help with Costa Rica Pacific shearwater ID

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JRZ

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Jul 12, 2008, 12:56:49 PM7/12/08
to Seabird News
Hello to all,

between March 21 and April 1, 2008 Bruce MacTavish observed several
all dark shearwaters while working off the Pacific coast of Costa
Rica. He took a series of photos of one bird on March 29. Five photos
of this bird can be seen at http://pajareroscostarica.blogspot.com/
He originally thought that this bird and the others were Short-tailed
Shearwaters but the identification seems to be more likely Christmas
Island Shearwater. This would be a new species for Costa Rica and so
we'd like to hear comments from anyone on the list who has experience
with Christmas Island, Short-tailed, and dark morph Wedge-tailed
Shearwaters or anything else that might match the bird in the photos.

Steve Howell has already commented briefly that he sees this bird as a
Christmas Island Shearwater based on the dark chocolate color and
longer tail (when compared to Short-tailed). He believes that it would
be the expected species at that time off Costa Rica.

Here also are comments from Bruce about the original sightings and
some thoughts from Guy Macaskie:

(note: Bruce is presently on another cruise, this time in the foggy N.
Atlantic, and has limited internet access, so I'm helping to get this
out on his behalf. I'm also interested as the NAB editor for Costa
Rica)

<bmact...@islandnet.com>

Hello,

This won't take long. I am Bruce Mactavish, a birder from
Newfoundland, Canada. I spent a month (mid March-mid April) this
spring off the west coast of Costa Rica and Nicaragua on a seismic
vessel. The seabirding was
excellent. It was my first time seeing Pacific pelagic species. I
saw a handful of dark shearwaters that I identified as Short-tailed
Shearwaters. According to the books if Short-tailed Shearwater were
even to occur off
Costa Rica it would be more likely in fall than spring. Attached are
photos of one bird taken on 29 March 2008. Christmas Island Shearwater
was the other species I was considering. It is difficult to get a
grasp on this species from the books I had. Apparently it is related
to the Audubon's Shearwater and should be relatively small. This bird
while a little shorter than the Wedge-tailed Shearwaters was overall
not really what you would call a small shearwater.

I am hoping this is a straight forward identification for you.

Thanks for your time
Bruce Mactavish
St. John's, Newfoundland

"Guy" <guy...@pacbell.net>

Bruce,

I have seen many Short-tailed Shearwaters, but no Christmas Island
Shearwaters. I do not believe the bird photographed is a Short-tailed
Shearwater. To me Short-tailed Shearwaters have decidedly shorter
bills
and noticeably "higher" foreheads than does the bird in the
photographs. In addition, the bird photographed looks longer tailed
than do Short-tailed Shearwaters. The under-wing also appears too dark
for an average Short-tailed Shearwater.

Guy McCaskie
954 Grove Avenue
Imperial Beach, CA 91932

thanks, and looking forward to your thoughts.

JZ

Greg Gillson

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Jul 13, 2008, 1:44:18 AM7/13/08
to Seabird News, ebi...@gmail.com
I agree with the comments already made about this bird not fitting
Short-tailed Shearwater. The tail is just too long and pointed. The
bill is on the long and thick side for Short-tailed as well.

The wing of the photographed bird seems too wide at the base and the
"hand" seems short and pointed. The wing on Short-tailed usually
appears thin and rather evenly wide throughout, coming more to an
abrupt point toward the tip. I cannot tell whether the step in length
between the outer two primaries and the others indicates molt or not.
The wing looks an odd shape for Sooty/Short-tailed.

The bill color does not fit Flesh-footed. There is no hint of Sooty
underwing pattern in the median primary coverts.

It has a definite shearwater bill, so it is not a dark fulmar,
Pterodroma, or one of the black petrels.

The description of Christmas Island Shearwater in the new Onley and
Scofield "Albatrosses, Petrels, and Shearwaters of the World" is of a
short round tail and rounded wings. That does not seem to be the bird
in the photographs. However, the photo in Harrison's Seabirds shows a
medium-length, pointed-looking tail, similar to the Costa Rica bird.

