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Pink Robin
Petroica rodinogaster
New Zealand Robin
Petroica australis
  (Sparrman, 1788)
Chatham Robin
Petroica traversi
  Owner:   C. Michael Hogan    

Petroica australis rakiura on the forest floor of Ulva Island, New Zealand

New Zealand Robin
Petroica australis

C. Michael Hogan PhD
October 21, 2009

Petroica australis is a characteristic bird of New Zealand's South Island and Stewart Island mixed hardwood/podocarp forests. Despite the IUCN characterisation of Least Concern, ground truth data suggests a more cautious outlook since (a) the Stewart Island subspecies rakiura has a habitat area that is quite small relative to the species range, and (b) the threat from mammalian predators on South Island is not clearly addressed by the IUCN. These predators are largely unchecked on the mainland of South Island, and some of whom were introduced only as recently as the nineteenth century. This robin is inherently vulnerable to mammalian predators, since its evolution occurred in the absence of any such predator, and humans only arrived on South Island within the last millennium.



SUBSPECIES AND DISTRIBUTION
There are two presently recognised subspecies of P. australis:

* P. australis australis (South Island Robin) occurs on South Island, New Zealand.
* P. australis rakiura (Stewart Island Robin) occurs on Stewart Island, New Zealand and neighbouring smaller islands.

P. australis australis is locally common north of Authur's Pass National Park, in Nelson, Buller, and coastal reaches of Marlborough, but is more sparsely and patchily found in the southern parts of South Island. P.australis rakiura is locally common in the densely forested areas of Stewart Island and smaller adjacent islands, in the habitats that are predator free.



MORPHOLOGY
The entire genus Petroica is not closely related to European or North American robins. P. australis is typically classified as an 18 centimetre length bird, (Butchart and Ekstrom) with a male body mass of approximately 38 grams. (Dunning) The rounded head features a thin bill, adapted for preying on insects; body shape is somewhat pudgey set atop longish delicate legs. Colouration is typically a deep slate gray. The P. australis male is dark gray except for a crisply defined whitish-yellow lower breast; females and juveniles exhibit more gray or brown than black plumage and display variable paler spots in throat and breast plumage. The P. longipes is closely related to P. australis (Miller and Lambert) and was earlier considered to be a subspecies of P. australis. The male of P. longipes is almost black, manifesting a grayish white lower breast and white patch above his bill; P. longipes females and juveniles are similar in appearance to their P. australis counterparts. Another similar species is P. macrocephala, which is a smaller robin, the male of which manifests white wing bars and very distinct whitish-yellow breast. A final close relative worth noting is the rare melanistic P. traversi, endemic to the Chatham Islands.



BEHAVIOUR
P. australis is found at forest fringes, but also in the depth of mixed hardwood/podocarp forests. Early note of its occurrence in the deep forest interior was first made during Trevor Chute's 1866 march during the Second Taranaki War. The species specialises in ground feeding, and this trait is borne out as prehistoric based upon archaelogical bone recovery (Worthy and Richard N. Holdaway) from pitfall deposits (e.g. digs in ancient vertical shafts within caves). Food hoarding and caching has been documented for P. australis with differing strategies for percentage of prey cached with respect to males versus females. (Burns) P. australis is an insectivorous bird, who also consumes moderate amounts of seeds and berries.

Powlesland conducted one of the most exhaustive studies of P. australis in a Leptospermum ericoides dominant forest along the Kowhai River seven miles inland from Kaikoura. (Powlesland) Male New Zealand Robins sing more frequently than females, with bachelor males singing more than paired males; in fact, females are not observed to produce full song. Adult males engage in song more than immature birds. The explanation for these facts is that adult males seem to sing largely to attract females, a thesis supported by the observation that bachelor singing decreases sharply once a mate is found. Furthermore, bachelor singing occurs at the highest perch levels, ostensibly to provide the greatest broadcast distance to attract the female. The male and female use calls to communicate, particularly in the early courtship stage and as they are defining territory. Some singing is, moreover, specifically associated with territorial defence, with males conducting the preponderance of such vocal territorial display. The full repertoir of song was first identified in 1975, documenting song, sub-song and 18 different calls. (Hay) Full song decibel amplitude is much louder than sub-song, the latter only being audible by humans within a few metres away. The female produced sub-song chiefly during nest site selection. The male song length may endure for up to 60 seconds, with song-bout lengths of up to eight minutes. Perching height of song production generally ranges from four to ten metres, with the higher perch singing taking place in the low light months of July to October.

