It started as a trickle and soon developed into a flood of devastating proportions. On 11 July 2011 I received an email enquiry from a family at Waikanae seeking help with identifying an unusual seabird that they had found dead on their driveway. It was a Salvin’s prion, a not-too-unexpected discovery near the coast during a winter storm. But the next day a Department of Conservation colleague phoned from Masterton reporting a dozen live prions found scattered inland in the Wairarapa, on the sheltered (eastern) side of the Tararua Range. If that number had reached the leeside, what was happening of the exposed western coast? It didn’t take long to find out.
By 14 July over a thousand live prions had been handed in to wildlife care centres in Wellington and Manawatu, an alarming number given that during prion ‘wrecks’, only a tiny fraction of the birds are still alive by the time they reach land. But what is a prion? and why do they wreck?
Prions (the singular is pronounced ‘pry-on’) are a group of six small closely-related seabirds that are hugely abundant in southern oceans. They are petrels, and like most petrels, typically breed in enormous colonies on remote islands free of introduced predators. They should not be confused with the other use of the word (in this case pronounced ‘pree-on’) used for a particularly nasty group of infectious proteins that cause the brain-wasting Creutzfeldt–Jakob disease in humans, plus mad-cow disease, and scrapie in sheep.
All prion (bird) species are very similar in size and plumage markings, with the most obvious difference being bill shape, which varies from broad through to narrow or chunky. Within this continuum of variation, some pairs of species are very difficult to distinguish from each other.
The three species with the widest bills have prominent lamellae (comb-like structures) along the edge of the upper mandible, used to filter tiny crustaceans and other small animals and their eggs from sea-water. When combined with a muscular tongue and an extendible pouch below the bill, these adaptations recall those of baleen whales, which feed in a similar way. Perhaps this is why prions are sometimes referred to as ‘whale-birds’.
Prions are well known to New Zealand birdwatchers, even if they are frustratingly difficult to distinguish at sea. Members of the Ornithological Society of New Zealand have for many years patrolled the New Zealand coastline recording the numbers and identities of birds cast ashore. For many, this is the only way to become familiar with prions, particularly in those years when large multi-species wrecks occur. The results of these ‘beach patrols’ are occasionally published in the OSNZ’s journal Notornis. Between 1960 and 1996, over 86,000 prions were found dead on New Zealand beaches; large wrecks occurred in 1961, 1970, 1974, 1975, 1984-86 and 2002, with over 10,000 birds cast ashore in 1974, 1985 and 2002. Earlier wrecks occurred in 1878, 1918 and 1932.
Like all petrels, prions are true seabirds, spending their entire lives at sea apart from the 4 months when they are tied to a nesting burrow and the care of their single egg and resultant chick. At other times they are constantly on the move, often in vast flocks, skimming the waves of the southern oceans in search of productive upwellings. Although frail-looking, they thrive in a part of the globe renown for strong winds. Until they encounter land…
Prions move with the wind, using the varying airspeeds on the windward and leeward sides of waves to fly long distances with great energetic efficiency. There are few land masses in the southern ocean, and it is usually easy for flocks of prions to slide around the few obstacles that present. Except, that is, for the 1500 km coast of New Zealand. For ten consecutive days in July 2011, persistent westerly gales in the Tasman Sea pushed prions against New Zealand’s western shores. To start with, the birds moved effortlessly with the wind. Then as land loomed, they started to fight the wind, trying to stay offshore. But the relentless gale continued, consuming the birds’ energy until they were exhausted and driven ashore in tens of thousands.
Although there is a long history of prion wrecks on New Zealand beaches, the scale of the 2011 wreck is unprecedented. Far more prions have been killed in this single event than the 37-year total recorded by the OSNZ. Details are still being collected and collated, but large numbers have been found from at least Dargaville to Okarito, 900 km apart. In places they have stranded at rates over 400 birds per kilometre of coast. And that ignores the birds blown inland.
Even more alarming is that nearly all the birds are broad-billed prions (91% estimated), a locally-breeding species. The two previous largest wrecks of broad-billed prions were between 1100 and 1400 birds. It will be difficult to estimate the full extent of the 2011 wreck, but it is likely to be up to 250 times larger than either the 1961 or 1974 events.
Desperate efforts are being made to save some of the birds, including a combined total of over 1000 being hand-fed at Wellington Zoo and Massey University. As the birds are exhausted and emaciated from their struggle against the gale, it is terribly difficult to revive them, and hundreds of those delivered have since died.
The 2011 prion wreck will have wreaked a terrible toll on the New Zealand broad-billed prion population. The species also occurs in the South Atlantic, but the birds in New Zealand waters are thought to come from the breeding populations on the Chatham Islands, Snares Islands, and islands around Stewart Island and off the Fiordland coast. Apart from the 330,000 pairs estimated on Rangatira Island in the Chatham Islands, none of these other populations are thought to number more than a few thousand pairs. The total New Zealand population is likely to be little more than a million birds, and so the tragic deaths of (probably) several hundred thousand of them will have a huge impact, especially if the birds in the Tasman Sea were mainly from the less numerous southern (non-Chatham) populations.