Variable oystercatcher

Haematopus unicolor J.R. Forster, 1844

New Zealand status: Endemic

Conservation status: Recovering

Other names: black oystercatcher, tōrea pango, torea pango, tōrea tai, torea tai, tōrea, torea, VOC

Geographical variation: No subspecies described, but the proportion of colour morphs differs in different parts of the country.

Variable oystercatcher. Black morph adult. Waipu estuary Northland, October 2012. Image © Thomas Musson by Thomas Musson tomandelaine@xtra.co.nz

Variable oystercatcher. Black morph adult. Waipu estuary Northland, October 2012. Image © Thomas Musson by Thomas Musson tomandelaine@xtra.co.nz

The variable oystercatcher is a familiar stocky coastal bird with a long, bright orange bill, found around much of New Zealand. They are often seen in pairs probing busily for shellfish along beaches or in estuaries. Previously shot for food, variable oystercatchers probably reached low numbers before being protected in 1922, since when numbers have increased rapidly. They are long-lived, with some birds reaching 30+ years of age.

The existence of different colour morphs (black, intermediate or ‘smudgy’, and pied) caused early confusion, and they were variously thought to be different species, forms, or hybrids. This confusion was compounded by a cline in morphs, with the proportion of all-black birds increasing from north to south. The colour morphs inter-breed freely and are now all accepted as being a single species. 

Identification

The variable oystercatcher is a large heavily-built shorebird. Adults have black upperparts, their underparts vary from all black, through a range of ‘smudgy’ intermediate states to white. They have a conspicuous long bright orange bill (longer in females), and stout coral-pink legs. The iris is red and eye-ring orange. Downy chicks have pale-mid grey upper parts with black markings and black bill. First-year birds have a dark tip to the bill, browner dorsal plumage, and grey legs.

Voice: variable oystercatchers are very vocal; loud piping is used in territorial interactions and when alarmed, and they have a loud flight call similar to other oystercatchers. Chicks are warned of danger with a sharp, loud ‘chip’ or ‘click’.

Similar species: the black and smudgy morphs are distinctive. Pied morph birds can be confused with South Island pied oystercatcher. If seen together, adult variable oystercatchers are noticeably larger, but first-year birds may be confused. The demarcation between black and white on the breast is generally sharper on South Island pied, and they have more white showing forward of the wing when folded, and a broader white wingbar in flight. The pied morph is similar to Chatham Island oystercatcher, but their ranges are not thought to overlap.

Distribution

Variable oystercatchers occur around most of the coastline of North, South, and Stewart Islands and their offshore islands. Strongholds are in Northland, Auckland, Coromandel Peninsula, Bay of Plenty, Greater Wellington, Nelson/Marlborough, and probably Fiordland. They occur  at lower average densities on west coasts of the two main Islands, and have not been recorded from any outlying island groups.

Habitat

Variable oystercatchers breed most commonly on sandy beaches, sandspits, and in dunes, but will use a wide variety of coastal habitat types, including shell banks, rocky shorelines, and less often gravel beaches. They forage in all these areas and also on inter-tidal mud-flats in estuaries, and on rock platforms. Variable oystercatchers are not usually seen far from the coast, but will forage in paddocks, and occasionally nest a short distance inland, usually on mown or grazed grassy areas or bare ground. A very few nest around lakes or a short distance up braided rivers.

Population

There were about 2000 variable oystercatchers in the early 1970s, this rose to about 4000 by the mid-1990s. There have been no nationwide surveys in recent years, but if the increase has continued at the same rate, the total population would now be 4500-5000. Population growth and delayed maturation mean there are many pre-breeders in the population, so the number of breeding birds is somewhat less than the total. Roughly two-thirds of the population is thought to be in the North Island.

Threats and conservation

Breeding success of variable oystercatchers is often low, with main causes of failure being predation of eggs or chicks by a range of mammalian and avian predators, flooding of nests by big tides, and disturbance resulting from human recreational use of the coast. Adults generally show high annual survival, but are occasionally killed by cats and stoats. No specific conservation measures are undertaken, but some birds on the east coast of the northern North Island benefit from protection programmes for New Zealand dotterels and fairy terns.

Breeding

Variable oystercatchers breed in monogamous pairs, and defend territories vigorously against neighbours. Nests are normally simple scrapes in the sand, often with a marker of driftwood, vegetation, or flotsam. The 2-3 eggs are usually laid from October onwards, and replaced if lost. Incubation is shared and takes about 28 days. Chicks fly at 6-7 weeks-old, and late chicks may not fledge until March. Chicks are vigorously defended by both parents, often well after fledging.

