G'day Daniel,
Thanks for your report, I enjoyed reading it and congratulations on the
Great Shearwater, I hope you will submit it to BARC.
Your description for odd Little Shearwaters could actually describe
Audubon's Shearwater, a species unknown to those waters thus far. Did you
consider that species when you were observing them, or were you able to
eliminate them as a possible ID.
Cheers Jeff.
-----Original Message-----
From:
On Behalf Of Daniel Mantle
Sent: Thursday, 4 December 2008 11:16 PM
To:
Cc:
Subject: Seabirds off WA and Christmas Island - incl.GREAT
SHEARWATER
Hi Evan, Birding-aus
I am sure the birds have no clue how deep the water is but there is no
doubting they will congregate in areas of richer feeding and are adept at
reading the oceans to this end - whether this is through smell, temperature,
sight or a combination of these thinbs. The density of birds is often much
higher along the shelf edge and continental slope than up on the shelf or
out in the open ocean. Commonly this is associated with areas of cold
upwelling along the shelf edges that brings with it nutrients and thus an
increase in planktics for seabirds to feed upon. It is not this simple off
the WA coast, as there are several currents running at different depths,
different directions, and at different distances from the coast. The most
important or well known of these is probably the warm Leeuwin Current that
runs south along much of the west and northwest coast of Australia. It is no
doubt this warm current that influences the rather subtropical/tropical mix
of seabirds off the west Australian coast - many Bridled and Sooty terns,
noddies, and Wedge-tailed and Little Shearwaters.
So why the increased density in the canyon systems off the WA coast (eg
Murchison Canyon off the Abrolhos and Perth Canyon off Perth)? Probably the
increase I observed in the Murchison Canyon also reflects the proximity to
nesting sites in the Abrolhos Islands but there was also a very noticeable
drop off in numbers as we transited to the next canyon systems north and
south everytime we movednalong this stretch of coastline. So there must be
something causing better feeding in these areas. There may be some limited
cooler, nutrient rich upwelling in these areas, or maybe the transfer of
sediment through these systems provides the nutrients for an increase in
planktonic species in these areas (although just how much sediment transfer
there is going on at present would be very debatable or even doubtful).
Sorry, I can't give a more definitive answer than this but I would have to
find out a lot more about the rather complex current patterns and influences
along this coastline.
Any other thoughts?
I would still be particularly keen to hear if anyone else has observed
'Little' Shearwaters with dark trailing edges off the WA coast? Have any of
the Perth seabirders any thoughts on Little Shearwater plumage wear to add?
Cheers Dan> Date: Thu, 4 Dec 2008 11:34:56 +1100> From: >
To: > Subject: Re: [Birding-Aus] Seabirds off WA and
Christmas Island - incl. GREAT SHEARWATER> CC: > >
Fascinating report indeed Dan.> > A question: How do the birds know how deep
the water is?> "By far the best areas for seabirds were in moderate to deep
waters> (800-4000m) within the canyon systems that run perpendicular to the>
shelf edge."> > My understanding was that pelagic species (fish and
invertebrate> marine) were pretty constant in the top of the water column...
so what> changes that causes the birds to pick the deeper water?> > EB> > On
12/3/08, Daniel Mantle <> wrote:> >> > I have just
returned from the 1st Leg of Geoscience Australia's current marine surveys
off the West Australian coast and thought I would write up a brief(ish) note
on some of the seabird observations during the trip.> >> Evan Beaver>
Lapstone, Blue Mountains, NSW> lat=-33.77, lon=150.64
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