A birding observation in Chicago resulted in a new proposal for classifying the European Sandwich Tern (Thalasseus sandvicensis) and the North American Sandwich Tern (Thalasseus acuflavidus), also known as Cabot’s Tern. Some of the features that differentiate both these species are their breeding behavior and the plumage of their young.
WHEN THE SMALLEST DETAILS CHANGE HOW WE SEE THINGS
On September 15, 2010 Greg Neise made an observation about a Sandwich Tern in Chicago, Illinois – a rare bird for the area: “The most exciting aspect of researching and documenting this bird were 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 the 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…”
This one observation become the tipping point in the bird’s likely identification and origin. Its combined features suggested it was a European Sandwich Tern, previously unrecorded in North America, in a plumage type deemed the most difficult, nigh on impossible to separate from the North American Sandwich Tern (Thalasseus acuflavidus, referred to as Cabot’s Tern from now on).
SANDWICH AND CABOT’S TERNS
Are they the same or different? Well, it all depends on your starting point. 130 years ago European Sandwich Tern, American Cabot’s Tern and Caribbean/ South American Cayenne Tern (Thalasseus acuflavidus eurygnathus) were classified as three separate species. Most birders of today have grown up with the idea that they are one species with 3 separate subspecies. This view is changing again, rapidly. When comparing the European and North American birds, most texts reference the similarity of adults, but until very recently have failed to notice the diagnosable differences between adults and even more so, the marked differences in plumages of young birds. There are also differences in their breeding biology. Observations of all kinds of birds which have led to changes in taxonomic thinking.
Here we cover the emerging story of Sandwich and Cabot’s Terns, a story at the frontiers of field identification, taxonomy, and vagrancy. We’ll leave the Cayenne Tern, currently best viewed as the southern subspecies of Cabot’s, for another day. Vagrant Sandwich Terns have reached North America from Europe. Vagrant Cabot’s Terns have reached Europe from North America. Both are likely to be overlooked except by the sharpest, most expectant eyes. Both are increasingly viewed as separate species and not each other’s closest relatives. The potential to discover transatlantic vagrants of both species has opened up.
The Chicago Tern raised the possibility of European Sandwich Terns reaching North America, but also raised the eyebrows of the skeptics. However just 3 years later a remarkable event changed perceptions for good. A Sandwich Tern ringed as a chick on Coquet Island off the northeast coast of England in 2002 turned up in late July 2013 in Massachusetts. The ring labelled DB67406 with the words “British Trust, London” was read by biologist Jeff Spendelow, who was studying the use of staging sites by Roseate Terns in the Cape Cod area of southeast Massachusetts. It was first seen by several observers on one of the study sites on July 31, but it wasn’t until August 21 that Jeff was able to read the ring.
On the other side of the pond, a “Sandwich Tern” found dead in Herefordshire, England, in November 1984 was only the third county record. Its silver ring told a different story: a Cabot’s Tern, the first in the UK! The only two European records are both of birds ringed as chicks in North Carolina, USA, and found dead later in the same year they were rung as first-winters. One was ringed at Cape Lookout on June 23, 1978 and found at Veerse Meer, Noord- Beveland, Zeeland, the Netherlands, on December 23, 1978. The other was ringed on June 25, 1984 near Beaufort, North Carolina and found dead by a Forestry Commission ranger at Newhouse Wood, Herefordshire, on November 28, 1984.
Recent field studies have shown that birds in juvenile, first-winter and adult winter plumages often have a distinctive set of features, making many individuals identifiable. To use taxonomist speak, they are “diagnosable,” a prerequisite condition for conferring species status in some schools of taxonomy. The exceptional wing-tip feature noted on the Chicago bird of 2010 further added weight to that “diagnosability.” Breeding habitat preferences also differ. Sandwich Terns prefer open, unvegetated sandy/shell-type habitats, Cabot’s (and Cayenne) Terns usually breed on vegetated rocky habitats, such as flat islands and dead coral. Finally and critically, a molecular study published in 2009 (Efe et al., 2009) found that Sandwich and Cabot’s Terns were as genetically divergent as other different species within the genus and were not each other’s closest relatives. The North American Cabot’s and Cayenne Terns were found to be sister taxa (more closely related) to Elegant Terns (Thalasseus elegans) – just like the juvenile plumages will tell you!
