The identification of tattlers, Pycroft's vrs Cook's and Westland vrs Black
Petrels - a reply to Hansbro.
After reading Phil Hansbro's post of 17 Sept. 97 on the identification of
tattlers and various tricky pairs of petrels, I feel that his post contains
so many errors of fact and is so misleading in parts that I feel I must
comment. Note that the following comments are offered in the spirit of
constructive critisism and in no way should be taken as a personal attack on
the no doubt well-intentioned comments of Phil (whom I've never met!).
First up, Tattlers: Most of what Phil said about the separation of
Grey-tailed and Wandering Tattlers in non-breeding plumages is correct
except for one point, the head pattern. Phil states that in WT the
supercilia meet across the forehead whereas they do not in GTT. In fact it
is the other way round (!) for it is in Grey-tailed that the white
supercilia meet rather broadly and obviously across the lower forehead cf.
Wandering, in which they are separated by the dark of the upper forehead
extending in a narrow point to the base of the upper mandible. It should
also be noted that this character applies only to adults and first-basics in
full non-breeding plumage. It does not apply to birds in juvenile or summer
(alternate) plumages, in which the head pattern difference breaks down and
there is overlap between the two sp. All this and more was covered in the
Field ID accounts I researched and wrote for the Handbook (HANZAB Vol 3). It
should also be noted that the longer wing-points of WT, while at times a
useful supplementry character, is not in itself an infallible one; it too is
subject to some variation (within both spp.) and overlap and should be used
with caution. I suggest that birders wishing to bone-up on their tattler ID
should refer to HANZAB for an A'asian perspective on ID and to the excellent
discussion of this tricky pair (with lots of beaut. photos) in Dennis
Paulson's excellent 'Shorebirds of the Pacific Northwest'.
Pycroft's vrs Cook's Petrels:
Hansbro states that the supposedly darker-capped appearance of Pycroft's is
not a good feature for separating this sp. from Cook's. While this is
essentially correct, his other comments about head pattern differences
between these spp. are wrong. Judging from a study of the recent ID
literature (see below), a number of skins and photos of others plus my own
experience of 100+ Cook's at sea, there appears to be no reliable difference
in the forehead pattern as both species can have a chain of dark mottling
extending through the forehead to the base of the bill. Further, this chain
of mottling varies considerably within both species, no doubt due in part to
such factors as plumage wear and moult. In any case, it is near impossible
to see this mottling in the field - one simply just doesn't see these
species at ranges of generally <100 m, making it impossible to evaluate any
supposed differences in forehead mottling in the field. Lighting effects
further compound the problem as in strong light the entire forehead of
Cook's invariably appears wholly white and somewhat bigger than its true
extent. I agree that the on-average different extent of white in the outer
tail-feathers is of little value as a field character as there is too much
overlap between the species. Nevertheless, the tail pattern has some value
as a supporting character under certain circumstances: a pale-plumaged
Cookilaria that looks in all other respects like a typical Cook's and shows
extensive white in the outer rectrices when the tail is partly fanned is
undoubtedly a Cook's! When watching many Cook's in the Tasman Sea in Jan-Feb
1996, I frequently saw them 'flash' the extensively white outer rectrices
and this is a very typical field impression of Cook's under such
circumstances; at other times (i.e. when tail is held tightly closed in fast
direct flight) the tail pattern appears no different b/n the two sp. as one
cannot then see the outer rectrices. The overlap and usefulness or otherwise
of the average tail pattern difference b/n Cook's and Pycroft's has been
adequately documented and discussed in a field ID context by Roberson &
Bailey in their seminal 2-part paper on the ID of Cookilaria petrels
(American Birds 45:399-403 and 1067-1081), to which interested readers are
referred. Such persons should also read the important subsequent papers on
Cookilaria ID by Spear et al, 1992 (Colonial Waterbirds 15:202-218; contains
excellent photos of museum specimens showing head and underwing patterns)
and by Howell et al, 1996 (Western Birds 27: 65-69; this one especially
important for separation of Cook's and Pycroft's). In reading these papers,
birders should also note that the more recent one incorporates commentry on
some of the field characters presented in the earlier ones, i.e. the Howell
paper especially has field-tested all of the major field characters proposed
