shearwater mortality

Subject: shearwater mortality
From: Peter Woodall <>
Date: Wed, 01 Nov 2000 12:46:40 +1000
Hi Birders,

I'm not a seabird person, although I've been out with Paul
on the Southport pelagics a few times, but I'd like to see
if I can draw a few threads together on this mortality thing.

As far as possible I'll stick to facts, although they will 
often end up with uncertainties and speculation.

1.  There is a high mortality this year, although there have
been similar losses in past years.

2.  It extends over a very large geographical area: Tas & Vic to
southern Queensland.

3. The postmortem reports do not seem to indicate any involvement 
of toxic chemicals  but report that many of the birds are emaciated.

For this to be the result of pollution, it would have to be a huge
spill over a very large area, which would also affect many other
species - no evidence of this.

Therefore the most likely alternative is that it is caused by
exhaustion and possible food shortage. 

Evidence for this is from 3 above. We probably need more freshly
dead birds weighed and then compared with the weights of "normal"
birds [probably obtained from banding operations!].

Incidential evidence of a food shortage comes from the recent pelagic trips
which have reported shearwaters being ravenous for the burley and
coming in much closer to the boat than usual.

It's hard enough to detect and quantify changes to food supplies on land
but at least one can look for flowering eucalypts, abundance of fruit,
nos. of rodents, etc.  Out at sea it would be very difficult, not least 
of all due to the great distances involved - just where was the food
shortage - at the start, middle or end of the migration??

If we accept that food shortage is the most likely explanation, then what
caused it can only be guessed at. It might be due to reduced foraging
success by the birds (sea conditions, etc) or actual reduction of the 
prey - potentially caused by a great many factors (changing temperatures,
nutrient levels, changes to any of the links in the food chain).

I think that this clearly shows  that natural systems are very complex.
Trying to unravel all the different factors and deciding which hypotheses 
are supported and which are excluded by the available evidence takes 

We should not jump to premature conclusions - 
even if this is what the media wants.


Dr Peter Woodall                          email = 
Division of Vet Pathology & Anatomy             
School of Veterinary Science.             Phone = +61 7 3365 2300
The University of Queensland              Fax   = +61 7 3365 1355
Brisbane, Qld, Australia 4072             WWW  =
"hamba phezulu" (= "go higher" in isiZulu)


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