sterna id (1 Viewer)

taken on a pelagic in southern California by a friend on July 23rd, id. was split between Common and Arctic.

http://mydogoscar.com/birdspot/

which is it and why??

hi julian,

looks like it has been sorted here: http://www.birdforum.net/showthread.php?t=208966&highlight=mydogoscar (esp. peter adriaen's post, no 6)

best,

I saw this bird first time round and fretted about it The inner primaries appear freshly moulted and hence give a suggestion of a Common Tern-like underwing pattern. Though the apparent inner wing panel is not an translucency effect, just a variation in the wear to the tips I think.

As the very knowing Peter said, the pattern of the primaries leaves little room for argument.
1st summer Arctic: http://www.pelagicodyssey.ca/styled-3/page20/files/page20-arttern1.jpg
1st summer Common: https://picasaweb.google.com/lh/vie...&cuname=tixbirdz&filter=1#5618791149319097682


I must admit that the structure of the bird while perched on its "seaweed raft" is perplexing, for me at least...it looks long-legged and long billed. But note also that the carpal bar is much less obvious on the flight shots (pro-Arctic), also, the face pattern looks more in line with Arctic (except for the white orbital ring).

Anyone else cares to comment on the middle bird of this thread: http://www.birdforum.net/showthread.php?t=208975 ?
It's been keeping me awake late last night...

Yes. Kenn K. says in first ed. of Advanced Birding (1990) that underwing pattern, in terms of the extent of dark on the tips of the primaries, is one of the few plumage features that is invariable across plumages and seasons. So Arctic for me too. Looks to be some feather displacement in the area of the carpal bar on the perched shots of the bird which might make it appear darker than normal.

I looked at the middle tern in the thread you linked to Tib78, and you well articulated my thoughts on the bird. But not sure how to go further on it either.

Jim

Well, I admit I was firmly in the Common camp, except for the full set of fresh primaries on a 1st-summer (and a thin of a white unmarked trailing edge) which are more pro- Arctic – that was the main issue I had with it being a Common.
I’m glad Peter added some comments since I respect his views. However, in my mind this couldn’t trump what I perceived as a long-billed, long-legged bird which didn’t strike me as a short-necked or short-legged.
Plus it has the pale orbital rings, which I thought were pretty atypical for Arctic – maybe a feature as reliable as I thought? Only juveniles?

You are all too kind!
To further address the points raised by Julian in his last post:

- I am not sure if leg length can be reliably assessed from these photographs. The bird is standing, or rather trying to stay balanced on floating kelp. In the first photograh, it is raising the visible leg, toes pointing downwards, which may create the impression of a longer leg than usual. Also, keep in mind that a bird that is trying to stay balanced, ready for taking off at any moment, may look slightly longer-legged than when it is relaxed. Compare, e.g. these two photos of one and the same Arctic Tern:
> relaxed: http://www.tarsiger.com/images/pirpa/ti1.jpg
> moments before take-off: http://www.tarsiger.com/images/pirpa/ti2.jpg

- Juv/immature Arctic usually lacks a pale eyering indeed, but there is some variation, and a few birds do show it:
http://www.tarsiger.com/images/masa/Steaea54.jpg

I would agree to the features mentioned by Peter - perhaps also that the black reaches further down under the eye (as often on juveniles), and a broader white forecrown and of course the impression of primaries being of the same generationas, looks good for a 1st summer Arctic indeed.
However, a bit concerned about the seemingly long legs.

JanJ

See that Peter has a good explanation to the leg-lenght

I agree with Julian's views, to me the bird looks like a Common Tern, despite the opinion of the majority of the others, because
1. the legs look way too long for Arctic Tern
2. also bill and neck look too long, like pointed out by Julian
3. like pointed out by Jane, it seems to have replaced the three innermost primaries, which would fit a first summer CT much better than a first summer AT, at least according to Pyle's identification guide
4. like pointed out by Julian, the eyelids are white, and to me also the rest of the head pattern looks better for CT (Arctic has more black around the eyes), compare with these first summer Arctic Terns:
http://www.tarsiger.com/gallery/index.php?pic_id=kunsa1123230906&lang=eng

Julian H said:

full set of fresh primaries on a 1st-summer

Click to expand...

A full set of fresh primaries on a 1st summer Common Tern should not be too unusual at this time of the year, as according to Pyle the moult of the primaries lasts until August. Arctic Tern completes the moult already by early summer, so the feature is more useful a bit earlier (but probably not completely certain even then).

