Weka and Seabirds

JOHN OGDEN

Rakitū, located off the northern east coast of Aotea was recently declared predator free by the (former) Minister for the Environment, Hon. Eugenie Sage. The 330-hectare island joins more than 40 other pest free islands in the Hauraki Gulf Marine Park. Rakitū, like much of New Zealand, once had thriving breeding colonies of seabirds, before rats were introduced.

The island is located on the northern New Zealand’s seabird superhighway, and restoration could see numerous seabird species becoming re-established, just as they did on Burgess Island in the nearby Mokohinau group following rat eradication(1).

New Zealand - ‘Seabird Capital of the World’

New Zealand is known as the ‘Seabird Capital of the World’ - almost half of the world’s 359 seabird species worldwide, 140 species occur within New Zealand’s Exclusive Economic Zone (out to 200 nm from the coast). Eighty-six of these species are endemic or native to New Zealand(2) . About a third of New Zealand’s seabird species(3) are found in the Hauraki Gulf.

Weka prevent seabird re-colonisation

The return of North Island weka (Gallirallus australis greyi) to Rakitū is likely to prevent recolonisation of the island by any of these seabirds. One of the clearest accounts of weka predation on shearwaters was provided by Harper in 2006(4).

“The first observation of weka predation on shear­water chicks was on Taukihepa (Big South Cape Island) on 17 Jan 2004. On 3 occasions an adult weka with 2 chicks was seen grubbing around burrow entrances, then entering rapidly and pulling out 2-3-day-old chicks. The adult weka killed the chicks by striking the back of the head with its beak. The weka ate just the head and discarded the body. Another 4 shearwater chicks with the same distinctive injuries to the head were found in the area. After this initial observation, 37 chicks with head wounds suggesting attacks by weka were found at various locations around Taukihepa. Intact dead chicks ranged from 66 to 283g. Seven chicks were found with at least 1 eye missing and the brain cavity cleaned out; 16 others had only head injuries (normally to the back of the head), but with no skin punctures. The preferred method for killing petrel chicks was by repeated blows to the back of the head, which has been observed for weka elsewhere (St Clair & St Clair 1992). Ten other chicks had had the brain removed through the back of the head …”

Over a three-week period in 2004, 43 weka predated chicks were recorded on Taukihepa, and more than 30 over a similar time the following year. The details leave no doubt as to the tenacity of the culprit: “A juvenile weka was seen killing a sooty shear ­water chick on Mokonui on 16 Mar 2005. At 1515 h, I was watching a weka foraging in tupare (Olearia colensoi) forest when it suddenly entered a burrow. The weka’s back was still visible as it grabbed a sooty shearwater chick by the mantle and, obviously struggling, dragged the chick to the burrow entrance. The weka repeated this procedure 6- 8 times, because the chick shuffled back down the burrow. After the last attempt, the weka began to strike the chick’s head with its beak, raising itself up to full stretch on its toes and then swiftly bringing the point of its beak down on the back of the chick’s head. After every 3-4 strikes, the weka pulled at the chick again, working it towards the burrow entrance.”

“The sequence of pulling and striking was repeated c. 6 times, then the weka grabbed the chick by the tail and pulled it further out of the burrow. The weka struck the chick c. 30 more blows on the back of its head, by which time it was not moving very much. The weka then pulled the chick under some punui (Stilbocarpa lyalli) about 10 m away, stopping regularly to strike the chick’s head in the same characteristic manner as before. After another 2-3 min of this ‘plunge-hitting’, the weka rolled the chick over, climbed on to its back, and began pulling at the chick’s head to feed. When it had finished feeding, the weka walked away, inspecting burrows as it left. The whole episode lasted c. 15 min.”

Harper concludes that: “…when populations of shearwaters decline below a few thousand individuals, colonies can become very vulnerable to extinction as a result of depredation by a few individual predators (Brothers 1984; Lyver et al. 2000; Jones 2002).

Populations of smaller petrels with restricted numbers or nesting distribution are more vulnerable to extirpation or severe declines as a result of weka predation (Falla 1948; Blackburn 1965)”.

As noted elsewhere in this issue (see guest editorial), the implications of these observations are clear. The seabird species most likely to recolonise Rakitū all weigh less than the sooty shearwater chicks predated on Big South Cape - storm petrel (Fregetta maoriana), prion (Pachyptila spp.), diving petrel (Pelecanoides urinatrix), Cook’s petrel (Pterodroma cookie), fluttering shearwater (Puffinus gavia) and little shearwater (Puffinus assimilis), all nesting on islands close to Rakitū. Grey-faced petrel (Pterodroma macroptera) has colonised one site on Aotea Great Barrier(5) , apparently since rats were eliminated on Cuvier. Even these large birds are smaller than sooty shearwaters, so they too must be vulnerable to weka predation.

