Black robin

Petroica traversi (Buller, 1872)

Order: Passeriformes

Family: Petroicidae

New Zealand status: Endemic

Conservation status: Nationally Critical

Other names: Chatham Island black robin, Chatham Island robin

Geographical variation: Nil

Black robin. Adult. Rangatira Island, February 2010. Image © David Boyle by David Boyle

Black robin. Adult. Rangatira Island, February 2010. Image © David Boyle by David Boyle

The black robin is endemic to the Chatham Islands where it is now confined to the southern extremity of its former range. It is a quiet, confiding forest-dweller, alert and almost always active in the lower forest strata. Birds are commonly attracted to human presence. The species is famous world-wide for its inspiring recovery from imminent extinction in the early 1980s.

Black robins are closely related to New Zealand’s other Petroica species (tomtit, and North Island and South Island robins). All are descended from Australian Petroica ancestors.

Identification

The black robin is a small, rotund bird intermediate in size and form between its tomtit and robin relatives. Both sexes are completely black at all ages, though juveniles have subtle pale streaking on the crown. Its tail, legs and bill are slightly longer and stouter than those of the Chatham Island tomtit with which it shares its range. Birds appear short and round through their habit of holding their large heads hunched close to their bodies. The sexes are indistinguishable to casual observation (weight and size ranges overlap).

Voice: a limited range of calls. Song, generally of 4-5 clear notes, is given mainly by males. It is not as rich or melodic as the calls of mainland robins. Males and females give a simple down-scale call, perhaps territorially. A single high-pitched note may be given in alarm or aggression. Contact calls when feeding resemble tomtit trills but are quieter.

Similar species: this is the only robin and only completely black Petroica species on the Chatham Islands. It shares its breeding and foraging ranges with the smaller Chatham Island tomtit.

Distribution and habitat

Black robins originally inhabited the five major forested islands in the archipelago: Chatham (90,040 ha), Pitt (6325 ha), Rangatira (South East) Island (218 ha), Mangere (113 ha), and Little Mangere (9 ha). Human activities eliminated all original populations, the last in 1976 when the survivors were moved from Little Mangere to Mangere Island for conservation purposes. Descendants of these seven survivors have been returned to Rangatira Island, which, like Mangere Island, is a protected sanctuary.   

The black robin breeds, forages and socialises almost exclusively within forest interiors. This habit is unique among Australasia’s temperate-zone Petroica. Birds are reluctant to venture outside forest perimeters and do not breed well in scattered or fragmented vegetation. The species favours large continuous tracts of forest with a closed canopy, mixed under-storey species and edges enclosed by dense sheltering vegetation. Optimum forest type comprises trees of complex structure, mixed height and age classes which offer abundant nesting and foraging opportunities.

Populations

During pre-breeding censuses in October 2011, 281 birds were counted. The Mangere Island population comprised 47 birds. At least 234 adults were alive on Rangatira Island.

Threats and conservation

The black robin is more extinction-prone than its Petroica relatives. Very low reproductive output, specialised forest-dwelling and foraging habits, narrow tolerances to habitat type and quality, and very limited powers of dispersal are key vulnerabilities. These traits exaggerate the effects of extinction pressures and suppress rebounds from population knock-downs.

Chronic inbreeding over more than a century and severe loss of genetic diversity further jeopardise resilience. Degrees of relatedness in today’s populations are among the highest recorded for any avian species in the wild. Inbreeding depression is expressed through lowered reproductive output. Long-term persistence of populations is uncertain.

Black robins are extremely vulnerable to external threats. Original populations were eliminated swiftly by introduced cats and Pacific rats. Mice and starlings may also threaten population viability. Historically, one large population was eliminated by declines in forest area and quality alone. 

Today’s populations are unnaturally small. Long periods of farming on both islands have reduced the extent of suitable forest cover available (about 8 ha on Mangere Island and 110 ha on Rangatira Island).

Conservation relies on quarantine of today’s small populations from predators, new pathogens and habitat loss. Genetic health is managed though increasing population sizes. Extinction risk will be reduced by translocating birds to new habitats within the Chathams.

Breeding

Black robins breed annually in spring and summer. Cup-like nests are built in tree cavities or dense Muehlenbeckia vines, usually within 2 metres of the ground. Most clutches comprise two eggs (range 1-3). Only the female incubates but both parents care for the young. Pairs are capable of raising two clutches to independence per season (one is usual).

