RE: Knots and Petrels

To: "'Trevor Ford'" <>, "'Rohan Clarke'" <>, "'Mike Carter'" <>
Subject: RE: Knots and Petrels
From: "Jeff Davies" <>
Date: Wed, 21 Oct 2009 13:57:14 +1100
G'day Trevor,


You have thrown the focus on a very important issue, namely the precision
required when allocating a limited number of words to a field ID description
in a book that is going to be taken very literally. Exactly the same applies
to field ID illustrations, they may be simplified but when viewed by the
reader they will pick up every brush stroke in an attempt to find features
to look for.

The photo in Harrison is very poor resolution and I would suggest the grey
on the outer webs of tail are pixilated out of existence. The impression in
this photo of darker grey towards centre of tail is correct, on the outer
tail feather the grey, which is actually vermiculation is confined to the
outer web, so actually gives stronger impression when tail is more closed
because you are not seeing the white inner web. This inner web however is
always overlaid by other feathers which are darker dorsally and never
matches the translucence of White-headed which shows a blindingly white tail
when fanned. So you can see it is much more complicated than just being a
grey tail, and depending on where the discussion goes dictates how specific
we need to be in describing it.

The white tips in the Harrison WH Petrel is an impression created  due to
translucence. When the tail is only partially fanned the areas not
overlapped are going to allow greater light penetration, I could do a
drawing of how this is manifested in this particular photo but I think you
can work it out, as the tail fans more the bits of feather not over laid
increases inwards on the tail until in an extreme fan only the bases are

I think the lesson in all this is that words are an imprecise tool so you
need take them in the context of the discussion and how the writer was
intending them to be taken. Grey tail is an impression and is OK to use in
context when comparing to another species which has a white tail, but if you
want to get down to specifics of the tail up close then it is complex grey
vermiculations distributed in a pattern that diminishes for each feather as
you move outwards from the central pair and this is in turn is effected by
the darker dorsal surface in relation to translucence. 

In regard to the Knot Danny Rogers has even thrown in the possibility that
this bird is a hybrid which would obviously be an easy way out but may in
fact be the true answer in this case. 


Cheers Jeff.


From: Trevor Ford  
Sent: Wednesday, 21 October 2009 1:05 PM
To: Rohan Clarke; Jeff Davies; Mike Carter
Cc: birding-aus
Subject: Re: Knots and Petrels




Many thanks for all the feedback and, yes, a wonderful shot of a
Soft-plumaged Petrel, Rohan. No disputing a dark undertail there! It's a
shame if Onley and Scofield refer to undertail coverts as undertail,
especially as the Harrison identification guide describes these features
correctly. Wondering how they would describe the undersides of tail
feathers, I looked at a few other species, such as Black-browed Albatross
and Blue Petrel, but as far as I can tell there was no mention of undertail
or undertail coverts. Incidentally, their illustrations of pale phase
Soft-plumaged Petrels show only a hint of pale grey on the undertail.


In the Harrison photographic guide, the Soft-plumaged Petrel shows broad
white outer undertail feathers and a white tip to the undertail, which is
otherwise pale grey. The White-headed Petrel appears to show a white tip to
a pale grey undertail. The photo of the White-headed Petrel that we've been
discussing appears to show a pale grey undertail. I'm not disputing the
identity, I'm just struggling to appreciate how definitive one can be when
viewing the undertail feathers shown in some photographs.


With regard to the Red or Great Knot, I'm also leaning to Jeff's Red Knot
suggestion. After recovering from the initial impression of long bill and
dark spotting, features such as size and jizz snap in. In fact, I "phoned a
friend" and he suggested that the "snouty look with a thick-based straight
bill" were good features for Red Knot. Great turn of phrase!


Maybe I should tackle Red v Great Knot for the next ID guide on my website?
I had thought it pretty straightforward but there are obviously a few
pitfalls out there.


Cheers - Trevor.

Trevor Ford




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