birding-aus

Seabirds off WA and Christmas Island - incl. GREAT SHEARWATER

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Subject: Seabirds off WA and Christmas Island - incl. GREAT SHEARWATER
From: Daniel Mantle <>
Date: Thu, 4 Dec 2008 12:16:11 +0000
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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