Phalacrocorax onslowi


Taxonomy [top]

Kingdom Phylum Class Order Family

Scientific Name: Phalacrocorax onslowi
Species Authority: Forbes, 1893
Common Name(s):
English Chatham Shag, Chatham Shag, Chatham Island Shag
Leucocarbo onslowi onslowi Turbott (1990)

Assessment Information [top]

Red List Category & Criteria: Critically Endangered B2ab(ii,iii,iv,v) ver 3.1
Year Published: 2012
Date Assessed: 2012-05-01
Assessor(s): BirdLife International
Reviewer(s): Butchart, S. & Symes, A.
Contributor(s): Bell, B., Bell, B., Bell, M. & Wilson, K.
Facilitator/Compiler(s): Benstead, P., Bird, J., Calvert, R., Harding, M., Lascelles, B., McClellan, R., Symes, A., Taylor, J.
Surveys in 2003 indicated that this species is in decline. This, in combination with the extremely small area occupied by its breeding colonies, which suffer disturbance from agriculture and feral mammals, qualifies the species as Critically Endangered.

Geographic Range [top]

Range Description: This species is restricted to the Chatham Islands, New Zealand. Four islands support breeding: Chatham, Star Keys, Rabbit and Pitt (Imber 1994, M. Bell in litt. 2012), with a further population on North East Reef (B. D. and D. Bell verbally 1999). In 1997, a census found a total of 842 pairs at 10 sites (Bell and Bell 2000), with the largest colony on Star Keys which, in 1980, had 358 nests containing eggs or chicks (Imber 1994). However, surveys in 2003-2004 estimated the breeding population to be 271 pairs, distributed at 13 colonies, with the largest colony on Star Keys holding 81 pairs (Bester and Charteris 2005, Wilson 2006). This represents a 67.8% decrease in total breeding pairs since 1997, but a poor breeding season or variability in the timing of breeding within and between seasons may have contributed to this apparent decrease, and further surveys are needed to confirm population trends (Bester and Charteris 2005). Although colonies are spread over three islands, the species's breeding range totals less than 1 ha (Wilson 2006). Its foraging range is assumed to be up to 24 km offshore (cf. New Zealand King Shag P. carunculatus).

New Zealand
Range Map: Click here to open the map viewer and explore range.

Population [top]

Population: A census carried out in 2011 counted 357 breeding pairs (M. Bell in litt. 2012), presumably equating to 714 mature individuals and c.1,070 individuals in total.
Population Trend: Decreasing

Habitat and Ecology [top]

Habitat and Ecology: It nests in colonies, usually high on exposed rocks on top of headlands or small islands, or on cliff-ledges (Marchant and Higgins 1990). It feeds mainly on small fish (Heather and Robertson 1997).

Systems: Terrestrial; Marine

Threats [top]

Major Threat(s): The largest breeding colonies are found on islands free of introduced predators (Taylor 2000). On Chatham, colonies are disturbed by humans, farm stock, feral cats, agriculture (Wilson 2006), feral pigs (Wilson 2006), Weka Gallirallus australis, brush-tailed possum Trichosurus vulpecula and dogs (Heather and Robertson 1997, Taylor 2000). Birds sometimes stampede from their nests when disturbed, causing egg breakage and subsequent predation by gulls (Taylor 2000), and several breeding colonies have been abandoned (Heather and Robertson 1997). Fur seals Arctocephalus forsteri may disturb the colony on Star Keys, possibly causing rapid declines (B. D. Bell in litt. 1994, Heather and Robertson 1997, Taylor 2000, M. Bell in litt. 2012), and have occupied former colony sites (Taylor 2000). Visits by tourists can cause disturbance to colonies if not supervised carefully (B. D. and D. Bell verbally 1999). Illegal shooting of birds occurs infrequently. Population declines may also reflect changes in the marine environment that affect their food supply (Bester and Charteris 2005). Having a distribution on relatively low-lying islands, this species is potentially susceptible to climate change through sea-level rise and shifts in suitable climatic conditions (BirdLife International unpublished data).

Conservation Actions [top]

Conservation Actions: Conservation Actions Underway
The first census of this species was completed in 1997 (Bell and Bell 2000), with a follow-up census carried out in 2003-2004 under the Chatham Islands Shag and Pitt Island Shag recovery plan (published 2001) (Bester and Charteris 2005). As yet no conservation action has been specifically directed towards the species (M. Bell in litt. 2012).

Conservation Actions Proposed
Census the entire adult population every five years and monitor a Chatham Island colony yearly to determine trends. Fence colonies from stock and pigs on main Chatham Island if agreement is reached with local owners. Conduct education and awareness-raising activities (M. Bell in litt. 2012), and educate dog owners about the possible impact of dogs on breeding grounds. Conduct research into the species's population dynamics, breeding biology, movements, foraging and diet (K.-J. Wilson in litt. 2008).

Citation: BirdLife International 2012. Phalacrocorax onslowi. The IUCN Red List of Threatened Species. Version 2015.2. <>. Downloaded on 30 August 2015.
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