shearwaters

["Mules, Michael" <>]

On the subject of previous die-offs of Short-tailed Shearwaters, I have no personal experience (being 25, and coming from Gippsland, I can hardly have had any experiences with beach-washed shearwaters in previous decades), but instead I have looked up a couple of books on the subject.

HANZAB (vol. 1A) gives the world population (and therefore the Australian population) at around 23 million individuals.  It also mentions that most beach-washed birds are thought to be 1st year birds, and that their mortality rate is between 30-63%.

The Handbook of Australian Seabirds, by D.L.Serventy, V.Serventy and J.Warham (1971), gives an average mortality rate of 50% for birds in their first year.  They also mention that mortality may be linked to storm conditions (p.133) and, elsewhere, that mortality rates are linked to the abundance of plankton in the Tasman sea (p.130).

I'll quote part of a paragraph from The Handbook of Australian Seabirds, as it seems to bring a number of the themes that have been written about this topic so far on birding-aus:

"In some years serious mortalities occur on the east Australian coast during the southward migration in late October to January.  Thousands of birds may wash ashore from southern Queensland to southern New South Wales, the deaths being correlated with a shortage of food during the transit migration, probably accelerated through exhaustion in battling through the S.E. Trades Wind belt.  Adults suffer far less.  Banding results have shown that most of such birds are yearlings.  Even so, though many thousands of birds may perish, in relation to the total number of individuals involved the death rate is not high." (THAS, p.133)

I wonder if there has been (or will be) any analysis of bands on the beachwashed shearwaters from this season?

Some very, very crude back-of-the-envelope calculations:

assumptions:    23 million s.t. shearwaters, 20% of which (4.6 million) are 1st years.
                50% mortality of first years on return journey (excluding at-nest mortality, which HANZAB puts at between 3%- ~30%)

Given the above, up to 2.3 million s.t. shearwaters may have died this season, depending on prevailing weather and food conditions.  As I said, these are very crude calculations, but may give an idea of what may be causing the large numbers of dead shearwaters this season.  Or, there could be an alternative explanation, or a combination of other factors.

Anyway, I'll stop rambling now.

Cheers, Michael

PS. I noticed that the Wollongong Pelagics trip report did not include any Short-tailed Shearwaters.  Have they already passed through?

Michael Mules
Curator, Science Program
Museum Victoria
Melbourne Museum, Carlton

Ph: 8341 7426

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