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Is a large number of Cayenne-type Terns invading North America and interbreeding with (1 Viewer)

Mark B Bartosik

Well-known member
Is a large number of Cayenne-type Terns invading North America and interbreeding with Cabot’s Terns?

Hi All,

A few months ago I reported a couple of Cayenne-type terns which were traveling with the large migrating Cabot’s Tern flock (on April 9). At that time I tried to summarize what was published on distribution and possible interbreeding between these two subspecies so there is no need to repeat references that are easy to find in archives. That post triggered some discussion that also continued offline. At that time (as well as I am now), I heavily depended on comparison with photos published by Floyd Hayes taken by him during his studies and showing intermediates that were present in breeding Caribbean colonies where both Cabot’s and “pure” Cayenne phenotypes can be found and possibly Cayenne are interbreeding with Cabot’s Terns. BTW I am very thankful to Floyd for all his observations and opinions he shared with me in email exchanges. I am also trying to check hundreds of photos available on the internet. Of course without further genetic studies nothing can be solved but, I think, my new observations can help to discuss probability of possible origin of intermediate forms and their dispersion.

A few selected photos to illustrate some of the subjects in this post:

1. A few bill examples (from all angles) of Cayenne-type Terns (chosen from 50+ birds I found on UTC during this year spring migration)

2. Just to show how striking these terns can look

3. Examples of orange-red vs. yellow bills

4. Example of orange-red/black bill

5. Example of yellow/black bill

6. Egzample of mating between intermediate (male) and ‘pure’ Cabot’s (female)

7. Example of yellow/black blotches % distribution – because of the bill surface curvature it is very difficult to make precise estimate in photos– so it is only very crude estimate

8. Example (from 2013) of one from only a few integrates I found in past years (and only with traces of yellow /orange blotches)

At present there are two major proposals to deal with Sandwich-Cabot’s-Cayenne-Elegant taxonomy.
1) 2 species, one with 3 subspecies (e.g. AOU)

Thalasseus sandvicensis sandvicensis (nominate) – Sandwich Tern; breeds on the Atlantic, Baltic, Black, Caspian and Mediterranean coasts, wintering from the Black, Caspian and Mediterranean Seas to the coasts of western and southern Africa, and from the south Red Sea to the north-west India and Sri Lanka.

T. s. acuflavida – Cabot’s Tern; breeds on the Atlantic coasts of North America (from Virginia) and north Caribbean; wintering in the Caribbean and further south in the South America, and on the Pacific coasts from southern Mexico to northern Chile.

T. s. eurygnatha – Cayenne Tern, breeds on the Atlantic coasts of South America from the Argentina north to the south Caribbean, currently expanding breeding range north in the Caribbean, integrating with acuflavida in the north of its range; several vagrants were found along the Atlantic coasts of North America and once, two birds, on the Pacific coast of Columbia. Interestingly there is a record from Panama entered this year (May 9) into eBird by the Sapsucker Team.

Thalasseus elegans – Elegant Tern; widely distributed on the Pacific coast of Americas from British Columbia to Chile but breeds only along coasts from southern California (USA) to Baja California (Mexico), (more than 95% species population breeds in colony on Isla Rasa); winters from the Mexico to Chile. Several vagrants recorded in Europe and one from South Africa. Unconfirmed records of possible elegans reported from South Africa, Canary Islands, Argentina and Christmas Island (Indian Ocean)

2) 3 species, one with 2 subspecies (e.g. Efe et al.)

Thalasseus sandvicensis – Sandwich Tern (closely related to Royal and Great Crested Terns)

Thalasseus acuflavida acuflavida (nominate) – Cabot’s Tern
T. a. eurygnatha – Cayenne Tern
(low genetic differentiation between both forms that are very likely integrating in Caribbean and imply non-assortative mating behavior)

Thalasseus elegans – Elegant Tern (sister species to T. acuflavida)

When searching the web one can find other possible combinations, more or less formal, from private opinions posted on blogs to pdf files uploaded by organizations.

