Trinidad and Tobago Field Naturalists' Club         Southeastern Caribbean Bird Alert         Trinidad and Tobago Rare Bird Committee
RESPONSES TO 'Mystery Tern' (Sterna / Chlidonia sp.)
The following responses are provided chronologically (and anonymously for private responses to protect the identities of those who may not have wanted their opinions publicized). If you wish to add your comments to this page, click here, and if you wish your name and e-mail address posted, you must explicitly state so.

     The bill, wingtips, and tail remind me of several specimens of Sandwich Tern in our collection; but the rest of it is pretty "washed out"...leucistic? or ill? At any rate, I'm not a tern expert but we get a few species here in NC and this looks most like a Sandwich to me.

RESPONDENT 2 [in response to answered query about when photo was taken]
     OK, thanks. It would appear to be a first-winter FORSTER'S TERN in heavy moult as it goes from winter to 1st summer summer plumage. The stout bill; clear 'bandit-eye' cheek patch; silvery primaries; carpal bar; reddish legs; and juv tail still very short (growing?) would all point to a molting Forster's.
     How many T&T records are there, anyway?

RESPONDENT 2 [in response to answered query about carpal bar of Forster’s Tern and comments on potential of Gull-billed Tern]
     By now I am convinced that at some stages in their lives nearly ALL terns can show (at least weak) carpal bars. GBT is large than CT, and also FT, which in turn size is also larger than CT. The red legs are off-putting, but they are also of a very odd hue. What did the  legs really look like?
     Another feature I have never seen mentioned is the location of the gonys: 2/3 way to bill tip in GBT, but only about halfway in FT. In  the photo it looks about halfway.
     Another excellent feature is that GBT has VERY long legs, Forster's much shorter, like Common/Roseate. This one's legs DO seem long, but it also looks alert, as if it is about to fly.
     Lastly, still another feature I have never seen mentioned is FT has no black mask in front of the eye, but GBT does. This one seems not to...
     I thought there were no SA FT records, but it is a tern and al larids wander. I have alerted Eddie and Martin to be on the lookout for FT on BDS, and there are now two Guadeloupe records.
     Having said all this, I was completely gulled (...) by an alleged juv GBT photographed in the UK this fall that several of us up here were 100% certain was a FT. It was seated only, and at odd angles, and it too had what appeared to be a very log bill, FT head patch, etc. But the observers had it feeding over fields and catching bugs: VERY GBTish, and not very FTish. Then finally, some flight pix were published, and there was no doubt it was indeed a GBT.
     So how was this bird behaving, and what was it with?
     I ALWAYS warn against IDing from pix, and then I blithely do it all the time, every now and then stumbling and falling.
     This is an interesting bird!

RESPONDENT 1 [after posted comment that a Sandwich Tern should be crested]
     I would have to disagree with the assessment that the bird should be "distinctly crested"; not in the plumage depicted in that photo; it's nowhere near what our crested birds look like; I suppose there are different populations that show molt variation. But given the plumage shown in that photo, I would not expect it to be crested at all. It's showing what is essentially a basic plumage, and crested is alternate. Also, I thought about Forster's, but concluded it could not be; that tail is very wrong-looking; outers look way too short. I also have to very much disagree with Gull-billed! (sorry; the coffee is good this morning). That's another that breeds here, and the bill is way too long and narrow for that species. I'm still left with Sandwich but I don't know the SAm terns well enough to compare.....

     Based on bill size, structure and long reddish legs, combined with the dark ear covert patch, I think the tern is a bleached Forster's Tern, rather than Common.

     If Gull-billed Terns occur regularly there, that would be my guess, but the photos are a little less than helpful. The bill looks pretty heavy and the plumage awfully white (especially on the head), both good for Gull-billed. The bill and bird both look too heavy for Forster's and the bill and head shape are wrong for Sandwich. Having looked at lots of Elegant Terns at Bolsa Chica (I assume you know the spot from your California days), I would say that leg color is the last thing that I'd use as a definitive character. It varies both individually and seasonally in lots of terns (and gulls for that matter). The tail also looked quite short and not really forked, at least as best I could determine (again not good for Forster's). If it was definitely a North American species, then I'd go with Gull-billed over the others. I'm less sure about other species. There you have it, though I'd admit that with only those photos to go on, a definitive ID is beyond me.

     the tern is definitely not a forster's. i see many of these--hundreds in fact just yesterday--and i've never seen one that looks like this. the plumage, soft part colors and the structure are not like forster's. it appears to be a juv gull-billed tern. we had one in louisiana last year that caused a stir as it was reported as a common black-headed gull! it looked very much like this bird, and threw the observers because of the pale legs. if i happen to come across the pictures of that bird i'll pass the link to you.

