Were broad-billed prions from The Snares part of the massive die-off of this species in 2011?

Were broad-billed prions from The Snares part of the massive die-off of this species in 2011?

This was one of the key questions that we were trying to answer when four Te Papa scientists – Colin Miskelly, Antony Kusabs, Jean-Claude Stahl and I – set off for the subantarctic Snares Islands in November-December 2013.  The Snares are one of the world’s great seabird islands and broad-billed prions – a small blue-and-white petrel – are just one of 13 seabirds that regularly breed there.

Broad-billed prions suffered a massive mortality event in July 2011, following a persistent onshore storm.  An estimated 200,000 broad-billed prions died on North Island west coast beaches at the time (see Riders of the storm – thousands of seabirds perish on New Zealand shores).  This was the largest documented seabird ‘wreck’ in New Zealand’s history and must have severely impacted on the species at many of its breeding sites.  However it was not known where all these birds came from.

BBP chick Snares
A healthy broad-billed prion chick on its nest in a cave, Snares Islands, December 2013. Image: Alan Tennyson, Te Papa

Broad-billed prions nest around the Chatham Islands and in the Stewart Island region, including on The Snares, which lie 105 km south-southwest of Stewart Island.  The largest populations occur in the Chathams area but initial monitoring indicated no change in numbers there.  So we turned our attention to the more southern nesting populations.  Fellow Te Papa Curator Colin Miskelly spoke with muttonbirders around Stewart Island and they reported a 90% decline in the species (known locally as ‘parara’ or just ‘paras’).  Colin also surveyed a small colony off Codfish Island/Whenua Hou, which indicated a similar level of decline (see Riders of the storm – the severely depleted next generation).

The best documented site for the species in these southern populations is The Snares.  In the 1980s we established some baseline information on this population by mapping two small cave nesting sites and estimating total numbers.  While these figures were only rudimentary, our 2013 team was able to resurvey the islands to see if numbers had changed markedly.  Our 1980s estimate was that 2,000-5,000 pairs nested on the main island (see Miskelly et al. 2001 Notornis 48: pages 1-40).  In November 1986 at South Bay – perhaps the densest nesting area on The Snares – 60 chicks were easily located and 3 or 4 birds were continuously circling over the site in the afternoon.  Another way that I assessed prion populations in the 1980s was to count the ‘middens’ left behind by predatory skuas, as these give an indication of the numbers and distribution of local prey species.

Skuas feeding
The main prey of subantarctic skuas on The Snares is burrowing petrels, such as prions – here they are eating a sooty shearwater (titi or muttonbird). Image: Alan Tennyson, Te Papa, November 2013
Dead skua prey remains
Counting remains of prey left behind in skuas’ middens gives an indication of the population distribution and size of the prey species. Image: Alan Tennyson, Te Papa, December 2013

So what did we find on our 2013 trip?  We found 3 and 4 nest sites respectively in the caves where we had counted 11 and 5 nests in the 1980s.  Although the numbers are small, this equates to a decline by two thirds.  In South Bay we found only 6 chicks after much searching and the species was notable by its absence in flight, with only a single bird seen.  This suggests a massive decline – perhaps more than 90%.  In skua middens we identified a minimum of 3,424 individual birds, of which broad-billed prions made up 3% of the total.  In 1986 I identified 2,402 individual birds of which 13% were broad-billed prions, suggesting a 75% decline.  So, while our figures are rough, our conclusion is that The Snares broad-billed prion population was badly affected by the 2011 wreck, with thousands of birds from this population dying.  Te Papa is continuing other lines of research, such as DNA identification, to find out more about the origins of the prions in the ‘big wreck’.

South Bay Snares
South Bay had a dense nesting population of broad-billed prions in the 1980s, but we found only a handful of chicks there in 2013. Image: Alan Tennyson, Te Papa, December 2013

Written by Alan Tennyson, Curator Vertebrates

For other Te Papa blogs on our Snares Islands expedition see:

Western Chain, Snares Islands – 1929 and 2013 – In the footsteps of Edgar Stead (Part 10)

Snares Islands – 1947 and 2013 – In the footsteps of Edgar Stead (Part 11)

Life through a burrowscope lens (Part 3) – subterranean Snares Island

Birds of the Snares Islands

Critters of the Snares Islands

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