At this moment we are still facing many unsolved questions. The Cayenne Terns bill variations are continuous and so not an example of polymorphism (as already pointed out by some authors). Recent published study is rather excluding a possibility of Cayenne to have rank of full species. The ‘pure’ Cabot’s type phenotype (black bill with only bill tip yellow) is found breeding in North America and Caribbean during the boreal-summer only. The ‘pure’ Cayenne type phenotype (whole bill yellow with no even traces of black blotches) and the intermediate forms (with clinal distribution and being most common in northern part to least common in the deepest south area of Cayenne distribution in Argentina) are breeding in Caribbean during boreal-summer (overlap range and time with Cabot’s Terns) and during austral-summer in South America. Intermediate forms (with yellow blotches on black bill; or black blotches on yellow bill) among ‘pure’ Cayennes are found quite commonly in West Indies from 1962 where due to recent Cabot’s breeding area southern expansion and Cayenne northern breeding area expansion these races can be found nesting together. The intermediate forms are, usually, treated as Cayenne in austral-summer breeding populations (strong pro arguments used are that Cabot’s Terns are not breeding there and there are no records of banded Cayenne vagrants from Caribbean found in that part of range). The intermediates from boreal-summer breeding populations on the other hand are treated as hybrids or as an unsolved problem as they may indicate introgressed alleles from Cabot’s Tern populations or simply genetic variability within the taxon. Also, it was suggested that some Cayenne Terns have a mostly black bill tipped with yellow; such birds closely resemble Sandwich Tern but the yellow tip is usually more extensive.

Status of intermediates inside mixed Cabot’s and Cayenne breeding colonies needs more studies. As Hayes suggested: a crucial, unresolved question is whether individuals with phenotypically 'intermediate' bill coloration (black with yellow/orange blotches or yellow/orange with black blotches) represent: (1) variant phenotypes of genotypic ally ' pure' Cayenne Tern; (2) hybrids between the two taxa; or (3) a mixture of both phenomena. BTW I also heard one more opinion (private one) that perhaps bill color variations might be a part of Cabot’s Tern phenotype as well – will back to that later.

Up to now only a few single Cayenne and Cayenne-type terns were reported from North America (first record 30 May 1983, North Carolina). There were a few mentions of Cabot’s Tern that have traces of yellow blotches – unfortunately I was not able to establish what exactly particular authors meant by ‘traces’ – my emails were unanswered but I do not think this is an important issue anyway. In my case I will use phrase “traces” when bill has only one or few very small but well defined yellow blotches.

After I found the first two Cayenne-type terns on April 9 I decided to continue search for more. During next few weeks (rest of April to beginning of May) I found many large migrating Cabot’s Tern flocks resting along the shore here. I tried to cover a quite long shoreline distance (part of the Upper Texas Coast) but, sort of expected, in most flocks I found only ‘pure’ Cabot’s. A few flocks brought a very nice surprise. I ended up with about 50 birds that shown full spectrum of bill variations from traces to about 50/50 yellow/black bills. Unfortunately it takes a lot of time to process all photos (I am still working on that) so I will only show a few examples. As I was trying to get bill views from all angles I ended up with quite large number of photos of many individual birds. Interestingly a few flocks had about 10-15 Cayenne-type terns and usually I sighted every bird (except one) only once despite trying to relocating flock with those birds the following days. As many terns were courting and copulating I was trying to note sex of some intermediates and also, as Hayes in his study, I observed that despite the availability of other intermediates (some flocks were large, a few hundred birds), no courting/mating was observed between intermediates). These observations imply non-assortative mating, supporting treatment of the two taxa as subspecies and not two distinctive species.

With all these Cayenne-type terns that seem to look like coming out of woodwork this year on Upper Texas Coast one has to start asking questions. Where they all came from? Are they a Cayenne, Cabot’s, integrates or mixture of above phenomena. If intermediate bill variations are part of ‘pure’ eurygnatha phenotype why there is a clinal distribution with most intermediates found within areas with mixed breeding colonies (e.g. Caribbean)? If they are ‘pure’ acuflavida than why there are so few records in the past, only in a very few places in North America, and none in most part of NA breeding areas. Birds with traces of extra yellow blotches can be easy ignored or even overlooked by most birders but what about birds with about 25-50% or even larger yellow areas in the bill? How this could be overlooked or ignored by so many people in the field for so many years?

In my opinion most plausible explanations could be: (1) most (if not all) intermediates are intergrades between Cabot’s and Cayenne Terns and some (most) of those are very likely already backcrosses that occurred in recent years in some North American Cabot’s Tern breeding colonies (2) single ‘pure’ Cayenne vagrants might inbreed with Cabot’s Tern for quite long time, producing offspring that was migrating back and forth and kept producing next generations of backcrosses in NA in following years remaining undetected (perhaps inside breeding colonies on isolated, not visited by humans, islands).(3) In recent years due to north expansion of Cayenne Terns and southern expansion of Cabot’s Terns breeding colonies both forms started to nest together more often and producing larger number of intergrades. Some of those could start to join wintering Cabot’s Terns in spring migrating flocks when they were coming back to breeding colonies in North America. This process could be initiated by courtship on wintering grounds, and as well as part of expanding due to running out of available breeding grounds in Caribbean. As we know available breeding grounds are not only limited but are also shrinking due to continuous habitat lost (island development), human disturbance (growing tourism) and even by natural need to leave old breeding grounds due to accumulated ground contamination during past nesting seasons. My observations (and others as well) of non-assortative mating support hypothesis of possible initial courtship between Cabot’s and Cayenne Terns on Cabot’s Terns wintering grounds and involved Cayenne Terns joining migrating flocks of Cabot’s Terns.