RESPONDENT 5 [additional comments]
     i couldn't find photos of the bird i referred to, but did find this writeup. a bird in this plumage would have all winter to get nice and bleached. bill color listed here is red, but i assume that they quickly acquire a black bill. thanks, **** ******* went down to Cameron today (9/3) in search of Labor Day migrants. I'll get to those in a minute.... At the Secret Place, we came across a bird that is likely to be a BLACK-HEADED GULL on the mudflats just before the pond on the southeast side of the ridge (in front of the pumping house). We stayed with the bird for about twenty minutes or so, shooting off an entire roll of film (with some definitive shots, either way). The bird appeared moribund, so we tried to approach the bird as close as it would let us. The bird would only fly a few feet at a time, so we decided to perhaps capture the bird. We got within ten feet of the bird at times, but were unable to capture it, as it sailed southward into the marsh. We were able to get a few shots of the bird in flight as well...and a brief description
follows: A small gull, a bit larger than a Bonaparte's...with a RED bill with black tip. Bill was longer and thicker than in BOGU. Plumage was overall "washed out" looking, but with field marks still present. A black post ocular spot was present with a bit more black smudging, perhaps suggesting the bird was obtaining adult plumage. The legs were dark red. Underwing primaries were dark, with P1 being white...and a trace of a subterminal band present on the tail. The shoulders had a small amount of what I would refer to as leftover juvenile plumage.

     Well, I am fairly certain your bird is neither a Gull-billed or Sandwich Tern. However, I do not really know Forsters, so can't comment on that possibility. Looking at the photos, there does not seem to be too much to convince me that an immature Common Tern is not a possibility as well. Bill and leg length seem to suit this species as do the strong carpal bar. Was that option ever considered?

     About the mystery Tern.. It does not look like a Gull-billed Tern. (Shape of bird, length of tail, shape and thinkness of bill). To me the bird has a clear Sterna look. Feet are too long etc for an Arctic and Roseate. Same goes for Sandwich tern. Wrong shape, and wrong leg color. In my mind I narrowed the choises of this bird almost immediately to Forter's and Commmon. The combination of black bill and red feet is OK for Common at least (have to check on Forter's). In winter plumage a Common Tern acquires a carpal bar also. I'm not sure if Forter's does. About the age of the bird. I'm not quite clear what are the strong arguments for claiming that the bird is moulting 1st summer one and not an adult bird. Clearly the plumage is quite worn, and bird seems to be moulting. May be in such a stage the carpal bar of the adult winter bird would not be this strong. So at this point my vote for Ste hir/for possibly 1st
     P.S. Is the outer web of the outer tail feather dark?

RESPONDENT 8: Martin Reid <>
     I feel that this is a COTE/FOTE type bird. The bill and apparent leg length do seem to favor FOTE, yet the strength of the lesser-covert patch - in March - seems extreme for FOTE, plus the P-molt seems better for COTE; the most obvious COTE feature (unless i am misinterpreting it) seems to be the tail pattern, which shows a blackish outer edge to the outer retrix (as in COTE/ARTE) - not the white outer edge plus dark tips to the inner retrices (as in FOTE). Looking again at the legs, the tern's efforts to perch on wires may be affecting the perception of length; there seems to be very little tibia showing....

     Why isn't the bird pictured a worn Roseate Tern? The bill shape looks better for that than Forsters (too long and thin), the dark outer primaries are right for Roseate, and the outer rectrices may be molting. It doesn't look like Forsters at all for me.

NOTE: The following comments were received after posting a summary of the first nine responses and concluding it was most likely an odd Common Tern.

RESPONDENT 3 [in response to posted summary]
     I looked again at the photos and agree that if the outer web is dark (can't really see it on my monitor) then it would identify the bird as a Common Tern, not Forster's, as I had initially thought.

     in my opinion the mystery tern with the red legs is an adult winter White-winged Black Tern (Chlidonias leucopterus). All features visible in the photo are good for this species: whitish plumage, grey cap restricted to crown and nape (not forehead), slim red legs, dark line on the forewing, short and only sligthly notched tail with, short and blackish bill (this could seem to be a bit wide-based than a typical W.-w. B. T., but adult birds can regularly show such a feature).

     I took a look at the mystery tern. And I think you need to round out your respondents to a statistically better sample of 10...
Assuming the features I see in the jpegs reflect what the bird actually looked like, I concur on the ID as a Common based on carpal patch, tarsus length, and dark outer web to the outer rectrices. Tarsus length eliminates Arctic. Dark outer edge on outer webs eliminates Roseate (and Forster's). Dark carpal patch also eliminates Forster's, Certainly NOT a Gull-billed or Sandwich based on bill, legs, plumage, etc.

