By Yoav Perlman
With special assistance from Mike Langman and Guillermo Rodriguez
These two tern species from both sides of the Atlantic look very similar, but in fact are not even the closest relatives – based on genetics, Cabot’s Tern was found to be a sister species of Elegant Tern. Sandwich Tern Thalasseus sandvicensis is monotypic, and Cabot’s Tern includes Thalasseus a. acuflavida and another taxon – Cayene Tern T. a. eurygnatha. This post deals only with nominate acuflavida.
In his Challenge Series Autumn book, Martin Garner dedicated a chapter to them. And of course there’s Garner et al. 2007 DB article. Though not a frequent visitor to Europe as other American gulls and terns, the potential to find a Cabot’s Tern in Europe remains, in light of records of both taxa on both sides of the Atlantic. Additionally, one individual in October 2016 on Tereceira, Azores in October 2016 was first claimed as Cabot’s Tern and caused a bit of a headache. Therefore, I think it is worth revisiting the main ID criteria to separate the two species. I must stress that I am no expert on Cabot’s Tern ID, so I hugely appreciate the valuable help I received from Mike Langman, Guillermo Rodriguez and Julian Hough who certainly are experts. In this post I will try to demonstrate some variation in both species regarding main ID criteria, but I think that still most individuals can be safely identified, within the known limits of variation. That’s why it’s important to document and acknowledge this variation in both species.
The thumb rule is that Cabot’s Tern has a thicker, heavier-based and straighter bill, with an obvious gonydeal angle. Not quite Great Black-backed Gull gonys, but it is still there. Most literature mentions that Sandwich Tern has a proportionately longer bill, without any gonys (really?), and a distinct drooping tip.
I don’t know much about bill development in terns, but I know that in gulls very often adults have heavier bills than young birds, with more pronounced gonydeal angle. The same goes for sexual differences in bill structure – male gulls have larger and heavier bills than females. So possibly some variation in tern bills is age and sex related?
This is a nice, heavy-billed Cabot’s Tern (in April, I know…) showing a pronounced gonys:
This Cabot’s Tern has a slightly thinner bill with almost no gonys, but it is still very straight, lacking the drooping tip of a sandwich.
This is a typical Sandwich Tern, with a long, slender bill, distinctly downcurved, and lacking any gonys:
Though some Sandwich Terns show a rather pronounced gonys, the bill always looks slender and downcurved:
Note also that in literature it is mentioned that on average, adult Cabot’s Terns shows a more extensive yellow tip to the bill than Sandwich Terns. There is lots of variation in this, and I suspect that like in other terns this variation in bill tip pattern may be related to breeding condition and sex.
Pattern an overall ‘darkness’ of primaries
Cabot’s Tern has generally darker primaries than Sandwich Tern. There is a difference between the two species in both the base colour of the primaries, and the width of the pale fringes to each primary, especially on the inner web. Sandwich tern also shows a pale ‘hook’ on the outer webs of old and new primaries, while Cabot’s tern apparently never has pale markings on the outer webs. It should be noted that in both species, old and worn primaries before moult are darker and have narrower pale fringes, so a worn Sandwich Tern just before moult can look as dark as a freshly-moulted Cabot’s Tern. Therefore, it is important to understand whether the primaries used for identification are fresh or worn.
This is a typically dark-winged Cabot’s Tern, in late September:
Check this individual, also in late September: it has an overall paler wingtip, with rather broad pale outer webs. Also, it is not easy to understand the exact pattern of the worn outer primaries. Seem to have no pale on the outer webs but I’m not sure what exactly is going on there:
Sandwich Tern typically gives a pale impression to wingtip, with prominent pale inner and outer webs to outer primaries, that it had almost completed moulting in October:
This September Sandwich Tern has a really dark wingtip, with very narrow pale fringes to visible outer primaries:
Moult (or molt?) timing
There is some conflicting information about this. Adult Sandwich and Cabot’s Terns perform an arrested moult, in which it replaced 5-6 inner primaries before migration, and then the rest after migration. In ‘Challenge Series Autumn’ Martin Garner wrote that Sandwich Tern moults earlier than Cabot’s and mentions that Cabot’s moults the outer primaries late, in December – January, with some individuals showing unmoulted primaries in March. Pyle mentions in his book that Cabot’s moult P8-P10 Between October and March. Malling Olsen and Larsson even write in their tern guide that P10 can be moulted between March and June. Quite a broad temporal window. And to make it even more complicated, according to the brand new BTO guide Identification of European Non-Passerines, some Sandwich Terns complete their moult by late October, while others complete this moult only in winter. So a complete or near-complete moult in mid-autumn (October) indicates Sandwich Tern, but it doesn’t seem to work the other way – late moult in late autumn (November – December) does not necessarily exclude Sandwich Tern. Using early moult to identify a vagrant Sandwich in North America is fine, but late moult is not necessarily a sign for a vagrant Cabot’s in Europe.
Basically, Cabot’s Tern has darker and more solid rear crown, with longer feathers and very few if any white tips. Crown is rather clean white. Cabot’s also tend to show more black around and in front of the eye. Not sure about this one… Sandwich Tern has a paler rear crown, with more white tips – often referred as ‘peppered’ rear crown. There are more dark feathers on the crown itself. However… variation here too. Quite a peppered rear crown on this September Cabot’s. Top crown is clean though.
This is a more normal-looking Cabot’s tern, with a rather solid black rear crown, though there are quite many white feathers mixed. Very clean white crown. But oh no, look at the pale ‘hook’ around the primaries…
A beautiful demonstration of the dirty crown of Sandwich Tern:
Check the rather solid dark rear crown of this October Sandwich Tern:
Terceira, Azores, October 2016 individual
This individual was on the Nearctic magnet archipelago last October. Some friends are writing about this individual so I will refrain from expressing my view on it. Here are some images – judge for yourself:
In the next episode – hatch-year birds…
Many thanks to all the talented photographers, to Killian Mullarney and to Nick Watmough who contributed to this post!