Sandwich Tern is two species, study suggests

2009 August 30
by David J. Ringer
Sandwich Tern

Sandwich Tern in Portugal by jvverde

VICKSBURG, MISS. — Version 2.2 of the IOC list, released earlier this month, drew my attention to an interesting tidbit that had so far escaped my notice: A genetic study published in 2009 indicates that New World and Old World populations of “Sandwich” Terns (Thalasseus sandvicensis) are not only not conspecific but aren’t even closely related within their genus.

I don’t have access to the paper itself — “Multigene phylogeny and DNA barcoding indicate that the Sandwich tern complex (Thalasseus sandvicensis, Laridae, Sternini) comprises two species.” (Efe et al. 2009) — but a fascinating summary is posted on Birdwatch: An armchair Sandwich.

Cabot's Tern

American Sandwich (Cabot’s) Tern by mitchmcc

A population of terns with yellow-tipped black bills breeds in the Palearctic and winters along the coasts of Africa and Asia. Similar-looking birds breed in eastern North America and intergrade in the Caribbean with a South American yellow-billed population, the “Cayenne” Tern, which some people want to elevate to species status. (See this global range maphivernage = winter range; nidification = breeding range; rĂ©sident = permanent resident.)

Most authorities treat these populations as Thalasseus (or Sterna) sandvicensis sandvicensis, T. s. acuflavidus, and T. s. eurygnathus respectively, but when acuflavidus was collected in Mexico in the 1840s and subsequently described (by a Dr. Samuel Cabot), it was treated as a full species: Cabot’s Tern. This treatment lasted until 1957, when the AOU lumped it with the Old World population. (I don’t know why this was done.)

However, Efe et al. found that the American populations are sister to the Elegant Tern (Thalasseus elegans), an orange-billed species of America’s Pacific Coast, while the Eurasian population is more closely related to the rest of the “crested” Thalasseus terns, which include Royal and Great Crested (Swift) Terns.

If these data are correct, then Sandwich Tern and Cabot’s Tern (or whatever — I’m not crazy about patronyms) need to be split into separate species again. I don’t know how quickly bodies like the AOU NACC or SACC might make this change.

Cayenne Tern

“Cayenne” Tern by barloventomagico

The paper also found low genetic differentiation between the “Cayenne” Tern and the black-billed American taxon. That, combined with evidence of interbreeding in the Caribbean, doesn’t bode well for those who would like to elevate this population to the species level. The Southeastern Caribbean Birds site has some pages with interesting discussion and photos illustrating variability and interbreeding.

So, we have yellow-billed and black-billed populations that apparently belong to the same species, but the other black-billed population is (relatively) unrelated in this clade of otherwise yellow- and orange-billed species. This is odd.

Birdwatch suggests that perhaps the black bill arose in the Palearctic species and was later introduced into the American population in some sort of hybridization event.

If that’s what happened, is it possible that the black-bill gene(s) are still in the process of spreading through the American population? Given the distribution patterns, I could see the trait being introduced from Eurasia into the northern part of the American population. Is it still spreading south, or has it hit some sort of barrier in the Caribbean? (If so, what, and why?)

In closing, here’s something from the DON’T LITTER, YOU JERK desk.

A congener of the Sandwich Tern, the Chinese Crested Tern (Thalasseus bernsteini), is one of the rarest birds in the world. It’s almost extinct; less than 50 birds are known to exist in the world.

Anyway, last year, a photographer caught an image of a Chinese Crested Tern with a piece of plastic impaled on its bill, which is very likely to have interfered with the bird’s ability to feed and led to its death. The species has enough problems without being killed off by garbage! Horrible.

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6 Responses
  1. September 2, 2009

    My gut feeling is that most species both continents share are ripe for splitting, e.g. the owls. This proposed split however is remarkable as it makes explaining the identical (and unique amongst terns) bill colouration of two forms that aren’t each other’s closest relative a difficult task. I don’t buy the hypothesis of a hybridisation event – but yet again, that’s just a gut feeling.
    It will be interesting to see how things will develop and I hope you’ll keep us updated.

    • David J. Ringer permalink*
      September 2, 2009

      Without a hybridization event, the options seem to be that 1. the trait is ancestral or 2. the trait arose twice. Either one seems rather unlikely, but this is hardly my area of expertise.

  2. September 3, 2009

    I know that both these options appear unlikely (I’d say the less unlikely is that it evolved twice). However, a recent hybridisation event that was so prominent as to change the bill colouration of an entire continent’s population should be detectable in the genetic analysis or not? The two aspects that both forms are not each other’s closest relative and a hybridisation event that affected the entire population seem quite contradictive.
    It is results like these that challenge the acuracy of genetic analysis, in my humble and not well-educated opinion.

  3. September 29, 2009

    will you please put there family class phylum and there order down?????

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