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Juvenile/Second-year Frigatebird, Tainan Taiwan May 2020

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Old Sunday 24th May 2020, 00:18   #1
SteveMM
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Juvenile/Second-year Frigatebird, Tainan Taiwan May 2020

I think this is an 'older' juvenile frigatebird as the black breast sides/tabs have not yet been replaced with white. I thought it must be Great Frigatebird in the field due to the shape of the belly patch (large and rounded below the breast tabs) and the absence of white axillary spurs. However, when I look at the photos, there are one or two isolated white feathers on the underwing well forward of the breast patches which should not be there on Great.

I see on average a couple of Lesser Frigatebirds a year, and these are obvious on account of their small and triangular-shaped belly patches (with much less white on the hindflanks) and axillary spurs. As the white belly patch should get smaller (not larger) with age in both sexes (and in both species), I'm not really sure which one this is!

Steve
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Old Sunday 24th May 2020, 04:44   #2
andyb39
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I believe it's a 2nd-year Christmas Island - buff head, lack of a breastband and lack of axiliary spurs makes it 2nd year, plus the diagnostic black breast "tabs" and heart-shaped belly patch make it CI. See plate 20 and the table on p.37 here:

http://citeseerx.ist.psu.edu/viewdoc...=rep1&type=pdf
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Old Sunday 24th May 2020, 08:44   #3
SteveMM
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Quote:
Originally Posted by andyb39 View Post
I believe it's a 2nd-year Christmas Island - buff head, lack of a breastband and lack of axiliary spurs makes it 2nd year, plus the diagnostic black breast "tabs" and heart-shaped belly patch make it CI. See plate 20 and the table on p.37 here:

http://citeseerx.ist.psu.edu/viewdoc...=rep1&type=pdf
Thanks, Andy.

I read the paper you link to before I posted and came to much the same conclusion. I had a hard time matching this bird to either Great or Lesser Frigatebird, but could see no difference between it and e.g. the one here: http://orientalbirdimages.org/search..._ID=&Location=

I am, however, wary about what variation might exist between these three, not least as CIF would be the first national record.

Steve
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Old Sunday 24th May 2020, 09:45   #4
andyb39
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It might be an idea to consult David James on this - you can contact him at [email protected]. He was very helpful indeed with my own chance sighting of CIF in Malaysia.

I thought it might be rare in Taiwan but I didn't know it would be that rare. However, your images are of a very high quality and clearly show the diagnostic breast tabs.

I wonder if this has anything to do with Cyclone Amphan. There have been numerous sightings of pelagic species quite far inland in West Bengal over the last few days - groups of Short-tailed Shearwaters flying up the Hooghly, for example. Our own Seth Miller reported a Wedge-tailed Shearwater in Dhaka!

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Old Sunday 24th May 2020, 11:18   #5
SteveMM
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Quote:
Originally Posted by andyb39 View Post
It might be an idea to consult David James on this - you can contact him at [email protected]. He was very helpful indeed with my own chance sighting of CIF in Malaysia.

I thought it might be rare in Taiwan but I didn't know it would be that rare. However, your images are of a very high quality and clearly show the diagnostic breast tabs.

I wonder if this has anything to do with Cyclone Amphang. There have been numerous sightings of pelagic species quite far inland in West Bengal over the last few days - groups of Short-tailed Shearwaters flying up the Hooghly, for example. Our own Seth Miller reported a Wedge-tailed Shearwater in Dhaka!
Thanks again, Andy. Will do!

The impression I got from the paper was that the breast band of juvenile frigatebirds gradually breaks down and whitens from the centre, which could then leave isolated 'tabs' on the breast sides in any of these three species in older/worn juvenile plumage. I had no idea that the tabs were considered diagnostic of CIF, but they are there and are indeed 'angled slightly forwards'.

I've attached some more (poorer quality) images which show that the apparent weak 'axillary spur' on the right side of the bird is on the body only and does not reach up onto the underwing. Lesser Frigatebird can at least be eliminated on a combination of this, the large white belly patch, and the rather long bill.

Fascinating to hear of so many Short-tailed Shearwaters way over there in the Bay of Bengal! I've always fancied this to be a cold water species as the birds that pass through here do not hang around at all. I wonder what happens to them? Do they successfully over-summer at that latitude, correct themselves, or just starve?

Steve
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Old Sunday 24th May 2020, 12:24   #6
andyb39
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Hi Steve,

I had always thought that the breast tabs were not remnants of the breast band but evolved independently of it and are in any case positioned slightly lower. I may be wrong. They are in any case still present in 3rd year birds according to the table, so presumably not just a remnant.

Again according to that table, Great has neither axillary spurs nor breast tabs, so should be ruled out. Lesser has no breast tabs and in any case lacks the hulking, heavy-billed shape of your bird.
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Old Sunday 24th May 2020, 13:23   #7
SteveMM
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Quote:
Originally Posted by andyb39 View Post
Hi Steve,

I had always thought that the breast tabs were not remnants of the breast band but evolved independently of it and are in any case positioned slightly lower. I may be wrong. They are in any case still present in 3rd year birds according to the table, so presumably not just a remnant.

Again according to that table, Great has neither axillary spurs nor breast tabs, so should be ruled out. Lesser has no breast tabs and in any case lacks the hulking, heavy-billed shape of your bird.
Cheers, Andy.

Yes, I do see what you mean. In view of the rarity of CIF here, though, I was looking for ways to explain how a Great Frigatebird might come to have an underpart pattern like this one, not a CIF. My speculation that the breast band may get 'partially moulted out' and leave a remnant 'tab' is me trying to fit a square peg into a round hole. The more inaccurate this is, the better, as it only strengthens the case for Christmas Island Frigatebird!

Steve
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