I think we also should take under consideration that we only have very little data and knowledge available about the past population ranges and contact zones between Cabot’s and Cayenne Terns. Especially data collected on the Cayenne populations is very poor and was mostly collected during past few decades. We have a very little clue what was going on during the past centuries and practically nothing about distribution in past millenniums; not to even mention earlier time when both subspecies were evolving. It is worth to mention that as recent as in 1955 (Junge and Voous) Cayenne was noted as one of the rarest and least known of the world’s terns and in 1965 (Sick) the Cayenne distribution was described as remained somewhat puzzling. We do not know if, and if so how many times, Cayenne populations were reduced in the past and bottleneck effect occurred. Like in other parts of the world seabirds and their eggs were, and still are, harvest for food consumption and folk cures; many people still believe that seabird eggs possess very strong aphrodisiac qualities. With so poor data and lack of genetic studies it seems to be impossible to draw final conclusions now.

If in fact intermediate forms are hybrids than, due to non-assortative mating behavior, many generations of backcrosses might already occurred in some Cabot’s Tern colonies in North America producing increasing number of birds with less and less traces of the yellow blotches on their bills compare to a number of the hypothetical F1 hybrid (‘pure’ Cabot’s x ‘pure’ Cayenne) with the hypothetical ~50/50% of yellow/black pigment distribution). I also hypothesize that the F1 hybrids are mostly produced in mixed colonies in the Caribbean and only a few between ‘pure’ Cayenne vagrants (that are very rare) and ‘pure’ Cabot’s parents in North America’s Cabot’ Tern breeding colonies. B1 backcrosses will occur in both places but chances for a ‘pure’ Cayenne backcross parent (to produce hypothetical ~75/25% (yellow/black) hybrids in North America colonies are practically nonexistent contrary to mixed colonies in Caribbean where they might occur from time to time; backcrosses with ‘pure’ Cabot’s parents probably are common and (again due to non-assortative mating behavior) more likely to occur as ‘pure’ Cabot’s Terns often greatly outnumber ‘pure’ Cayenne Terns so Cabot’s Tern backcross parents (versus ‘pure’ Cayenne or intermediate form) are statistically most probably choice in random courting and mating. In mixed breeding colonies where both subspecies are interbreeding the number of intergrades will grow in the successive generations so odds for intergrades to mate with other integrates will become more probable.

I am well aware that my described scenarios are speculative. Without further genetic studies all possible proposed explanations remain undecidable for as long as we do not determinate genotypes of intermediate forms.

The leg coloration of Cayenne Tern adults. From published papers: most of the Cayenne adult birds have the legs, including the feet and the webs, black, but a small number, have the legs bright yellow. In addition there are birds showing irregular patches of black or yellow in which either black or yellow is predominating. There are also birds with uniform black legs and toes, but with the underside of the webs conspicuously yellow. When checking photos available on the internet I found all possible leg coloration variations in ‘pure’ Cayenne (yellow-billed) and mostly black legs in intermediates (none with predominantly yellow coloration). All intermediates I found in Texas had black legs. Unfortunately almost all intermediates’ photos I checked were taken in Brazil. I found only a very few photos taken in Caribbean – area I am most interested to make comparison and to find out if indeed black legs are practically almost exclusively dominant in intermediate forms from mixed colonies.

The bill coloration of Cayenne and Cabot’s Tern adults. The color of pale parts of the Cayenne bill is usually yellow but in some birds can be orange or red. Especially the birds with orange-red bill coloration look similar to Elegant Terns making identification of vagrants difficult. I did not find paper that describes Cabot’s Tern bill tip color other than yellow (including BNA account). In extreme case I found one Cabot’s with bright red bill tip. Also I found that some juvenile Cabot’s can have orange-reddish bills. BTW when I started to write this post I titled it first: “Large number of Cayenne-type terns migrating through Texas this year, and bare parts color aberrations in some terns”, and, for some reason, I thought it is going to be relatively short post. Now I see it is not going to be a case so I decided to write more about some interesting aberrant (bills and legs) cases separately. I also gave up on including references as this would take an extra page as well.