The home range for a nesting pair is approximately two hectares. A clutch typically consists of two to three eggs. Chicks may remain in the nest for about two weeks, but require parental guidance outside the nest for an additional four weeks. After that age a young bird may be seen foraging on the forest floor by itself. I made a number of observations of P. australis rakiura in the primary mixed hardwood/podocarp forest on Ulva Island. Both adults and juveniles manifested no fear of humans. As one stands motionless, the robins will forage on the ground within a metre. Even gentle movements of humans will not discourage the birds from proximity; moreover, this bravery is understood by noting the species evolved in the total absence of mammalian predators.



ECOLOGY
P. australis occurs in wet montane and subalpine forests; moreover, these habitats may be primary mixed hardwood/podocarp forests as well as secondary growth forests. (Sibley and Monroe) Chief prey of the New Zealand Robin are ectothermic invertebrates. The relative inactivity of such prey species in the cool early morning may help to explain the accentuated vocalisation frequency of the early morning, since prey are then less mobile and hence less identifiable. Another reason for more early morning singing may derive from the tendency to prey cache, such that early morning dining can occur with ease without time consuming hunting for prey.



CONSERVATION
P. australis, P. longipes and P. traversi were all more abundant prior to European arrival; however, destruction of New Zealand birdlife began in earnest with arrival of the Maori and attendant forest clearing and burning. Chief ongoing threats to the species is the presence of non-native predatory mammals, particularly rats and stoats. The vulnerability to mammalian predators was demonstrated most dramatically with extirpation of Big South Cape Island robin populations after accidental arrival of black rats on this small island situated off of Stewart Island. Other threats to P. australis include indiscriminate use of toxins such as brodifacoum, which in broadcast form has annihilated approximately half of the robins in a given area. (Orueta and Ramos)

The IUCN classifies the taxon P. australis as of Least Concern; however, I contend this is misleading, since the subspecies P. australis rakiura occurs on a relatively small area, and hence the genetic survival of this subspecies is much more treacherous than the areal range data applied to the full species. In fact, the survival of P.a. rakiura is highly dependent upon keeping Stewart Island and associated surrounding small islands free of rats and stoats. Relocation of this species is not simple based upon transport of birds from Southeast Island to Little Mangere Island, with attendant stress and weight loss of individuals. (Caughley and Gunn) Moreover, relocation to Codfish Island is thought to be infeasible, since P. australis is thought to lack the transportation skills required to cross even low mountain ranges. (MacPhee)



REFERENCES

* Stuart Butchart and Jonathan Ekstrom. 2009. Species factsheet: Petroica australis, BirdLife International
* John Barnard Dunning. 2008. CRC handbook of avian body masses, CRC Press, 2nd edition, 655 pp.
* Jonathan Alderfer. 2009. National Geographic Complete Birds of the World, National Geographic Society, 384 pp.
* Hilary C. Miller and David M. Lambert. 2006. A molecular phylogeny of New Zealands Petroica (Aves: Petroicidae) species based on mitochondrial DNA sequences''. Molecular Phylogenetics and Evolution 40(3): 844-855.
* T. H. Worthy and Richard N. Holdaway. 2002. The lost world of the moa: prehistoric life of New Zealand, Indiana University Press, 718 pp.
* K.C. Burns. 2009. Fine-scale food hoarding decisions in New Zealand Robins ( Petroica australis). Journal of Ornithology, Springer Berlin / Heidelberg, ISSN 0021-8375, volume 150, number 2
* R.G. Powlesland. 1983. Seasonal and diurnal variation in vocal behaviour of the South Island robin, New Zealand Journal of Zoology, vol. 10: 225-232
* R. Hay. 1975. The vocal behaviour of the New Zealand robin (Petroica australis) and its local congenerics, Msc thesis, University of Auckland, New Zealand
* Charles Gald Sibley and Burt Leavelle Monroe. 1990. Distribution and taxonomy of birds of the world, 1111 pp.
* Jorge Fernández Orueta and Yolanda Aranda Ramos. 2001. Methods to control and eradicate non-native terrestrial vertebrate species, Council of Europe, 66 pp.
* Graeme Caughley and Anne Gunn. 1996. Conservation biology in theory and practice, 459 pp.
* R. D. E. MacPhee. 1999. Extinctions in near time: causes, contexts, and consequences, Springer Publishers, page 201