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eBird: Sandwich Tern in North America
Wednesday September 4, 2013   |
Sandwich Tern is a rarity as far north as Massachusetts, and eBird has just a handful of reports. Historically, most reports have come following late-summer and early fall hurricanes. In recent years a few Sandwich Terns have been found each summer among the throngs of Sterna terns that stage in the area prior to their over-water hop to South American wintering grounds. This year the Sandwich Tern story from Massachusetts became much more interesting as we may now have good evidence for the occurrence of a Eurasian Sandwich Tern (Thalasseus sandvicensis sandvicensis).

Although Sandwich Tern is a primarily southern species in North America, the European subspecies is common north through the British Isles. The nominate form, widespread in the Old World from Europe to South Africa and east to India, was recently split from the two New World subspecies by the British Ornithologists' Union (Sangster et al. 2011) based on a genetic study by Efe et al. (2009). Cabot's Tern includes two subspecies: northern T. s. acuflavidus has the typical black bill with a yellow tip (just like Eurasian Sandwich Tern) while the more southerly T. s. eurygnathus-also known as Cayenne Tern-has a yellow or orange bill. Cabot's Tern (subspecies T. s. acuflavidus) has occurred in the United Kingdom twice (records discussed by Garner et al. 2007). The North American Classification Committee considered a split in Sandwich Tern this past year but elected to await more compelling evidence. Despite this decision, it still seems likely that New World and Old World Sandwich Terns will eventually be universally adopted as a split.

On 31 July, a long, thin-billed Sandwich Tern was seen on Cape Cod, Massachusetts at South Beach (see photo-illustrated checklist by Blair Nikula). Also of interest, the bird sported a metal band on its right leg. An unbanded Sandwich Tern was spotted at nearby Coast Guard Beach/Nauset Marsh-about 10 miles north-on 3 Aug (illustrated checklist by Ben LaGasse). On 18 August, the banded Sandwich Tern was seen again, this time at Nauset Marsh. It now had a much whiter crown but otherwise appeared similar, with a very long drooping bill; it was accompanied by a bird (also seen 15 August) that looked similar in size and structure but with a more extensive dark cap (illustrated checklist by Keenan Yakola). Finally, on 21 August, Jeff Spendelow of the U.S. Geological Survey was able to read the band number on the banded Sandwich Tern: the band read "British Trust, London". Full information on this bird and where it was banded has not yet been received, but it seems that this is likely to be North America's first documented Eurasian Sandwich Tern. In light of this, it would be worthwhile to review other North American Sandwich Terns, especially those seen out of normal range or out of season.

Eurasian Sandwich and Cabot's Terns are extremely similar. Garner et al. (2007) point out striking differences in juvenal plumage, with Eurasian birds having strong black chevrons on the scapulars in comparison to much more muted upperparts of juvenile Cabot's Terns. In adult plumage they are much trickier to identify. Potential field marks include: 1) long, thinner, more drooping bill with less of a gonydeal angle in Eurasian Sandwich; 2) broader white fringe to the fresh primaries in Eurasian Sandwich; 3) typically earlier primary molt in Eurasian Sandwich; 4) more 'salt and pepper' speckling on the crown of Eurasian Sandwich, with the speckling extending farther foreword on the crown.

The Massachusetts bird has a very white forehead and central crown, more like Cabot's. However, the very fine speckling towards the rear crown may suggest Eurasian Sandwich. The freshly molted inner primaries visible on the 31 July photos by Blair Nikula could be a point in favor of Eurasian Sandwich and the pale tip, although hard to discern, does seem quite broad on the freshest primary. But overall, identification from plumage and structure may not be diagnostic; whether this individual can be confirmed as North America's first Eurasian Sandwich Tern may depend on what is learned from the band details.

There is just one prior suggestion that Eurasian Sandwich Tern has occurred in North America. Greg Neise, in a blog post discussing Illinois's second record in September 2010, pointed out the very long, drooping bill of that bird and the molt pattern of the crown, making a case for North America's first Eurasian Sandwich Tern. Chicago is so far outside the normal range of Cabot's Tern, that either taxon might be almost equally likely here, so this was an interesting suggestion. Whether or not the ID of the Chicago or Massachusetts birds can be confirmed from the existing photos remains to be seen, but we strongly encourage observers to be aware of the possibility of Eurasian Sandwich Tern in North America and to extensively document any suspected occurrences.

Within eBird, carefully identified terns can be entered as Sandwich Tern (Cabot's), Sandwich Tern (Eurasian), and for the yellow or orange-billed population in the Caribbean, Sandwich Tern (Cayenne). Under all circumstances, we recommend identification at the subspecies level to be based on careful assessment of field characters, rather than assumptions about what subspecies seems more likely. As evidenced by this story, we still have much to learn and a significant proportion of the Sandwich Terns in the Northeastern U.S. and Atlantic Canada may in fact turn out to be trans-Atlantic strays rather than wanderers from further south on the Atlantic coast.

REFERENCES:

Efe, M.A., Tavares, E.S., Baker, A.J. & Bonatto, S.L. 2009. Multigene phylogeny and DNA barcoding indicate that the Sandwich Tern complex (Thalasseus sandvicensis, Laridae, Sternini) comprises two species. Mol. Phylogenet. Evol. 52: 263-267.

Garner, M., Lewington, I. & Crook, J. 2007. Identi
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