The long pointed tail leads me to lean toward Wedge-tailed Shearwater.
The tail isn't excessively long, so not definitive. The wing looks
wider at the base. But I've already discussed the possibility of wing
molt creating the wrong impression. My experience with these is
minimal, but think that Wedge-tailed holds its wings more forward than
this bird with wider secondaries. I am not sure of status of dark-
morph birds in the Revilla Gigedo population.

My guess is Christmas Island Shearwater or perhaps, without anything
to judge size, a Wedge-tailed Shearwater. I'd even choose an aberrant
dark underwinged Sooty over Short-tailed. In fact, why not an aberrant
Sooty?

If I may, my recent Birding article on separating Sooty and Short-
tailed may help with descriptions of shape that are hard to describe
in words: http://www.americanbirding.org/birding/v40n2p34.pdf

Greg Gillson
Hillsboro, Oregon

JRZ

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Jul 14, 2008, 12:52:32 PM7/14/08
to Seabird News
Thanks to all those who have responded so far, both on and off list,
although my hope was to have a good discussion on list so that we can
all learn from it.

There seems to be a pretty good consensus that the bird in the photos
isn't a Short-tailed Shearwater. There have been a few votes for dark
morph Wedge-tailed Shearwater. I'll list here those comments as
anonymous as they were received off list, but I think they are
important the discussion. Also some new comments from Bruce. Note that
light morph Wedge-tailed was by far the most common shearwater during
Bruce's cruise off Costa Rica (trip total of over 11,000 with a few
daily highs of 2500+). Unfortunately, the photos of WTS he mentions
didn't make it out on his poor internet connection.

As far as considering Christmas Island Shearwater as an unexpected
species just because it isn't on the Costa Rica list, I certainly
think we don't know enough yet about what's going on out there to say
that. Based on what I've seen on this list about occurrences at well
birded areas, such as off the N. Carolina coast, it seems there can be
fairly short windows of opportunity for pelagic species that would be
"expected" at a certain time but considered rare at others.

Anonymous 1:

Hey there,

That dark shearwater is not a Christmas, or a Short-tailed, but a dark
morph Wedge-tailed Shearwater. Christmas is a rather small and rounded
winged shearwater, actually more closely related to Galapagos
(Audubon's) Shearwater than to the Sooty/Short-tailed pair. It is also
not a Short-tailed for some similar reasons, and the ones noted by
Guy. Compared to Sooty, Short-tailed is stubby looking, short and
small billed, high forehead, tends to have a pale chin, dark hooded
look, and more round-tipped wings. The Costa Rica bird is long and
slim, with a long and wedge-shaped tail, as well as a crooked wing
structure that you see on wedgies. The weird look of the bird is due
to molt, with outer primaries in molt, and some patchiness above due
to molt. The bill is long, and this is good for Wedge-tailed.
Additionally you can see the pale fringes on the humerals that are
classic on WEdge-tails, as well as pale edging to the secondary
coverts. These are patterns not shown by other all dark shearwaters.

cheers

Reply from Bruce Mactavish: Thanks for the response. I am surprised
you think this is a Wedge-tailed Shearwater. In life the dark
shearwaters looked smaller, shorter, than the Wedge-tailed Shearwater
which were abundant for comparisons. I just went through many WTSH
shots I took in Costa Rica and could find none with a tail short and
thick as the 'dark shearwater'. Compared to a WTSH, the dark
shearwater in the photo is shorter and broader winged, the rear/tail
of the bird is thicker based coming more abruptly to a taper and looks
shorter. I lighened up the one picture to make sure the tail feathers
were in fact present. WTSH have such narrow and long rear bodies and
very long tail feathers.

The dark bird looks slightly heavier chested, shorter necked(?)
thicker necked(?) more solid compact over all. The outer two most
primaries are not yet moulted so the wings should not look much longer
if the full primary set was there. By the way for what it is worth,
WTSH still in primary moult didn't seem to have this notch where the
older outer primaries were noticably longer then the new primaries
growing in next to them. In fact looking through my shots this morning
I saw only one WTSH that still had an old primary. There were however
a number still growing out the outer most primaries.

