Hi Mick,
Sorry for the delay in replying to you. Perhaps you and Nikolas discussed this
further on the pelagic on Saturday. I thought I should clear up some issues you
raised that, in my understanding, rule out Lesser rather than Greater
Frigatebird.
I said I thought the spurs of the Tanzanian bird originated off the sides of
the belly patch (consistent with Great). Conversely, you thought they
originated off the front of it, which would eliminate Great. I think this comes
back to a limitation or interpretation of my descriptions in James 2004.
However, there are other ways to word the differences. Bare in mind, that the
descriptions of triangles and ovals refer to 2-dimensional shapes, but the
under surface of the birds is highly 3-dimensional, so the descriptions are
simplifications.
In Lesser the margin between the triangular belly patch and the breast band is
straight(ish). This line forms the base of the rear-pointing triangular belly
patch. This straight line also continues (more or less) to the outer tips of
the spurs; no part of the belly patch extends WELL forward of an imaginary line
between the points of the two spurs (assuming the angle of view is
appropriate). On Great the margin between the oval belly patch and the breast
band is rounded, with the convex belly patch intruding into the breast band.
The spurs technically cannot originate from the front of the belly patch
because the front is confined to the very centre of the chest. I have termed it
that they originate from the sides, but technically there are no sides on an
oval (well, one side only). In this case, perhaps you could say that the spurs
originate from the side between the front and the mid-point of the belly patch.
It starts getting a bit grey and
murky, and becomes impossible to capture all the variation in a simple
sentence.
There are some other points that are more important than the origin of the
spurs:
The shape of the belly patch: The Tanzanian bird has both a rounded front edge
and the broadly rounded rear edge creating an oval (if you hide the spurs -
colour them in black using a graphics program if you like). Juvenile Lesser
always has the belly patch narrowing to a blunt point at the rear. Together
with the straight front edge, this makes a distinctly triangular shape, if you
cover up the spurs. Actually, however, the spurs tend to add to the triangular
shape of the belly patch by stretching out the front (basal) corners. On this
particular bird note that the the rear of the belly patch is fairly broad. It
is a little bit square at the rear, not perfectly rounded as would be
expected in a true 'oval'. The important detail is that there is a fairly broad
separation of the black on the flanks right to the back, rather than a
gradually narrowing.
The spurs themselves are pointed or triangular on Lesser and blunt-tipped on
Great. On Great they are often also jagged. I've not looked into why this is,
but because each feather is either black or white (no white-tipped feathers) it
must be something to do with the shape, size and number of feathers
involved). The Tanzanian bird is completely inconsistent with Lesser in this
detail.
On the Tanzanian bird, it is fair to say that the spurs are longer than normal
for a Great Frigatebird, apparently reaching beyond the flanks and onto the
axiliaries. Also the spurs originate well forward on the belly patch. These are
the only two points that I can construe against Great, and I think they can be
explained.
I have seen one (almost) definite example of a Great with spurs this long
(longer in fact) photographed by Grant Penrhyn at Ashmore Reef in October 2007.
It was first thought to be a Lesser, then a CI, but in my view it was a Great.
I have a discussion paper on it which I'll send to you separately. Therefore,
if you accept that the Ashmore bird was a Great then the Tanzanian is not
unprecedented. This might be a circular argument however.
The spurs of the Tanzanian bird might be further forward than usual because we
are dealing with a different subspecies to 'usual'. Subspecies are
unfashionable at present in Great Frigatebird, but there is a lot of
geographical variation with at least 3 subspecies in the Pacific, 2 or 3 in the
Indian and 1 in the Atlantic Oceans. The form in the western IO, 'aldabrensis'
might have spurs further forward than the forms I'm used to (palmerstoni in the
SW Pacific, listeri or nominate minor on CI, and unnamed forms (possibly
aldabrensis or minor?) at Asmore and Cocos (Keeling) Islands. But this is
speculation. Perhaps it is part of the variation across all subspecies.
Overall, frigatebird ID is in it's infancy, the birds vary greatly, and James
(2004) certainly does cover all variation and nail every aspect. You can expect
variation. I think the Tanzanian bird just shows that some Greats go a little
outside the descriptions in James (2004) that were based on my experience and
language limitations at the time.
Cheers,
David James
Sydney
==============================
________________________________
From: Mick Roderick <>
To: Nikolas Haass <>; Mike Carter <>;
Steve Clark <>; ""
<>
Cc: 'David James' <>
Sent: Wednesday, 16 January 2013 10:06 AM
Subject: Tanzanians need help with Frigatebird id
Nikolas,
Would you mind expanding on the 'axillary spur business' as I'm now thoroughly
confused and would have though that this ID could have ruled out Great on the
basis of the spurs (hence my confusion).
David comments that juvy Greats (when they actually have spurs) have "spurs
that originate off the side of the belly patch and extend only onto the flanks,
just like this bird." I am actually seeing the spurs originating from the front
of the belly patch, and this combined with the prominence of the spurs and the
possibility that they are also just reaching the underwing has me confused -
maybe this is what you're referring to?
I'm very happy to concede the subject bird is not an Ascension (and I
originally thought Lesser would be a better fit than Greater but I take on
board David's comments there) - moreover I'm curious about your comment on the
spurs (i.e. should we not be using them as a reliable character?).
Mick
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