I included Elegant Tern in this discussion because of similar problem with bill coloration patterns (black blotches on yellow/orange/red bills) that occurs in the Elegant populations. These bicolored bill forms are usually identified as (probable) hybrids between Elegant and Cabot’s Terns in published papers but this view is opposed by some people (in private opinions) who think that this phenomenon is just a part of the ‘pure’ Elegant phenotype variations. One suggestion that seems to sound as a strong argument used by the opposition is that the Elegant and Cabot’s Tern breeding grounds do not overlap. On the other hand there is several published records of possible interbreeding between both species on both coasts (Atlantic and Pacific) – and as well between Sandwich (sandvicensis) and Elegant in Europe (confirmed by DNA study). Elegant Tern can be confused with Cayenne in South America and with Lesser Crested Tern in Europe. Especially confusion with Cayenne can lead to easy overlook Elegant vagrants on the Atlantic side in South America (and Cayenne vagrants on the Pacific side as well). There are several well documented and accepted Elegant records in Europe (plus many Elegant-type that were not accepted) and even places like South Africa. Personally I think that many Elegant Tern vagrants can, and do, travel far from what is thought to be their normal population range. Even here in Texas I found two Elegant Terns in past two years (2013 and 2014; both records accepted) – there were only 3 accepted records prior to my records (1889, 1985, and 2001) and one between (2014). Again poor data collected only in recent decades and lack of genetic studies should prevent us from excluding a possibility of Elegant inbreeding with Cayenne occasionally, now and especially in the past. These integrates could sport bills with orange/red coloration; uniform or with black blotches.

There is a problem with entering Cayenne-type terns into the eBird database. I entered first two records as Cayenne (with proper annotation and supportive photos) only to find out that these were not accepted by reviewer so they are not showing at all when somebody is searching for it. This was not making sense to me to enter any more additional records. On the other hand there are a couple of old Texas records from a different review region, most likely of the same bird, without supporting material that evidently were confirmed as they are showing in search results. It is making even less sense to confirm some and not all, giving totally misleading scenario. Another example is that there are several northern Cayenne records (along Atlantic coast in North America) showing in eBird (most were published) but nothing is showing in Florida where the record validating committee and eBird reviewers are very restrictive and refuse to accept any Cayenne-type tern (even with traces of black blotches) so it looks on eBird map like in Florida Cayenne-type vagrants were never recorded. IMHO, for as long as problem is not solved, all intermediate forms (including these from South America)should be entered as Cayenne-type so anybody researching the data could have a clear picture of intermediates distribution and frequency compare to the ‘pure’ form inside and outside eurygnatha population range. This approach could produce much more valuable data sets whatever these individuals are hybrids or only a part of eurygnatha phenotype variations. Optimally two new entry choices should be offered (1) Cayenne-type and (2) Cayenne/Cayenne-type (e.g. birds observed from far away, etc.). I am aware that most birders do not pay much attention to subspecies (no extra ticks) and even less attention to intermediate forms but the records from those who do will stand up in the crowd and provide important insights. These extra choices could help reviewers as intermediates are easy to recognize in the photo or base on detailed description included in the entry. Once the genetic studies are done it would be an easy task to move all records to proper categories if the intermediates belong only to one category (hybrid or ‘pure’ genotype). If not (being a mixture of both phenomena) it might be impossible to safely identify intermediate forms in the field so all records should remain as Cayenne-type.
As my observation of so large number of the Cayenne-type Terns entering North America seems (to my best knowledge only single birds were observed before) to be never recorded before I hope that my post will trigger discussion on this phenomenon.


At present there are two major proposals to deal with Sandwich-Cabot’s-Cayenne-Elegant taxonomy...
Also, HBW/BirdLife: Thalasseus sandvicensis with two sspp: T s sandvicensis; T s acuflavidus.
Form eurygnathus ("Cayenne Tern"; often misspelt eurygnatha [David & Gosselin (2002)]) previously considered a separate species, but S Caribbean data indicate it is a race or perhaps morph of T. sandvicensis, and that all New World populations (except those of extreme N) have at least small percentage of "Cayenne"-type birds; based on recent genetic study [Efe et al. (2009)], eurygnathus here treated as a synonym of acuflavidus; the two interbreed freely in S Caribbean, and mixed pairs occur in Patagonia and Puerto Rico; in Curaçao typical acuflavidus make up 20% of population, while in French Guiana they comprise less than 1%.
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