The pale fringes on upper greater cing coverts could be due to wear.
If Christmas Is Shearwater is really related to Audubon's (Galapagos)
Shearwater, then maybe they share the pale fringed greater coverts in
worn plumage shown by many Audubon's off CR in March and April with a
pale bar on upper wing formed by bleach tipped greater upper wing
coverts.

Attached are three Wedge-tailed Shearwater shots. I can only send
three per email. I have lots more, none field guide quality!

Cheers,
Bruce Mactavish
Grand Banks, Newfoundland

Anonymous 2:

Hi all,

Re the mystery shearwater, the underwing photos unfortunately aren't
very clear, but from what I can make out, I don't see why the bird
(and others mentioned as seen on other dates of the trip) isn't a dark
morph Wedge-tailed. Call me a spoilsport, but in these matters I
always figure that you should eliminate all possibility of it being
the most common/likely species to occur in the given geographic area
and/or habitat, and only then start considering rarities. Although, I
suppose a dark morph Wedge-tailed could still be considered a "good"
bird for our area.

all for now, JZ

J. BRIAN PATTESON

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Jul 14, 2008, 1:57:05 PM7/14/08
to JRZ, Seabird News

"Based on what I've seen on this list about occurrences at well
> birded areas, such as off the N. Carolina coast, it seems there can be
> fairly short windows of opportunity for pelagic species that would be
> "expected" at a certain time but considered rare at others."


There some truth to that, but there is also is a widespread
misconception about ocean off North Carolina. There is actually a
fairly big window of occurence for many species which occur here in very
low numbers. And there is considerable variation from year to year
depending on a number of factors across the ocean in addition to the
local winds and ocean current conditions. Birders and pelagic trips are
more numerous here in late May and early June. Late June, July, August,
and September are all good months to look for most of the "spring
rarities" here. If we ran daily trips here for two weeks in August, I
think it would open people's eyes. As it is, that is not going to
happen because there is not enough word of mouth advertising generated
by our usual four to six trips in August w/ a skeleton crew of
participants. We run more trips here in two weeks in spring than we do
for the next three months because people will go then but not a much
later on. The same sort of thing happens on the U.S. West Coast, but in
the fall. There are some great trips offered in August off Northern
California, but getting participants before September can be a
formidable challenge.

If birders around the world would take advantage of the present
opportunities for going on pelagic trips instead of putting it off for
sometimes years, so as to schedule at the "peak season", we would all be
better off. We can learn a lot more about pelagic seabirds and their
movements with more uniform coverage year round.

Brian Patteson
Hatteras, NC
br...@patteson.com
http://www.seabirding.com/

George Armistead

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Jul 14, 2008, 3:09:37 PM7/14/08
to JRZ, Seabird News
Bruce, Jim et al.,

I've been having some hotmail problems of late w/ messages not getting to their intended destinations. Below is a note that I sent last night. I'd only add that I'd echo Bruce's comments about the tail appearing rather short (not skinny enough about the waist) for a Wedge-tailed. As I say I haven't seen many Christmas Shearwaters but the birds do not appear rangy enough for a Wedge-tailed to me, and appear far too compact, and rather resemble Galapagos Shearwaters in structure. (I can provide Galapagos Shearwater images for comparison, if anyone is interested.)

Though Christmas and Galapagos Shearwaters are apparently more closely related (than Galapagos is to Audubon's) I wouldn't necessarily expect Xmas and Galapagos Shearwater to appear similar in the coverts. I don't have enough experience w/ Christmas Shearwater to comment, but the pale fringes to the secondary coverts on Galapagos Shearwaters is most evident on fresh birds and wears away with time.

Very best,
-George Armistead

CR Shearwaters‏
From:George L. Armistead (georgea...@hotmail.com)
Sent:Sun 7/13/08 9:51 PM
To:ebi...@gmail.com; bmact...@islandnet.com

Hey Guys,

I don't have a ton of experience w/ Christmas Shearwater, having seen but one off of Honolulu a couple of years ago but I have seen a lot of Wedge-tails, Sooties and Short-tails. I agree with Steve that these birds look like Christmas Shearwaters. Structurally, and geographically that is the only thing that makes sense to me. They look sort of compact with a relatively broad-based wing, and the head profile looks rather sloping w/ sort of a longish bill given the birds's size. Everything seems to fit for Christmas to me. I wonder if they are more common out there than folks realize. That's what I love about seabirding... still so many frontiers.

