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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