Efe et al. come to the following conclusion: “Our analysis indicates that the Old World (T. s. sandvicensis) and the New World (T. s. acuflavidus/eurygnathus) tern populations are genetically as divergent as different species in the genus, and do not form a monophyletic group. Instead, the latter are sister to the Elegant Tern (T. elegans). These results strongly suggest that the current taxonomic treatment of the T. s. sandvicensis/ acuflavidus/eurygnathus complex as subspecies within a single species or as a northern hemisphere (T. s. sandvicensis) and a southern hemisphere species (T. s. eurygnathus) are phylogenetically inappropriate. The new arrangement should be one in which the Old World (Sandwich) Tern T. s. sandvicensis and the New World (Cayenne and Cabot’s) Terns T. s. acuflavidus/eurygnathus are considered two different species.”*
Two years after this genetics paper, the BOURC (taxonomic governing body in Britain) split the two former subspecies as Sandwich and Cabot’s Terns and the position is now being adopted by other national authorities. The split has been recommend within the American Ornithologists Union: “I recommend that we follow Efe et al. (2009) and recognize the American birds as a distinct species, Thalasseus acuflavidus (Cabot, 1847) and call it Cabot’s Tern.” Richard C. Banks, July 2012. Being elevated to full species status, with confirmed records of Cabot’s in Europe and Sandwich in North America, it is surely only a matter of time before there are more well-documented sight records of vagrants of each species. Now we turn to the emerging field characters most useful to birders.
FEATURES COMMON TO ALL AGE CLASSES
Cabot’s averages smaller in overall size than Sandwich Tern, and the bill, especially when fully developed in adults is shorter, thicker and straighter (Fig. 1). Conversely Sandwich has thinner, more curved and more “drooping” at times even “needle-like” bill (Fig. 2). From Neise’s account of the Chicago Sandwich Tern:
“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. F 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.”
In winter plumage the black rear crown and nape feathers on Cabot’s are normally longer, blacker and more “greasy-looking” with white fringes/tips either very small or lacking, and forehead and crown unmarked white. In comparison, on winter plumaged Sandwich Tern the black rear crown and nape feathers look shorter and have obvious white tips giving rise to a “white-peppered” effect.
FIRST-WINTER CHARACTERS (OCTOBER-MARCH)
Juveniles undergo progressive molt into first-winter plumage of variable extent. Features to note are structural differences, differences in head pattern and retained juvenile dark patterning especially in tail, coverts and tertials.
JUVENILE CHARACTERS (JULY-OCTOBER)
In Cabot’s the mantle and scapulars vary from being plain-looking and almost unmarked to having blackish marks restricted to shaftstreaks or prominent roughly spade-shaped black subterminal marks. In Sandwich Terns the mantle and scapular feathers have a different pattern of conspicuous blackish “U” or “V”-shaped coarse-looking marks.
In Cabot’s the wing coverts are notably plain gray on the closed wing apart from blackish marginal and lesser coverts which create a darker leading edge to an otherwise plain upperwing. Some have plain black innermost greater coverts. In Sandwich Terns the wing coverts are usually more obviously covered in “U” and “V”-shaped blackish subterminal marks. Some individuals have a weaker “ghosted” pattern of internal dark marks. When present, internal dark marks in the wing coverts can be best detected through the winter.
Fresh juvenile Cabot’s have black-centered outer tertials with a wide white fringe. Juvenile Sandwich have an entirely different pattern of coarse-looking, variegated dark, and pale feathers in the outer tertials, molting in new dark gray-centered and plainer-looking tertials.
Structural differences in size and especially bill can be critical to identification as can differences in the black and white feathering on the crown and nape in non-breeding plumage.
A major difference between Cabot’s and Sandwich is the width of the white fringe to the inner web of the outer primaries in fresh plumage. On Cabot’s, the white fringe is very narrow (1-1.5 mm/0.04-0.06 in), whereas on Sandwich the fringe is wider (2-4 mm/0.08-0.16 in), each primary also having 3-5 mm/0.12-0.2 in of white at the feather-tip (lacking in Cabot’s). The pattern at the tip on Sandwich also has a dark “hook” in the darker part off the web, which remains even when the feather is very worn and the white fringe missing. Cabot’s, when fresh or worn, does not show a dark “hook” at the primary tips.
The adult winter Sandwich usually has a full set of new fresh primaries by December- January, whereas Cabot’s often does not complete its molt until early spring. The adult Cabot’s usually (but not always) has a more obvious darker gray oval on the tertials and a tendency for a dark secondary bar. In comparison, adult-winter Sandwich lacks the dark secondary bar found on many Cabot’s but may show a slightly darker gray oval on the tertials.