RESPONDENT 12: Alvaro Jaramillo <>
     I thought that it looked like a Common/Forster's/Roseate based on shape, size and structure. While initially I thought Foster's due to the eye patch, I took it back after pondering a bit more. First of all this is a bird in its first basic plumage and its actively in wing moult. It looks like P5-6 are new, 7 is growing in, 8-9 are missing and 10 is old and worn (retained juvenile feather). Its hard to decipher this due to the blurry quality of the photo but I think I am in the ballpark. It is odd that the innermost primary looks darker (more worn?) than the next and I can't explain that unless moult was suspended. The tertials are new, and the tail may be in active moult. The outer rectrix has a dark tip, which is a Juvenile trait, while the inner ones look paler and fresh (new feathers?). The lesser coverts are retained, but the pale area in the median coverts appears to be active moult in that feather group. I bet the head is moulting too, perhaps creating part of the odd appearance of the bird. The fact that the covert bar is dark and bold and that the outer edge of R6 is black (best seen on the third photo down) would eliminate Forster's for me. Also what is left of the ear patch looks better for Common than Forster's. Forster's has a bit more black before the eye (on average) than Common, usually reaching a bit further towards the bill than the level of the gape, while on Common it reaches to the gape at the most. The mystery bird is consistent with the Common Tern pattern. Also on Forster's Tern there is usually a good amount of black below the eye, while on Common the black is entirely absent below the eye or is restricted to a fine line, again your bird fits the Common Tern pattern. The moult timing tends to be earlier in Forsters and later in Common, but with much variation in this first complete moult; my thought is that it fits a young Common Tern better but I am uncertain. But before deciding its a common, I must admit that I know nothing about the appearance and moult timing of Roseate Tern at this age. I have never seen a Roseate Tern (that I know of) so my experience is sorely lacking on this species. I don't think that any of the general points that appear to exclude Forster's Tern exclude Roseate Tern so it would still be a contender in my opinion. I think that is as far as I can take it - Common/Roseate. The adults differ in moult timing, and general strategy of moult suspension; this is what gives them their different wing patterns in summer. Perhaps the moult timing and extent differs in younger birds and this could aid in the identification of the mystery tern.

     I'm in Texas and not near any reference material but have you considered Whiskered Tern? That was what flashed through my mind when I first glanced at the top image. Perhaps it was the fact that the bird was perched on a rope, a "characteristic" of marsh terns (but of course not limited to them) and had those obviously sturdy, reddish legs.. The bill seems too long but the bird is so frayed and the head so small that I suspect it's lost a lot of feathering around the base of the bill. The tail seems grayish as well...
     I regret I'm not in a position to do more than raise this question for your consideration.

     My comments below are based on the assumption that the mystery tern is a species that occurs on the East Coast of the United States (i.e. is not some species from South America--or somewhere else--I'm not familiar with)
     If I saw a bird that looks like that in my area (coastal North Carolina), especially as it looks in picture with the gulls, I would dismiss it right off as a Forster's Tern. Many birds in late winter have this blotchy washed out look.
     I can see that the outermost tail feather appears to have the pattern of a Common Tern--dark outer web (I think I'm remembering this point correctly). Is there any chance that a couple of feathers are overlapping and what we think we see isn't what is actually there? It's hard to believe that shape and bill size goes with a Common Tern. Really hard to believe.
     My second guess (if I had to make one) would be a Sandwich Tern. In the close up pix, the bill looks more like a Sandwich Tern. but in the pic with the gulls, the bird looks too small for a Sandwich. What is the outermost tail feather of a Sandwich like?
     I wouldn't be surprised if Common and Forster's terns hybridize occasionally. In fact, I would expect it.

RESPONDENT 15: Klaus Malling Olsen <>
     Having seen the mystery tern from Trinidad, I think Gull-billed and any Sandvish Tern could be ruled out - the bill looks to slender for  Gull-billed and I have never heard of any Gull-billed nor Sandvish with reddish legs.
     The strongly abraded plumage suggest 1 winter tern. The carpal-markigns are too heavy to fit Forster's Tern. With a few reservations I think what fits best (I am always cautionous in judging birds from pgotoes alone) is Common Tern. This would explain the reddish looking legs, the strong carpal bar and the head-pattern. The size seems good too for Common Tern.
     I am sure others have come up with some iD ideas, and hope my views will be of interest.

NOTE: The following comments were received after posting an announcement that the above responses were posted here and that White-winged Tern and Whiskered Tern had been suggested.

