Chatham Island shag

Leucocarbo onslowi (Forbes, 1893)

New Zealand status: Endemic

Conservation status: Nationally Critical

Other names: Chatham Island cormorant, Chatham shag

Geographical variation: Nil

Chatham Island shag. Adult. Star Keys, Chatham Islands, August 1968. Image © Department of Conservation (image ref: 10035213) by John Kendrick, Department of Conservation Courtesy of Department of Conservation

Chatham Island shag. Adult. Star Keys, Chatham Islands, August 1968. Image © Department of Conservation (image ref: 10035213) by John Kendrick, Department of Conservation Courtesy of Department of Conservation

The Chatham Island shag is a critically endangered shag which, as its name suggests, is restricted to the Chatham Islands. It is the only large black-and-white shag in the island group. Despite their declining population, Chatham Island shags can still readily be found roosting on headlands along the rocky coast line. They are entirely marine, dispersing from colonies and roost sites to forage in coastal waters and within parts of the brackish Te Whanga Lagoon. During breeding, compact colonies form on rocky headlands and offshore islets located throughout the Chatham Islands.

Identification

The only large black-and-white shag, and the only one with pink feet, found in the Chatham Islands. Adult Chatham Island shags have a black head, hind neck and back, with white patches on the wings that appear as a white bar when the wing is folded. The chin, throat, breast and under wing are white. In flight, they have a white bar on the inner part of the upper wing, and another white patch on the back. The head is held lower than the body when flying. During the breeding season, the facial skin is dark purple with prominent orange-red caruncles above the base of the bill; this colour fades outside the breeding season. The upperparts of juveniles are dark brown rather than black, and the wing and dorsal patches are pale brown rather than white. Juveniles lack facial caruncles, and have pale purple facial skin.

Voice: Chatham Island shags are noisy at colonies, with a range of ticking, gargling and barking calls given by males; with females uttering soft puffing calls. They rarely call away from colonies.

Similar species: The Chatham Island shag is unmistakeable within the Chatham Islands. It is very similar to the New Zealand king shag and the pied morph of the Stewart Island shag, and would be difficult or impossible to separate from them if one ever reached the South Island.

Distribution and habitat

As implied by its name, the Chatham Island shag is endemic to the Chatham Islands group, with birds breeding on Chatham Island, Pitt Island (and offshore islets including Rabbit Islands) and The Star Keys. Colonies and roost sites are situated on exposed rocky coastal headlands or offshore islets. The Chatham Island shag is a marine feeder and is typically found foraging in coastal and inshore waters usually within a few kilometres of shore. One colony is found within the brackish Te Whanga Lagoon, where birds also forage within the southern deeper portion of the lagoon.

Population

The Chatham Island shag population has undergone significant decline in the past 15 years. In 1997 the breeding population was 842 pairs, but had declined to 355 pairs in 2011.

Threats and conservation

Populations of Chatham Island shag are threatened by loss of breeding habitat, disturbance from stock, human persecution, introduced predators, and gull predation. However, given that population declines have been recorded at colonies on remote predator-free offshore islands, it is likely that some type of at-sea effects are impacting on the population. There has been no direct conservation management action to benefit Chatham Island shag other than co-ordinated island-wide population censuses. The recent increase in fencing coastal habitats on the Chatham Islands will prevent stock from gaining access to some colonies, thereby reducing disturbance.The conservation status of this species was changed from nationally endangered to nationally critical.

Breeding

Chatham Island shags are colonial breeders, forming densely-packed colonies of 5 to 350 pairs. Colonies are found on exposed rocky headlands or offshore islets. Laying is mostly from October to December, but there is much variation both between and within colonies. Birds in the colony in Te Whanga Lagoon breed 3 months earlier than those in other colonies. Little is known about breeding biology. The 2-4 eggs are incubated for about 30 days, with chicks fledging in 4-6 weeks. Birds probably start breeding when three years old.

Behaviour and ecology

Chatham Island shags gather in small to medium-sized flocks when roosting, but usually feed solitarily. Birds swim on the surface, diving to forage underwater for small fish. Chatham Island shags are non-migratory, remaining within coastal Chatham Island waters throughout the year.

Food

The diet is mostly small fish (<25cm long), especially flatfish, opalfish and bullies, along with occasional squid, cuttlefish and octopus.

Weblinks

http://www.birdlife.org/datazone/speciesfactsheet.php?id=3691

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

References

Aikman, H.; Miskelly, C. 2004. Birds of the Chatham Islands. Department of Conservation, Wellington.

Bell, M.; Bell, D. 2000: Census of the three shag species in the Chatham Islands. Notornis 47: 148-153.

Bester, A.J.; Charteris, M. 2005: The second census of Chatham Island shag and Pitt Island shag – are numbers declining? Notornis 52: 6-10.

Heather, B.D.; Robertson, H.A. 2005. The field guide to the birds of New Zealand. Viking, Auckland.

Marchant, S.; Higgins, P.J. (eds.), 1990. Handbook of Australian, New Zealand and Antarctic birds. Vol. 1, ratites to ducks. Oxford University Press, Melbourne.

Robertson, H. A; Dowding, J. E; Elliott, G. P; Hitchmough, R. A; Miskelly, C. M; O’Donnell, C. F. J; Powlesland, R. G; Sagar, P. M; Scofield, R. P; Taylor, G. A. 2013. Conservation status of New Zealand birds, 2012. NZ Threat Classification Series 4. Department of Conservation. Wellington.

Recommended citation

Bell, M. 2013. Chatham Island shag. In Miskelly, C.M. (ed.) New Zealand Birds Online. www.nzbirdsonline.org.nz

Chatham Island shag

Social structure
monogamous
Breeding season
  • Jul
  • Aug
  • Sep
  • Oct
  • Nov
  • Dec
  • Jan
  • Feb
  • Mar
  • Apr
  • May
  • Jun
Nest type
ground-level platform
Nest description
A roughly constructed platform of twigs, ice plant and seaweed, about 50 cm in diameter.
Nest height (mean)
0 m
Nest height (min)
0 m
Nest height (max)
0 m
Maximum number of successful broods
1
Clutch size (mean)
2-3
Clutch size (min)
1
Clutch size (max)
4
Mean egg dimensions (length)
61 mm
Mean egg dimensions (width)
39 mm
Egg colour
Pale blue with chalky white coating
Egg laying dates
  • Jul
  • Aug
  • Sep
  • Oct
  • Nov
  • Dec
  • Jan
  • Feb
  • Mar
  • Apr
  • May
  • Jun
Interval between eggs in a clutch
Unknown but probably 2-3 days
Incubation behaviour
shared
Incubation length (mean)
Unknown probably 28-32 days
Nestling type
altricial
Nestling period (mean)
Unknown
Age at fledging (mean)
Unknown
Age at independence (mean)
Unknown
Age at first breeding (typical)
Unknown but probably 3 years
Maximum longevity
Unknown
Maximum dispersal
15 km