Behaviour and ecology

The breeding, foraging and social behaviour of the black robin resembles that of its Petroica relatives. Water and open spaces are barriers to movement, so that colonising of detached habitat is slow, and natural migration between islands does not occur. They have a patriarchal social system based on territories, which are defended most aggressively during breeding. Birds are monogamous and may mate for life. Site and pair fidelity is strong between years. Juveniles are usually evicted from territories in their natal season.

Black robins are always alert and active, moving about in brief direct flights. Birds forage diurnally in sub-storey vegetation and on the ground (usually pouncing from low branches, or flicking ground litter over with their bills). Large prey items (such as wetas < 20 mm) are thrashed against objects first to remove limbs. Unlike mainland robins, black robins do not cache food.

Black robins make a long parental investment in a few offspring, typical of the slow breeding strategies of temperate southern hemisphere birds. Reproductive output is the lowest recorded among known Australasian Petroica because clutch sizes are unusually small, the breeding cycle is long and the breeding season is shortened climatically. 

Males live 4.2 years on average, slightly longer than females (3.7) but a very small number of both sexes may reach ages up to 14 years. Both can breed successfully as yearlings and in old age (≤ 11 years).

Food

Black robins are wholly insectivorous. Prey items include many winged and wingless invertebrate taxa at all life-stages, including spiders, flies, wetas, worms, beetles, moths, cockroaches and caterpillars.  

Weblinks 

http://www.doc.govt.nz/conservation/native-animals/birds/land-birds/black-robin/

http://www.doc.govt.nz/conservation/land-and-freshwater/offshore-islands/mangere-and-rangatira-islands/

References

Ardern, S.L.; Lambert, D.M. 1997. Is the black robin in genetic peril? Molecular Ecology 6: 21-28.

Butler, D.; Merton, D.V. 1992. The black robin—saving the world’s rarest bird. Oxford University Press. Melbourne.

Hay, R. 1975. The vocal behaviour of the New Zealand robin Petroica australis and its local congeners. Unpublished MSc thesis. University of Auckland.

Heather, B.D.; Robertson, H.A. 1996. Field guide to the birds of New Zealand. Viking. Auckland.

Higgins, P.J.; Peter., J.M. (eds) 2002. Handbook of Australian, New Zealand and Antarctic birds. Vol. 6: pardalotes to shrike-thrushes. Oxford University Press. Melbourne.

Kennedy, E.S. 2009. Extinction vulnerability in two small, chronically inbred populations of Chatham Island black robin Petroica traversi. Unpublished PhD thesis. Lincoln University, NZ.

Miller, H.C.; Lambert, D.M. 2006. A molecular phylogeny of New Zealand’s Petroica (Aves: Petroicidae) species based on mitochondrial DNA sequences. Molecular Phylogenetics and Evolution 40: 844-855.

Recommended citation

Kennedy, E.S. 2013. Black robin. In Miskelly, C.M. (ed.) New Zealand Birds Online. www.nzbirdsonline.org.nz

Black robin

Social structure
female-only incubation and brood-care
Breeding season
  • Jul
  • Aug
  • Sep
  • Oct
  • Nov
  • Dec
  • Jan
  • Feb
  • Mar
  • Apr
  • May
  • Jun
Nest type
woven cup
Nest description
Woven cup concealed in tree cavity, underside of dense Muehlenbeckia mantles or rarely rock crevice. Bowl lined with moss and feathers.
Nest height (mean)
1.84 m
Nest height (min)
0.1 m
Nest height (max)
5.5 m
Maximum number of successful broods
2
Clutch size (mean)
2.02
Clutch size (min)
1
Clutch size (max)
3
Mean egg dimensions (length)
22 mm
Mean egg dimensions (width)
17 mm
Egg colour
Creamy-white, sometimes pinkish, with purplish-brown blotches.
Egg laying dates
  • Jul
  • Aug
  • Sep
  • Oct
  • Nov
  • Dec
  • Jan
  • Feb
  • Mar
  • Apr
  • May
  • Jun
Interval between eggs in a clutch
1 day days
Incubation behaviour
female only
Incubation length (mean)
18 days
Nestling type
altricial
Nestling period (mean)
22 days
Nestling period (min)
19 days
Nestling period (max)
25 days
Age at fledging (mean)
22 days
Age at fledging (min)
19 days
Age at fledging (max)
25 days
Age at independence (mean)
42 days
Age at independence (min)
35 days
Age at independence (max)
60 days
Age at first breeding (typical)
1 year
Age at first breeding (min)
1 years
Maximum longevity
14 years
Maximum dispersal
Unlimited within available habitat