the identification process is high. However, it may only be obvious on fresh grown feathers. The effects of wear will make this difference less apparent on older, darker outer primaries as the white fringes and tips wear off, so an evaluation of the age of the feathers based on their overall appearance, combined with a knowledge of moult patterns and time of year, is crucial.

My investigation included not only examining hundreds of photographs, but also ~60 specimens of acuflavida at the Field Museum in Chicago. In particular, I was looking at the primary tips of birds taken in late summer/fall to see if any showed the hooked shape of seen in sandvicensis. The result of examining hundreds of photos and dozens of specimens of adult acuflavida with worn primaries was that not a single bird showed the distinct hook-shaped tips of sandvicensis.

T. s. acuflavida, September 15, Texas.

Unfortunately, the Field’s collection only included 5 specimens of sandvicensis (and only three adults), all very old and in poor condition. But, the shape of the primaries did hold true (although the sample is admittedly small).

T. s. sandvicensis, June 12, Netherlands.

The wingtips of the Chicago tern clearly show the hook-shaped tips characteristic of sandvicensis:

Chicago tern, September 11, 2010

Chicago tern, September 11, 2010


Chicago tern, September 30, 2010

The rest of the wing of the Chicago bird was also in alignment with what would be expected of sandvicensis. Garner et al again:

[Winter adult acuflavida]…there is usually (but not always) a more obvious darker gray oval on the tertials and a tendency for a dark secondary bar. Combined with the spikier-looking bill, the overall appearance is distinctive.

[Summer adult acuflavida]…as in some other large terns, such as Elegant Tern S. elegans, some acuflavida have a dark secondary bar in otherwise full adult breeding plumage…adult sandvicensis normally lacks such a secondary bar.

Chicago tern, September 11, 2010


• The bill of the Chicago Sandwich Tern is longer and proportionally thinner than acuflavida, and appears to have a slighly drooped tip. Garner et al:

“The bill [of acuflavida] often appears subtly, but distinctly different from that of sandvicensis. Biometrics indicate that it averages shorter and proportionally slightly thicker. The effect is often of a distinctly straighter-looking and obviously thick-based spikey bill on acuflavida. On average, the gonydeal angle appears to be slightly more pronounced. In contrast, the bill of sandvicensis tends to appear longer and slightly more slender overall, with a fine and slightly droop-tipped  or at least with an overall slight decurvature to the bill (particularly the upper mandible).

•  The feathering on the forehead and crown of the Chicago Sandwich Tern—being “grizzled” gray vs bright white—is at odds with acuflavida in late summer/fall, but fits well for sandvicensis.

• The shape/pattern of the outermost primaries, with the hook-shaped tips discussed above are well seen and documented on the Chicago Sandwich Tern. This shape/pattern was was not found in any adult acuflavida, at any time of the year—both in hundreds of photos and dozens of specimens examined. The hook-tip shape/pattern was found to be easily discernible when one knew what to look for on virtually every photo of adult sandvicensis examined (~100), regardless of the time of year.

The three major visual field marks outlined by Garner et al for separating acuflavida from sandvicensis, all point to the Chicago bird being the nominate subspecies: Thalasseus sandvicensis sandvicensis. I think there can be little doubt that this bird was of European origin, and represents the first North American record of a living Thalasseus sandvicensis sandvicensis.

The question is, what to do about it?



M. Garner, I. Lewington, J. Crook, Identification of American Sandwich Tern; Dutch Birding Vol:29:5

Flickr image search: Sandwich Tern

Flickr image search: Thalasseus sandvicensis

Flickr image search: Sterna sandvicensis


I would like to thank Rick Remington and Sam Burckhardt for making their excellent images of the Chicago bird readily available to me.


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  • Nathan Goldberg

    This is awesome! Thanks for your hard work. How will we know if the record is “accepted”?

    Nathan Goldberg

  • Chris W

    Dammit…. Now I wish I had chased the thing…..

  • Alex Lamoreaux

    Fantastic post, thanks very much…I learned a ton!

  • Nicholas Sly

    This is an excellent analysis – thank you!

  • Robert Hughes

    Very impressive detective work Greg. The next question is can the people who saw the bird count it as  Thalasseus sandvicensis sandvicensis if the relevant field marks weren’t seen? To my mind the answer is no.

    • Well, I know of about a dozen who can…those who were standing next to Larry Balch and I as we were discussing the “hook-shaped tips” on the primaries. I asked if he’d ever seen anything like that, and he said no…and then the others took interest in our conversation and looked at them also.

    • Well, I know of about a dozen who can…those who were standing next to Larry Balch and I as we were discussing the “hook-shaped tips” on the primaries. I asked if he’d ever seen anything like that, and he said no…and then the others took interest in our conversation and looked at them also.

  • Nathan Goldberg

    I feel that I can count it, since I saw the bird in question. Even if the detective work is done later. My 2 cents.

  • Nathan Goldberg

    I feel that I can count it, since I saw the bird in question. Even if the detective work is done later. My 2 cents.

  • Robert Hughes

    I’d also like to hear what the Europeans have to say about this bird. They know this form far better than we do. Greg, have you received any feedback about this bird from the Europeans?

  • Kirk Roth

    Good work, Greg.  Nice to see some closure on this.  I had passed along the photos and materials to David Sibley, so I know that he was looking into this bird also, although he is very busy.

  • Mark Gawn

    Photo taken in Switzerland in July, 2009, where Sandwich Tern “sandvicensis” is rare but regular so I assume that this is what it was. As far as I can tell, it does not have the hook shaped primaries, or the broad white edging noted above for sandvicensis. The bill, assuming this is a good feature, is spot on. Time for the Europeans to chime in! 

    • Actually, if you blow up the image it does have at least two of the new primaries with the extra white at the tip (which creates the “hook”)…looks like P6-7, and you can see the large white fringe on P5.


      • Mark Gawn

        The “hooks” in the photos of the Illinois birds are structural, the white accents them. The only primary feather in the swiss bird that is “hooked” is a heavily worn retained primary. The fresh primaries lack this feature, thus suggesting that it is due to wear. I think a lot more is needed to be known about wear patterns before a determination can be made. This is probably a useful field of inquiry as moult patterns are already know to be critical id features in some terns (Spring Arctics vs  Commons, to cite a well know North American example). For the moment the Swiss photo would seem to invalidate the literature cited. 

        • I’m not sure I follow you. The main point I make in the article above is that the “hooks” on sandvicensis are structural, and acuflavida does not have them.

          The fresh primaries do have them, but as a white fringe that widens at the tip of the inner web. When the white wears away, it leaves the “hook” shaped tip.

          The photo of the Switzerland bird you posted shows this very well actually, both on the worn primaries (particularly P8), and a fresh primary (P6, with the arrow).

  • Sam Burckhardt

    I corresponded with Klaus Malling Olson about this bird. He didn’t feel comfortable to make any definitive identification to subspecies. As far as the lateness of the molt is concerned, he mention as well that birds who are far off course often times have delayed or interrupted molt, however, a geographic origin cannot be established by that fact. He mentions that the majority of Northern European birds have molted their heads to winter plumage by early August.

    • I asked Martin Garner to take a look at it and said it was “very interesting”, but I haven’t heard back from him.

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  • Pim Wolf

    Any news on the acceptance/rejection of the Chicago bird Greg? Attached a picture of an adult brooding a chick in the Netherlands, taken a few weeks ago. One much older primary showing the typical hook-shape where the white has worn away next to a fresher primary where you can still appreciate the shape of the white “rim”.

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