     Your mystery tern is categorically NOT a Gull-billed Tern.
     I'm not sure what it is, but it certainly looks like a Chlidonias -- I would guess Whiskered based on large bill (but I have no experience with that species).
     Because of faded plumage I'm not sure it's identifiable, and I wouldn't be shocked if it turned out to be a Common/Roseate/
Forster's sort of thing (of those, I'd give the nod to Roseate), but it just has the look of a marsh-tern and thus I'd agree with the commentor who initially suggested that.

     I hadn't looked at your tern before but just did so. Where I live I see Forster's yearround and Commons regularly in migration; your bird does not recall them at all. It is the wrong shape being much too short-tailed compared to the winglength, no matter the degree of wear and/or bleaching.. I am also content it is not a Sandwich for a wide variety of reasons.The bill is much too large for White-winged Tern.
     I think the possibility of Whiskered Tern is the best suggestion I saw in the comments, but I don't know that species well. Yet it does strike me as the right size/shape for that choice.

     WWT has a moderately small bill --- quite smaller than CT and much smaller than FT. The bill in the photo is VERY long, and so I think one can rule out WWT on that feature alone (and yes, they do have carpal bars, but then so do most terns).

RESPONDENT 12: Alvaro Jaramillo <>
     I must admit, that Whiskered Tern came to my mind as well. I don't know the species, but the 'jizz' of your bird was suggestive of photos I have seen of Whiskered Tern. However I eliminated it from the running due to the bold carpal bar. The books suggest that this does not occur on Whiskered, but books can be wrong. Also the bill may be too long for that species, and I don't know if the outer rectrix is showing too much of a 'streamer'?

RESPONDENT 15: Klaus Malling Olsen <>
     Again today I have seen the photoes of the mystery Trinidad Tern. Judging the bird from photoes is not that easy. However, I think it is most probably a Common Tern immature (judging from moult it could be a retarded 2 winter bird, as the outer primaries are much worn and there seems to be 3 generations of flight feathers. The very short tail look odd, but it seems that the outer webs of the outernost are dark - a good Common/Arctic character. Arctic could be ruled out by the too long bill and legs and the extensive dark leading edge to upperwing, looking perfedt for Common Tern. The overall shape is again good for Common, and I think it will never fix even the longest-billed Whiskered Tern. The bird looks too slender and not that chunky as any chlidonias. Also, I think Whiskered would look smallar compared to the Lauging Gulls present. Hope this will be of any use

RESPONDENT 18 [in response to another respondent's query]
     With regard to the Tern I'd have to agree with you that its a 1st-winter/1st-summer ('portlandica'-type) Common Tern (albeit abnormally pale-crowned). It looks too large and long-tailed for a 'Marsh Tern' and somehow more reminiscent (having the 'feel') of a Sterna to me. Perhaps more importantly, it appears too long-billed for Whiskered Tern (and thus even further removed from WwBT) (though admittedly bulky even for Common), the neck appears white with a broad complete white collar which looks good for Common (would expect hindcrown to meet grey-mantle on Whiskered) and the wing-coverts appear contrastingly white between the carpal bar and the secondaries (typical of both juv, 1st-w, 1st-summer Common) - would expect the wings to appear more uniformly all-grey (lacking the apparent contrast) in Whiskered. I would think that the leg colour possibly helps eliminate Roseate (esp. combined with bill size/shape) and definitely Sandwich & Gull-billed. The carpal bar presumably eliminates Forster's (and again
helps eliminate Whiskered). I would guess your thoughts were along the same lines? Clearly a very tricky bird though.

RESPONDENT 8: Martin Reid <> [in queries to respondent 15: Klaus Malling Olsen]
     3) there has been much controversy about the tern at this site: ttmysterytern - just take a look at the suggested IDs posted there anonymously, to see what I mean! I feel it's a COTE that appears to be a 2cy (March) but the three generations of primaries would have to make it a 3CY bird - i.e. a 2nd winter bird that  retained the juv. outer primaries - is this possible?; also, could the tail still be so short for such an age-diagnosis?

RESPONDENT 15: Klaus Malling Olsen <> [ in response to queries of respondent 8: Martin Reid]]
     My judgement was Common Tern (see comments 15) but I was less reluctant in judging age for sure. The bird is extremely worn and might suffer from disease. The stronlgy worn outer primaries might be retained juvenile, fitting with the known moult pattern of 2. years (= 3 cal year), that two moult cyckles are present in innerwing, whereas juvenile primaries might occur onto 3 cal year spring (although rarely, not so frequent as stated in the general intro in Terns)
     The tail looks extremely worn to me, especially confusing were the abnormal short outer tail feathers. Must be worn down to the shortest possible degree.