by earlier authors and presents caveats to earlier discussions of ID
characters as well as suggesting which of these is the most reliable for ID
in the field. This brings me to a further and very important point about the
field separation of Cook's and Pycroft's, namely another aspect of the
head/neck pattern. Both Spear et al and especially Howell et al point out
that the single best (i.e. most reliable) character for separating these two
species is the presence in Pycroft's of an obvious notch of white that
extends upward behind the dark ear-coverts, creating an obvious division
between the dark ear-coverts and the dark half-collar on the neck-sides
immediately behind. This white notch effect is typically absent in Cook's or
at best, only vaguely suggested thus the two species have a very different
head/neck pattern that can be summarised as: Cook's has a well-defined grey
cap and no half-collar effect whereas Pycroft's has a dark cap plus an
obvious dark half-collar. The resulting head pattern of Pycroft's is in fact
much closer to that of Stejneger's Petrel than it is to Cook's. This
actually makes sense since Pycroft's is actually taxonomically closer to
Stejneger's than to Cook's; Pycroft's is more correctly considered the
south-west Pacific representative of Stejneger's rather than forming a
taxonomic species pair with Cook's. A few further points: Phil H reckons
that the slightly thicker and more extensive dark carpal bar on the
underwing is another difference from Cook's. Birders should note that
Roberson & Bailey, Spear et al and Howell et al all have field-tested this
subtle in-the-hand difference and found it useless in the field! This is
chiefly because the effects of lighting, distance and the so-called
sea-glare illusion effect can make it very difficult if not impossible to
discern such a fine difference in average field conditions; I fully agree
with them. What's more, none of those authors has discussed another and
possibly more useful difference in the underwing pattern, namely the
apparently large dark wing-tip area shown by some (but not all!) Pycroft's.
My evidence for this character is based on only one museum specimen so it
remains to be fully checked out so I won't bother going into it here.
Finally, Phil H points out that differences in the timing of the primary
moult may be a further useful supplementry character for separating Cook's
from Pycroft's. What he doesn't say is that this difference (first pointed
out by Spear et al) does not apply to birds in Australasian waters because
adults and sub-adults moult their wings while on their non-breeding grounds
in the Eastern Tropical Pacific and never while in our area; and juveniles
don't moult their primaries during their time in our waters either. Thus out
of the 100 or so Cook's and of the 1000's of Gould's Petrels Ive seen in the
Tasman Sea, not one has ever shown any sign of active wing moult (and yes, I
do keep extensive moult records of these and all petrel species)! As far as
I know, locally-collected specimens in Aust. and NZ museums support this point.
By the way, should anyone think that I have read these ID papers and just
accepted the authors views uncritically, nothing could be further from the
truth! Being a seabird freak, I have invested an enormous amount of time
thinking about and researching these matters, both in the museum and during
the many research cruises I've undertaken. These have allowed me to field
and museum-test many of the characters discussed above and, in fact, both
Chris Corben and I quite independantly came up with many of them ourselves
in the years before they were published by the Americans. I should also add
that I paid particular attention to the field characters of Cook's while on
a 36-day cruise in the Tasman Sea and NZ waters in 1996 and was able to
critically assess many of the characters outlined in the above papers. The
result is that, while fundamentally correct, even these quite detailed
papers don't actually describe the many nuances and subtle variations of
cap-shape, colour and contrast that a Cook's can show. For example, I noted
many times that a Cook's seen in strong light can appear quite dark-capped
momentarily; that some Cook's can show a hint of a pale notch behind the
ear-coverts (nothing like as big and obvious as on a Stejneger's or a
Pycroft's though); and that the bottom half of the dark area on the sides of
the neck can often appear darker than the rest of the grey cap; and so on...
The point I am making is: birders should field-test any published ID
criteria for themselves as far as possible as this will prepare them for any
future encounter with a suspected rarity as well as giving them an insight
into any limitations of published data.
Westland and Black Petrels:
Alas, as with his comments on Cook's v Pycroft's, many of the points
mentioned by Phil about this tricky pair are also misleading or wrong.