Rakitū - restoring a seabird ecosystem

Seabirds once nested everywhere on the New Zealand mainland coast, even well up into the mountains – millions of birds. A few mainland colonies remain – Hutton’s shearwater (Puffinus huttoni) in the alpine zone of the Kaikoura Range, Westland petrel (Procellaria westlandica) in the forests of the Paparoa Range, black petrel (Procellaria parkinsoni) and Cook’s petrel on Hirakimata (Mount Hobson) and Hauturu (Little Barrier Island) in the Hauraki Gulf.

Once, on Rakitū, and the hills of Aotea Great Barrier, seabirds were digging in the soil, creating habitat for lizards, tuatara and invertebrates. These colonies were bringing vast amounts of phosphates and nitrates from the marine zone (in droppings, disgorged fish, feathers, broken eggs, even dead birds) – their additions compensating for leaching losses and maintaining forest growth rates(6) . This has all gone from the mainland but can still be experienced on some off-shore Islands. Rakitū could readily become such a seabird ecosystem, if it was weka-free.

Weka, one species but four subspecies

The North Island weka introduced to Rakitu is one of four subspecies in New Zealand.

• North Island weka with strongholds on Russell Peninsula, Kawakawa Bay (Auckland) and the Opotiki-Motu region in the Bay of Plenty (At Risk-Recovering).

• Buff weka (G. a. hector) is now found only on the Chatham Islands where it was introduced and is now plentiful, although is extinct in its natural distribution in the eastern South Island (At Risk-Relict).

• Stewart Island weka (G. a. scott) is largely confined to offshore islands around Stewart Island where it was introduced. On the main island, this subspecies has largely died out save for occasional reintroductions (Nationally Vulnerable).

• Western weka (G. a. australis) is relatively widely distributed on the West Coast, some populations being abundant, while others are sparse (Not Threatened).

Historically, weka was a significant resource for some iwi and also used by early European settlers, who gave it the name ‘woodhen’. If conditions are suitable, weka can breed prolifically, as seen on Rakitū since their introduction in the 1950s.

Acknowledgements:

Thanks to June Brookes for useful discussion and background information on this article and my guest editorial.

References:

1. Cronin, E. 2017. Seabird Super Highway – a return to the Hauraki Gulf. Environmental News 37: 4-7.

2. Forest and Bird, 2014. New Zealand Seabirds: Important Bird Areas and Conservation. The Royal Forest and Bird Protection Society of New Zealand, Wellington, New Zealand.

3. Gaskin, C. P. and Rayner, M. J., 2013. Seabirds of the Hauraki Gulf: Natural History, Research and Conservation. Hauraki Gulf Forum.

4. Harper, G. 2006. Weka (Gallirallus australis) depredation on sooty shearwater/titi (Puffinus griseus) chicks. Notornis 35: 318-320.

5. Ogden, J. 2012. Environmental News 29: 1-2. The greyfaced petrel population at Awana is still increasing and now has pest management by volunteers.

6. Waterhouse, K. 2017. Seabirds: Canaries in the mine or elephants with wings? Environmental News 38: 8-13.

Additional Material Consulted:

Beauchamp, A. J. et al. (Eds). Department of Conservation 2010. Weka (Gallirallus australis) Recovery Plan 1999- 2009. Dept. of Conservation.

Beauchamp, A. J., Blick, A. J. & Chambers, R. C. 2002. Report. Dept. of Conservation, Great Barrier Field Office Archives (reported in Waterhouse, K. 2017. Environmental News.37 Autumn 2017, p.10).

Beauchamp, A. J., Chambers, R. & Kendrick, J. L. 1993. North Island weka on Rakitu Island. Notornis 40: 309- 312.

Bell, D. B. & Braithwaite, D. H. 1964. The birds of Great Barrier and Arid Islands. Notornis 10: 363-386.

Bellingham, P. J., Hay, J. R., Hitchmough, R. A. & Mccallum, J. 1982. Birds of Rakitū (Arid) Island. Tane 28: 141-147.

Brookes, J. 2016 Rakitū/Arid Island Rat Eradication. Environmental News 35.

Landcare Research, 2010. Impact of Rats on Seabirds. Environmental News 21.

Maturin, S. 2014. Seabirds rule the roost. An account of the seabird island Rangatira in the Chatham Islands. Forest& Bird Magazine 352: 30-32.

Ogden, J. 2012. The Great Barrier slaughterhouse, Environmental News 29.

Ogden, J. & Brookes, J. 2017 Rakitū, Rats and Weka – moving ahead in 2017? (including a time-line of lost conservation opportunities on Rakitū). Environmental News 37.

Waterhouse, K. 2012. Rakitū, Where the Light meets the Sky. Environmental News 29.