The Chatham Island oystercatcher (torea) is an endangered species found only on the Chatham Islands, 800 km to the east of mainland New Zealand. Although pied (black-and-white), and similar in appearance to the pied morph of the variable oystercatcher, it is a smaller and stockier bird. Pairs defend coastal territories throughout the year in rocky and sandy sites around the islands of the Chathams.
The lack of suitable breeding habitat, as a result of clearing the land for farming and introduction of marram grass to the dunes, and predation by introduced animals apparently took a large toll on the oystercatcher population since the 1850s. By 1970 there may have been as few as 50 birds left. Sporadic management improved the situation but by 1998 there were still only about 140 birds in the population. More intensive management over the next 7 years resulted in the population more than doubling to over 300 birds.
The Chatham Island oystercatcher is the only resident oystercatcher species on the Chatham Islands. It is a sturdy shorebird with pied plumage, and a smudgy border between the black and white feathers on the breast. The eyes and bill of adults are red and the legs and feet are pink. Juveniles have paler skin and a dark-tipped bill; adult characteristics are gradually attained in about 2 years.
Voice: the staccato high-pitched piping call is used in territorial defence and courtship. Adults also use piercing screech and squawk calls and softer calls are made in flight.
Similar species: the pied morph of the variable oystercatcher has similar plumage but is a larger bird. The South Island pied oystercatcher (which has been recorded on the Chatham Islands) is also similar in appearance, but smaller and less sturdy, and has a more distinct demarcation between the black and white feathering of the chest.
Distribution and habitat
Chatham Island oystercatchers breed around the rocky and sandy coasts of Chatham, Pitt, Rangatira (South East) and Mangere Islands. More sheltered coastlines are favoured, where there is access to a food supply throughout the tidal cycle, for example a rocky wave platform exposed at low tide, kelp debris or a stream mouth. Occasionally, birds will feed on adjacent farmland, particularly in winter. Territories vary in size from 100 m to 1 km of coastline depending on the habitat quality and protection from predators. Breeders defend their territories throughout the year, but juveniles tend to move around the islands and may form small flocks, and spend more time in non-breeding areas, including lagoon and pond shorelines. Some Rangatira breeding pairs also regularly forage on Pitt Island, 2 km away.
There were possibly as few as 50 Chatham Island oystercatcher in 1970, 110 in 1987, and 142 in 1998. Intensive management of predators, stock exclusion, and protection of nests from high seas in northern Chatham Island resulted in a rapid population increase by the mid-2000s to over 300 birds, including 89 pairs, for the population as a whole. By 2006 there was a minimum of 194 birds in northern Chatham Island, 60 birds elsewhere on the island, and 59 birds on the other three islands. Sporadic, less intensive management at key sites has apparently maintained the population at a similar level, although recruitment of juveniles has returned to pre-management levels.
Threats and conservation
The most significant threat to Chatham Island oystercatchers on Chatham and Pitt Islands is predation by introduced cats. Cats prey on eggs and chicks, and to a lesser extent, adults. Introduced weka also prey on eggs. Trampling of nests by farm animals also occurs. Farming has changed the vegetation on the coast, particularly through the establishment of marram, an introduced grass, to stabilise the dunes. The resulting steep-faced dunes force birds to nest close to the high tide mark, where they are vulnerable to being washed away by storm seas.
An intensive period of management in northern Chatham Island from 1998-2004 showed that predator control, stock exclusion and re-locating nests away from the high tidemark could greatly improve productivity. An influx of young birds entering the breeding population at 2-6 years of age quickly more than doubled the population. Trial dune restoration also showed that replacing marram with native vegetation and providing more space in the foredune created a safer nesting environment.
Chatham Island oystercatchers are monogamous and share incubation duties. Nests are a simple scrape in the sand or gravel, or a lined depression on a rocky outcrop. Incubation of the 1-3 eggs is shared and takes about 29 days. The chicks are mobile as soon as they are dry, and rely on hiding and camouflage amongst tidal debris to escape predation. Juveniles can fly at 6 weeks of age and may disperse or stay with their parents for several months. Management of nesting areas (particularly control of cats) boosts productivity from an average of 0.35 to 1.0 chicks per pair per year.
Behaviour and ecology
Chatham Island oystercatchers defend their breeding and feeding territories throughout the year. Juveniles disperse around the islands but tend to return to their natal areas, or nearby coastline, to breed. The oldest known adult bird was 30+ years of age.
Oystercatchers use their sturdy bills to prize, hammer, probe or pick for invertebrates in rocky or sandy substrate. Food includes amphipods, polychaete worms, nemerteans (ribbon worms), molluscs, crustaceans, echinoderms, ascidians and anemones.
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.
Moore, P.J. 2008. The recovering population of Chatham Island oystercatcher (Haematopus chathamensis). Notornis 55: 20-31.
Moore, P.; O’Connor, S.; Hedley, G.; Goomes, R. 2001. Chatham Island Oystercatcher - report of 1999/2000 field season. Science & Research Internal Report 189. Department of Conservation, Wellington.
Moore, P.J.; Reid, C. 2009. Effectiveness of management on the breeding success of Chatham Island oystercatcher (Haematopus chathamensis). New Zealand Journal of Zoology 36: 431-446.
Moore, P.J. 2013. Chatham Island oystercatcher in Miskelly, C.M. (ed.) New Zealand Birds Online. www.nzbirdsonline.org.nz
Chatham Island oystercatcher
- Social structure
- Breeding season
- Nest type
- Nest description
- A simple scrape in soft sandy substrate with an edging of tidal debris for camouflage and or shelter. On rocky habitat birds nest in depressions that contain finer substrate with a lining or prostrate vegetation. Occasionally nest in short vegetation near the shore. On offshore islands nests are often under small bushes or rock overhangs away from skua territory.
- Nest height (mean)
- 0 m
- Maximum number of successful broods
- Clutch size (mean)
- Clutch size (min)
- Clutch size (max)
- Mean egg dimensions (length)
- 57 mm
- Mean egg dimensions (width)
- 40 mm
- Egg colour
- Olive-grey with dark brown spots and blotches
- Egg laying dates
- Interval between eggs in a clutch
- 2 days days
- Incubation behaviour
- Incubation length (mean)
- 29.4 days
- Incubation length (min)
- 26 days
- Incubation length (max)
- 33 days
- Nestling type
- Nestling period (mean)
- 1-2 days
- Age at fledging (mean)
- 42-49 days
- Age at independence (mean)
- Variable, some disperse as soon as they can fly, most stay with parents for several months
- Age at first breeding (typical)
- 2-5 years
- Age at first breeding (min)
- 2 years
- Maximum longevity
- At least 30 years
- Maximum dispersal
- 80 km