Very best,
-George


George L. Armistead
Philadelphia, PA
georgea...@hotmail.com
http://www.flickr.com/photos/seabirder/

Field Guides, Inc.
Birding Tours Worldwide
http://www.fieldguides.com/

JRZ

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Jul 15, 2008, 11:39:44 AM7/15/08
to Seabird News
Hello,

Bruce's comparison photos of wedge-tailed shearwaters came through and
they are posted at the same site with the photos of the unknown bird:

http://pajareroscostarica.blogspot.com/

thanks,

JZ


On Jul 14, 9:09 pm, "George Armistead" <armistead.geo...@gmail.com>
wrote:
> Bruce, Jim et al.,
> I've been having some hotmail problems of late w/ messages not getting to
> their intended destinations. Below is a note that I sent last night. I'd
> only add that I'd echo Bruce's comments about the tail appearing rather
> short (not skinny enough about the waist) for a Wedge-tailed. As I say I
> haven't seen many Christmas Shearwaters but the birds do not appear rangy
> enough for a Wedge-tailed to me, and appear far too compact, and rather
> resemble Galapagos Shearwaters in structure. (I can provide Galapagos
> Shearwater images for comparison, if anyone is interested.)
>
> Though Christmas and Galapagos Shearwaters are apparently more closely
> related (than Galapagos is to Audubon's) I wouldn't necessarily expect Xmas
> and Galapagos Shearwater to appear similar in the coverts. I don't have
> enough experience w/ Christmas Shearwater to comment, but the pale fringes
> to the secondary coverts on Galapagos Shearwaters is most evident on fresh
> birds and wears away with time.
>
> Very best,
> -George Armistead
>
> CR Shearwaters‏ From:*George L. Armistead* (georgearmist...@hotmail.com)
> Sent:Sun 7/13/08 9:51 PM To:ebir...@gmail.com; bmactav...@islandnet.com
>
> Hey Guys,
>
> I don't have a ton of experience w/ Christmas Shearwater, having seen
> but one off of Honolulu a couple of years ago but I have seen a lot of
> Wedge-tails, Sooties and Short-tails. I agree with Steve that these
> birds look like Christmas Shearwaters. Structurally, and
> geographically that is the only thing that makes sense to me. They
> look sort of compact with a relatively broad-based wing, and the head
> profile looks rather sloping w/ sort of a longish bill given the
> birds's size. Everything seems to fit for Christmas to me. I wonder if
> they are more common out there than folks realize. That's what I love
> about seabirding... still so many frontiers.
>
> Very best,
> -George
>
> George L. Armistead
> Philadelphia, PA
> georgearmist...@hotmail.comhttp://www.flickr.com/photos/seabirder/
>
> Field Guides, Inc.
> Birding Tours Worldwidehttp://www.fieldguides.com/

Angus Wilson

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Jul 15, 2008, 1:38:04 PM7/15/08
to Seabird News
The four new shots of Wedge-tailed Shearwaters are instructive and emphasize the more rakish look of these birds compared to Bruce MaacTavish's all-dark shearwater. Somehow, the long, angular wings of Wedge-tails remind me of frigatebirds and I just don't get the same impression from the 'mystery' bird. The shorter inner wing is most evident if you compare photos 4 and 6. Like Greg Gillson, I do think Sooty Shearwater needs to be considered. Apart from the unfamiliar underwing pattern, I see nothing solid to exclude it from the analysis. Of course, Bruce may not have got the impression of a Sooty viewing it in life, hence the query. The wing shape, bill length and general proportions seem to be at odds with my expectations for Christmas Shearwater but I lack field experience and defer to the comments of others.

Has anyone noted differences in measurements of Wedge-tails from different parts of their range or any differences in measurements associated with color morph?

Thanks to Jim Zook for instigating the discussion and hosting the pictures on his blog. Seabirds are never dull.......

Angus Wilson
New York City, USA
http://oceanwanderersnews.blogspot.com/


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