The attached photo is one of 13 taken over a 10 second period 60 miles
offshore Coos Bay on September 20, 2016. This was a cruise ship with a
high vantage point enroute from Vancouver BC to Los Angeles CA. 3000
passengers and 30 sharp-eyed birders were aboard.
This photo (a) compares Sooty vs. Short-tailed Shearwater ID; (b) points
out an unidentified storm petrel or ? and (c) discusses inadequate field
guide treatment of Sabine’s gull.
(a)SOSH vs. STSH Photo 6708 B,E,F,G&K ‘appear’ to be Sooty Shearwaters,
the default and most abundant species on many or most west coast pelagic
trips. All dark, they can appear very different with plumage wear and the
angle of the sun. However, familiarity is soon developed with them since
they are so abundant and (with chumming) will come right up to a fishing
boat, land on the water, and then dive underwater, using their wings to
swim. However, there is a second species, the Short-tailed Shearwater
which is very similar in all plumage details and only a little smaller in
size. As season progresses from spring through summer and into winter,
Short-tailed are generally considered to replace Sooty in abundance,
although the difficulty of identification adds some uncertainty to the
exact numbers.
In this photo, the red lines (not marking birds) are of a length equal to
the wing spread of Shearwater F, presumed to be Sooty. This line is then
rotated in comparison to other wing lengths. Note that the line on F
underestimates the wing length because the wings on this bird are slightly
flexed.
When I looked at this photo the difference in appearance between E and G
was immediately apparent; but was it just an optical illusion? The
wingspread of G would be close to that of E once the wings were spread at a
right angle, but those of E are already at a right angle and significantly
smaller. In addition, E has a generally chunkier look than F which is in
almost the same position, has a slighter bill and a steeper forehead. I’m
calling this a Short-tailed Shearwater, the only one recorded for the trip
(assuming it is accepted by the Cruise Records Committee, whoever they
are). Short-tailed is not a rare bird, just difficult to separate from
Sooty. The relative winglengths given *in Petrels, Albatrosses and
Storm-Petrels of North America*, Steve N.G. Howell, 2012, are Sooty
97-106” and Short-tailed 91-99”. (This is a must-have book for serious
pelagicers). To quote SNGH (p.67 “As a rule, if you *think* you see a
Short-tailed off the West Coast in Sep-Oct, it’s far more likely a Sooty
…..). In this photo, I think I *know* it’s a Short-tailed. But, hey; prove
me wrong. Yellow lines are of the same length and there is not much
difference between E and G. Bird B is presumably also Sooty Shearwater but
the image could not really separate the two. However, J has significantly
flexed wings, stated to be a mark for Sooty but much less so for the
shorter-winged Short-tailed. There are some rare petrels which
superficially resemble Sooty and/or Short-tailed Shearwaters, but these
have proportionately shorter, stouter bills and are ruled out in those
birds whose bills can be seen. They usually have different flight styles.
None were seen on this cruise, in spite of much hope. A very good mark
(SNGH) makes it somewhat ‘easy’ to pick out a single Short-tailed
Shearwater in a flock of Sooties due to a different ‘flight style’. I’ve
used this field mark several times on a bird which I eventually
photographed and ‘confirmed’ the ID. In this 10-second sequence there was
no time for such observation and this ID is based solely on the photo.
Bird H appears larger, browner, and with more of a white underwing than
Sooty and is likely a Pink-footed Shearwater, which is confirmed in the
next photo taken 2 seconds later which has a slightly better view.
(b) Unidentified ?. Bird A is very much smaller than the
shearwaters and clearly is a smaller species. But which one? A
storm-petrel seems the most likely species and lots of Fork-tailed were
seen on the trip. To me A looks a little too brownish-gray to be a
Fork-tailed. A very interesting bird. Any ideas?
(c) Sabine’s Gull. I (at least) saw more than 10x Sabine’s Gulls on
this cruise than the total I had seen in my previous birding life of many
decades. This beautiful species is generally considered not to be an
identification challenge. Most birds seen at sea in southbound migration
are adults; a rare but regular continental movement of juveniles turns them
up at isolated lakes and reservoirs inland, usually singly and always a
great find. Many field guides do not even illustrate the winter plumage,
saying that these gulls don’t acquire that plumage until south of the US.
The abundant Sabine Gull flocks seen on this trip did not read the field
guides.
But more interesting than the many winter plumage birds were the tails of
all the birds. Most guides show Sabine’s gull with a shallowly forked tail
and in fact that is one of the factors contributing to its ‘beauty’. *Gulls
of the Americas*, another ‘must have’ compendium, (by, you guessed it,
Steve N.G. Howell (& Jon Dunn)) groups it among the ‘fork-tailed’ gulls;
only two in this group, including the Swallow-tailed Gull of South
America. Again, the Sabines didn’t read the book. Of all the many Sabines
that I photographed, there were all possibilities: forked (concave) tails
(as advertized), flat tails, convex tails, and almost pointed tails. Now,
the possibility of partially molted tails must be considered, but I don’t
think that is the explanation.
I’ve only marked a few of the Sabine’s. The two birds at C are adults. At
left is a bird whose forehead has molted to lose the black feathers
(relatively flat tail) and the bird at right seems to be showing lightening
of the top of the head with impending molt (convex tail). The two birds at
D are both juveniles, as shown by their dark backs. Juveniles are also
show with forked tails but with a black terminal band. The black band
seems to show on the bird at right. Tail-end-shape is not really visible
on these. The adult at H has a beautifully convex tail. The adult at J at
first seems to have a pointed tail, but closer inspection shows this is due
to the angle of the tail which appears flat in this view. Bird L is a
juvenile with a flat tail and no apparent dark band at the end. Near the
top above L (but not marked) is a better view of a dark juvenile which
shows the dark band on a convex (not forked) tail.
Note: Comments on tail shape are based upon all 13 photos, not just this
one. This photo by itself gives the impression that Sabines with the
commonly illustrated forked tail were not seen. They were photographed,
just not in this photo.
Attachment:
IMG_6708H-lines.jpg
Description: JPEG image