Before I spell these out, let me begin by stating that the fundamental ID
characters for separating this pair in the field are already published in
some detail in Vol 1 of HANZAB. I researched and wrote those accounts using
(A) my own extensive field experience; (B) study of museum skins including
the largest available collection of specimens of these species, namely those
in the National Museum of NZ in Wellington; (C) correspondance and review of
texts by Mike Imber (both spp.) and Sandy Bartle (Westland mainly), the
acknowledged world experts on these species at their breeding grounds. Mike
Imber also kindly examined all of the NMNZ skins and drew the bill pattern
and colour variation for me as well as supplying me with unpublished photos
of birds of known age and provenence. All of this adds up to what still
remain fundamentally accurate accounts of the field characters and to my
knowledge, no one has yet improved upon them in the literature. Moreover, my
own subsequent field experience and that of various colleagues continues to
bear me out on all important points. Thus any birder wishing to get a handle
on this pair should start with the HANZAB accounts. Tony Palliser and I are
currently researching a forthcoming paper that will deal with this tricky
pair in some detail, and we intend to include many photos and some
illustrations to help get the important points across. And now to the specifics.
Size and shape: Phil H is correct in stating that Westland (hereafter WP)
has a more barrel-chested appearance than Black P (BP) but that is not the
only structural character. In fact, these two species are best separated not
only by the marked size difference, WP being essentially the same size as
White-chinned P, but also by a combination of the following structural
characters: Westland differs from Black by: (A) a noticeably bulkier-bodied
or barrel-chested appearance (with a more rotund or 'pregnant' belly
profile); thicker bull-necked appearance; (C) a relatively bigger head; and
(D) proportionately slightly broader wings. Taken together, these add up to
a noticeably bulkier looking bird when directly compared with a Black P,
admittedly something that is rarely possible to do in our area. Importantly,
WP appears essentially identical in size and shape to a White-chinned P
(though it can look marginally smaller and slimmer) whereas Black appears
not only somewhat smaller than either but, more especially, it appears
slightly slimmer-bodied and distinctly slimmer-necked with a rather smaller
and often more rounded head; the wings appear somewhat narrower and longer
proportionally but this difference is only really possible to evaluate when
either a WP or a WCP is available for direct comparison. Likewise, the other
structural and size differences are much better appreciated with a direct
comparison and thankfully, White-chinned P's are sometimes around to help
out! However, it should be stressed that these differences can often be very
hard to evaluate on lone birds (I've been tricked initially more than once
or twice myself!). But with experience, the differences can usually be
discerned, provided that the bird stays round the boat long enough for it to
settle into it's natural relaxed flight mode i.e. the one in which these
structural differences are most apparent and easily/reliably judged.
Otherwise it becomes easy to think that e.g. a Procellaria that looks
slim-necked is a Black when in fact it is just a Westland craning its neck
in excitement as it manoeuvres round the boat in search of food scaps etc.
Similarly, all three dark Procellaria typically look slim-necked when
swimming in their typical alert posture with neck up-stretched. The ability
to 'pick the moment' or flight mode and then correctly judge the shape,
structure and typical jizz of the species comes only with experience and
practice. I should also add that the structural differences are often hard
to discern in anything but a reasonably long series of flight photos as just
one or two shots may not capture the bird in the ideal flight mode from
which to make a good assesment of these points. Thus birders seeking to
document their Procellaria should take as many flight photos as possible in
addition to close-ups of the bill characters when a bird is on the water. By
the way, I disagree with Phil's statement that there is less difference in
size between these two spp. than there is variation within them. I would
argue the opposite and I think that the measurements support my view. The
thing of fundamental importance when judging matters of size, shape and
structure in the field is to have a direct comparison with a species of
known size and shape. In the case of this pair, only a direct comparison
with another dark Procellaria species is really helpful; comparison of size
against a similarly dark Puffinus such as a Fleshy-footed Shearwater might
help; but comparison with a completely different species and genus such as a
Great-winged or Providence Petrel is of little or no value as these spp. are
just too small and so very different in shape as to make anything compared
with them look radically different! I also disagree with Phil H that size
[and shape] is not so important as concentrating on bill shape, colour and
pattern. Rather, establishing the size, structure and jizz of a WP or BP is
the fundamental first step in the ID process and much effort should be
expended in working these things out.