Adults show high fidelity to mate and site. Many breeding pairs remain on territory year-round, but some join flocks of pre-breeders for the autumn and early winter, typically at larger estuaries.

Behaviour and ecology

Variable oystercatchers are often highly aggressive towards people close to nests or chicks, dive-bombing (sometimes making contact) and screeching. They also undertake distraction displays on the ground in defence of eggs and chicks. On the North Island east coast, territories often overlap with those of New Zealand dotterels. There is normally little aggression, but there have been a few cases of nearby dotterel nests being taken over (usually after variable oystercatchers have lost their own nest), and a few birds have been seen killing small dotterel chicks.

Unlike most shorebirds, variable oystercatchers feed their young. Small chicks often remain hidden under vegetation, rocks, etc, with parents bringing food; this probably reduces the risk of predation by gulls and harriers. As chicks grow, they gradually find more of their own food, but are often seen begging from parents well after fledging. Sibling rivalry is common; when hatching is asynchronous, there is often large variation in size of the chicks within a brood. Adults commonly false-brood to conceal the location of nest or chicks.

In North Canterbury, a few mixed variable x South Island pied oystercatcher pairs produce viable offspring.

Food

Variable oystercatchers eat a wide range of littoral invertebrates, including molluscs, crustaceans, and annelids. They favour bivalve molluscs (e.g. mussels, tuatua, cockles) when these are available; these are opened either by pushing the tip of the bill between shells and twisting, or by hammering. They occasionally take small fish. On grass, variable oystercatchers eat a range of terrestrial invertebrates, including earthworms. When foraging, they are sometimes accompanied by kleptoparasitic red-billed gulls.

Websites

http://en.wikipedia.org/wiki/Variable_Oystercatcher

http://www.nzbirds.com/birds/toreapango.html

References

Baker, A.J. 1973. Distribution and numbers of New Zealand oystercatchers. Notornis 20: 128-144.

Baker, A.J. 1974. Criteria for aging and sexing New Zealand oystercatchers. New Zealand Journal of Marine and Freshwater Research 8: 211-221.

Dowding, J.E.; Moore, S.J. 2006. Habitat networks of indigenous shorebirds in New Zealand. Science for Conservation 261. Department of Conservation, Wellington.

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

Marchant, S.; Higgins, P.J. (eds) 1993. Handbook of Australian, New Zealand and Antarctic birds. Vol. 2, raptors to lapwings. Oxford University Press, Melbourne.

Robertson, C.J.R.; Hyvonen, P.; Fraser, M.J.; Pickard, C.R. 2007. Atlas of Bird Distribution in New Zealand. Ornithological Society of New Zealand, Inc., Wellington.

Rowe, L. 2008. Breeding of variable oystercatchers (Haematopus unicolor) at Kaikoura Peninsula, South Island, New Zealand. Notornis 55: 146-154.

Recommended citation

Dowding, J.E. 2013. Variable oystercatcher. In Miskelly, C.M. (ed.) New Zealand Birds Online. www.nzbirdsonline.org.nz

Variable oystercatcher

Social structure
monogamous
Breeding season
  • Jul
  • Aug
  • Sep
  • Oct
  • Nov
  • Dec
  • Jan
  • Feb
  • Mar
  • Apr
  • May
  • Jun
Nest type
scrape
Nest description
Simple scrape in sand, shells or occasionally soil and normally unlined.
Nest height (mean)
0 m
Clutch size (mean)
2-3
Clutch size (min)
1
Clutch size (max)
3
Mean egg dimensions (length)
59 mm
Mean egg dimensions (width)
41 mm
Egg colour
Background buff to pale green-blue, variable dark brown-black splotches and streaks
Egg laying dates
  • Jul
  • Aug
  • Sep
  • Oct
  • Nov
  • Dec
  • Jan
  • Feb
  • Mar
  • Apr
  • May
  • Jun
Interval between eggs in a clutch
2 days days
Incubation behaviour
shared
Incubation length (mean)
28 days
Incubation length (min)
25 days
Incubation length (max)
32 days
Nestling type
precocial
Nestling period (mean)
Up to 2 days
Age at fledging (mean)
40-50 days
Age at independence (mean)
Highly varia ble, many leave natal territory within 2-4 weeks of fledging while a few remain with parents over winter.
Age at first breeding (typical)
4-6 years
Age at first breeding (min)
2 years
Maximum longevity
At least 30 years
Maximum dispersal
Juveniles up to 300 km