I can definitely see where the others are coming from regarding the narrowly black tips of the primaries, but that's just a single feature against a host of others. Looking at photos of Common Terns the perceived width of the black trailing edge varies a lot, which I presume stems partly from the angle and how the primaries overlap each other. Compare with this adult Common Tern:
http://www.tarsiger.com/images/Jniemi/stehir108copy72.jpg

Tib78 said:

Anyone else cares to comment on the middle bird of this thread: http://www.birdforum.net/showthread.php?t=208975 ?
It's been keeping me awake late last night...

Click to expand...

Also that one looks like a Common Tern to me (long legs).

[Edit: posting at the same time as CAU].

Hmmm.... Seems as though the photos have disappeared from the page they were linked to.

As for leg length, can we actually see the legs in any of the photos? I was assuming the long-legged impression was just an illusion created, e.g. by the bird being on a rock that is hidden by the seaweed. If the observers at the time actually saw unusually long legs then I'd have to reconsider the ID.

Jim

Apparently a new blog entry has removed the photos from the front page, but you can still see them here:
http://mydogoscar.com/birdspot/2011/08/18/an-easy-id/

I also took the liberty to attach lightened versions of two of the photos (the legs should be easier to see now).

I hate to disagree with someone like CAU... because he is usually right!
Also, I know that the pattern on the underside of the primaries can be deceptive in photographs, depending on the angle towards the camera, as rightly pointed out by CAU.

However, I would like to add the following to the discussion:

- Moult: I think there is still a lot we do not know about moult in Arctic Tern. All the books say that they conclude their primary moult in the winter quarters (i.e. Antarctica), but in the field, I have noticed that a few birds replace inner primaries during early summer. Here is an example:
http://waarnemingen.be/waarneming/view/55478216

- Tail: I think noone has mentioned it so far, but 1st-summer Common Tern normally has short outer tail streamers. In the subject bird, the tail streamers are long, and reach the end of the wingtip at rest, as they do in 1st-summer Arctic.

- Secondaries: 1st-summer Common Tern has distinctly dark secondaries, often visible on the underwing too as a dark bar. If you look at the white tips on the secondaries of the subject bird (particularly in this photograph), they become wider inward, towards the body - a feature of Arctic Tern.

- Underwing: It is not just the narrow dark tips to the primaries that attract my attention, but also (and perhaps even more so) the contrast between these (black) tips and very pale (white) underwing.

- Structure: Nobody has mentioned the short arm and very long hand in flight?

- Additional features: The distinct white scapular and tertial crescents may be worth noting. For some reason, these usually seem to be narrower and less distinct in 1st-summer Common Tern.

It is always good to have comparison material in discussions like this, and I think that Shaibal Mitra's collection of 1st-summer terns is one of the largest on the internet.
Common: https://picasaweb.google.com/109808209543611018404/CommonTernsOnLongIsland#
Arctic: https://picasaweb.google.com/109808209543611018404/ArcticTernsOnLongIsland#
Shaibal's photos of 1st-summer Arctic include a bird moulting its inner primaries, and another with prominent white eyering:
https://picasaweb.google.com/109808209543611018404/ArcticTernsOnLongIsland#5197640919303875010
Kevin Karlson also has a few shots of 1st-summer Arctic:
http://www.kevinkarlsonphotography.com/gallery/v/Gulls/terns/

smiths said:

I hate to disagree with someone like CAU... because he is usually right!

Click to expand...

Thanks Peter, I might sometimes have been right in easy cases, but this time I believe I was wrong. On a second look, you are probably right about the bird balancing on the seaweed, making the legs look longer than usual (as well as the neck). I believe the colouration of the secondaries and the long streamers are important points favouring AT, in addition to the trailing edge of the hand (and after comparing with photos, I also changed my mind regrading the bill size). Many features are still difficult to compare with a generic 1st summer Common Tern, as a 1st summer bird may show three different generations of feathers (juvenile, formative and alternate, if we use North-American terminology). For example worn juvenile and fresh formative outer tail feathers must obviously differ greatly in length (but at least according to Pyle's guide, the first alternate moult does not include streamers, which probably reduces the chances of finding especially long-tailed 1st summer Common Terns).

By the way, I found another picture of a 1st summer Arctic Tern moulting the inner primaries:
http://artportalen.se/birds/gallery_imageinfo.asp?imageid=306608

I think Peter's post contained solid information and put forward a good case for the id. as Arctic..I thought it may have gone the other way for a second though!