Bill shape: I have some real problems with some of the assumptions
underlying what Phil H has to say about these characters. I believe that
many of his points about bill shape and stucture are based on
misinterpretation of photos and perhaps even skins. For example, he says
that the latericorn of WP has straight edges whereas photos I have on file
of both live and dead birds and of museum skins and a check of the skins
themselves shows that the top and bottom edges of this bill plate are quite
curved in all Westland P's as well as in all species of the genus
Procellaria, the bottom edge of the latericorn being the more strongly
curved of the two edges. I believe that his notion of straight sides is
based on a misinterpretation of a photo of Tony Pallier's (shown to me) in
which the actual (curved!) bottom edge of the latericorn is faint but the
top edge of the relatively straighter ramicorn is visible and has been
mistaken for the former! Similarly, no skin, photo or live Procellaria that
I have studied (and indeed no petrel species!) has what could even remotely
be described as a rectangular or square-shaped mandibular unguis! The very
fact that this bill plate has a strongly-curved cutting edge forming part of
the strongly hooked bill-tip proves this to be the case; and the bottom edge
of the plate is also strongy arched. Again, the explanation lies with a
misinterpretation of the same photo or series of photos in which the pale
area on the basal part of the mandibular unguis does appear somewhat
square-shaped but the unguis as a whole does not even remotely appear so. In
summary, all of the points Phil mentions about bill shape etc are suspect or
wrong with the sole exception of the general impression of bill proportions
between these two species: it is true that WP has a relatively long and
strongly-hooked appearance c/w the relatively short stout bill of a Black.
However, I must emphasise that such shape and length differences are, in the
absence of a direct comparison, really quite subtle and subjective ones that
are further complicated by slight sexual differences in overall length etc.
I would advise any one trying to ID this pair on bill characters to
concentrate as much if not moreso on bill colour and pattern.
Bill colour and pattern: on this subject, Phil's opening statement that
there is more black on the bill of a Westland P requires some qualification.
There appears to be little or no recognition within either the literature
(excepting FID in HANZAB) or the Aust. birding community that there is
almost complete overlap b/n these species in the extent and pattern of dark
areas on the bill. This has been fully established by study of skins, photos
of birds of known identity and discussion and correspondance with NZ workers
who know these species well at their breeding grounds. Every photo of a
Westland P taken off NSW that has been shown to me (mainly of the 1996
winter birds) features a bird with a bill pattern that could fit either
species. It is only those Westland P's that show a wholly black culmen and
maxillary unguis that have a diagnostic bill pattern; and this pattern is
found only in full juveniles (which usually also have all-dark nostrils) and
in some adults at the darkest end of the bill pattern spectrum. Thus a
Westland with any significant pale areas on the basal parts of either unguis
invariably has a bill pattern that fits nicely within the more limited bill
pattern variation of Black P. By the way, Phil H is also incorrect in
stating that the cutting edge of the bill is black and trumpet-shaped. This
character is actually the sulcus and, whats more, there is absolutely no
difference b/n WP and BP in its extent! Finally, the colour of the pale
areas of the bill: once again PH has it the wrong way round for it is Black
P (adults and perhaps some or all sub-adults but not juvs) that has these
areas typically yellow with a greenish tinge. In contrast, adult and imm.
Westland P's have the pale areas a distinctly cream, ivory or horn colour
and any greenish tinge detected in the field is usually the result of
reflection off the waters surface! Should anyone doubt me on this, I have
checked these colours out with Sandy Bartle and Mike Imber and the latter
even sent me a nice slide of an adult and a juvenile Black P side-by-side
proving that Black has a strongly yellowish tone to the bill while a fresh
juv (never seen in Aust. waters) has a ghostly bluish-white colour to the
pale areas. All good colour photos (and live) Westlands Ive seen have shown
the typically creamy or horn-coloured bills said by Sandy Bartle to be
diagnostic of adult and perhaps some sub-adult Westland P's. Perhaps Phil
has been confused on these points by not realising that there is
considerable age-related variation in both bill colour and pattern in both
species (see HANZAB). I should also stress that evaluating the true colour
of the pale areas on the bills of these species is almost always very
difficult due to lighting conditions. In strong light, all 3 dark-plumaged
Procellaria spp. can appear to have the pale areas of the bill a very pale
whitish colour and I have already remarked above on how reflection off the
waters surface can impart a slight greenish tinge to the bill at times. The
colour of the pale areas of the bill and the extent of any dark areas is
best judged at close range and in good even light and, preferably, under a
cloudy sky as the reflection and burn-out factors that affect perceptions of
bill colour and pattern are then much reduced.
Well, thats it. If I have been too harsh on Phil's no-doubt well-intentioned
comments, then I apologise. I hope that these comments will be taken as
constructive ones and I am happy to debate any points further.
David W Eades
Sub-editor Field Identification section
HANZAB
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