EN
Black-billed Gull Larus bulleri



Justification

Justification of Red List Category
Surveys indicate that this species may have undergone a very rapid decline over three generations (32 years). It is therefore listed as Endangered.

Population justification
The most complete nationwide census was carried out in 1996-1997 (G. A. Taylor per R. Coumbe in litt. 2000), and counted 48,000 nests (Powlesland 1998), thus the number of mature individuals is estimated to be 96,000; however, more up-to-date survey data are required.

Trend justification
Very rapid declines have been noted in a number of this species's populations over the last couple of decades. Despite increases in numbers of a few smaller colonies, the species is believed to be undergoing a continuing and very rapid decline overall.

Distribution and population

Larus bulleri is endemic to New Zealand. The majority of the population (78%) breeds in Southland (Taylor 2000), mostly on the Mataura, Oreti, Aparima and Waiau rivers (Powlesland 1998). On the Oreti and Aparima, the number of breeding birds appears to have plummeted by as much as 90% in the last one to two decades (Powlesland 1998, Taylor 2000, McClellan in litt. 2007). Upper Waitaki catchment populations declined between the 1960s and 1990s, with breeding colonies disappearing from six rivers (Maloney 1999). Recent surveys at one minor colony in the Hunter Valley, Otago, showed numbers had dropped from 581 in 1969 to just 12, with the same trend seen in the nearby Makarora catchment area (R. Hitchmough in litt. 2005). Overall, Southland counts estimated a minimum of 57,000 pairs in 1985-1986 (Taylor 2000), declining by c.40% to 33,500 pairs in 1996-1997 (Powlesland 1998). Its numbers and range continue to increase in the North Island, but these colonies are small and the increase does not offset the South Island declines (R. Hitchmough in litt. 2005). The most complete nationwide census was carried out in 1996-1997 (G. A. Taylor per R. Coumbe in litt. 2000), and counted 48,000 nests (Powlesland 1998). Some birds remain at colonies throughout the year, others move from inland breeding sites to the coasts (Higgins and Davies 1996).

Ecology

In the South Island, it breeds mainly on braided river systems (Higgins and Davies 1996, Taylor 2000). In the North Island, it uses sand-spits, shellbanks, lake margins and riverflats (Taylor 2000). It often roosts and feeds on farmland, and scavenges in urban areas where refuse is available (Higgins and Davies 1996). It has a varied diet of terrestrial, freshwater and marine invertebrates, fish and shellfish (Higgins and Davies 1996, Heather and Robertson 1997). Breeding can begin after two years (Heather and Robertson 1997), but many individuals do not start until six years old, and adults may live over 30 years (R. Hitchmough in litt. 2005).

Threats

Brown rats Rattus norvegicus take eggs and chicks in the North Island. Remote video cameras have shown that mustelids Mustela spp. and feral cats are major predators on South Island colonies, often taking hundreds of chicks in a season (Biswell 2006). Hedgehogs may also take eggs. The recreational use of riverbeds and coastal areas is increasing, causing greater disturbance of nesting colonies (Taylor 2000). River modification (including hydroelectric development, and water and gravel extraction) also has a significant impact. The spread of weeds is a major threat, reducing suitable nesting habitat on riverbeds (Maloney 1999, Taylor 2000).

Conservation actions

Conservation Actions Underway
Studies of breeding biology, movements and dispersal are being undertaken. Localised and nationwide counts are on-going. Habitat restoration and protection in the MacKenzie Basin is undertaken as part of Project River Recovery, including predator research and a public awareness campaign (Taylor 2000).

Conservation Actions Proposed
Monitor key breeding populations. Initiate nest protection and trapping of introduced predators at key colonies. Initiate riverbed weed control if nesting habitat continues to be lost. Assess the possible impacts of further hydroelectric dam projects, and gravel and water extraction proposals (Taylor 2000).

Identification

37 cm. Pale grey-and-white gull. Adult, pale, silvery-grey back and wings. Thinly black-bordered wing-tips. White underparts. Long, thin, black bill. Black to reddish-black legs and feet. White eye. Juvenile, more extensive black on wing-tips. Pale, flesh bill with dark tip. Pinkish to reddish-black legs. Brown eye. Similar spp. Red-billed Gull L. novaehollandiae has shorter, deeper bill - red in adults, darker grey wings, more extensive black on wing-tips.

Acknowledgements

Text account compilers
Benstead, P., Harding, M., Khwaja, N., Mahood, S., McClellan, R., Taylor, J. & Symes, A.

Contributors
Hitchmough, R., McClellan, R. & Taylor, G.


Recommended citation
BirdLife International (2020) Species factsheet: Larus bulleri. Downloaded from http://www.birdlife.org on 04/07/2020. Recommended citation for factsheets for more than one species: BirdLife International (2020) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 04/07/2020.