Data Zone
Justification of Red List category
Inconsistencies and a lack of transparency in the methods used to census the population in the past have meant that it is difficult to assess changes in this species's population size. Some past analyses have indicated a rapid decline, but a more recent analysis concluded that it is unlikely to have undergone a severe decline over recent decades, and is probably slightly declining or stable. Therefore, although the species is unlikely to have undergone a large population reduction over the past three generations, there is a large amount of uncertainty. For this reason, the species is listed as Near Threatened.
Population justification
A nationwide census was carried out by the Ornithological Society of New Zealand in 1995-1998 (G. A. Taylor per R. Coumbe in litt. 2000), and counted 48,000 nests (Powlesland 1998), thus the number of mature individuals was estimated to be 96,000. The methods used for these surveys are poorly-documented, but appear to have involved ground counts on the North Island, and a combination of ground and aerial surveys on the South Island (Hawkins and Powlesland 1995, in Mischler 2018b). The omission of some known current breeding colonies and sites from this census indicates that the population size may have been underestimated (Mischler 2018a).
Following a series of ground counts made across four river systems in Southland from 2004 to 2006, calibrated with aerial photography, the total population size in New Zealand was estimated at approximately 90,000 mature individuals (McClellan 2009). However, it has been suggested that gaps between the timing of some of the ground counts and aerial photographs, together with the application of inaccurate conversion factors for nests and individuals, could have led to inaccuracies in the population estimate, resulting in an overestimate (Mischler 2018a,b).
A further nationwide census took place from 2014-2017. Aerial surveys were used to locate, photograph and count nests, and ground surveys were used to determine a correction factor to apply to the nest counts. A total of 60,256 nests were recorded (Mischler 2018a,b), which is here assumed to equate to 120,512 mature individuals.
The population size is here placed in the band 90,000 - 121,000 mature individuals.
A genetic analysis showed that the species has two distinct genetic groups, with individuals in the Rotorua region on the North Island being distinct from those in the rest of New Zealand (Mischler et al. 2018). The species is therefore likely to have at least two subpopulations.
Trend justification
A nationwide census was carried out by the Ornithological Society of New Zealand in 1995-1998 (G. A. Taylor per R. Coumbe in litt. 2000), and counted 48,000 nests (Powlesland 1998), thus the number of mature individuals was estimated to be 96,000. The omission of some known current breeding colonies and sites from this census indicates that the population size may have been underestimated (Mischler 2018a).
A comparison of surveys of sections of nine rivers in the Upper Waitaki Basin in 1962, 1965 and 1968 and in 1991-1994 found lower densities in the 1990s in six rivers, with significant declines in two of these (Ahuriri and Hopkins). Six rivers had lost breeding colonies (Maloney 1999). A long-term dataset from the Ashburton River, Canterbury, found a significant decline of 3.6% per year (C. O'Donnell unpubl. data, in McClellan 2009), which would equate to a reduction of 58% over three generations (23.55 years).
An analysis of ground counts made across four river systems in Southland from 1974 to 2006, calibrated with aerial photography of colonies and population trends, found that between 1977 and 2006, the number of breeding individuals in Southland underwent a rapid decline, at a rate equivalent to 6% per year (McClellan 2009). This would equate to a reduction of 77% over three generations (23.55 years). However, the overall trend in the number of breeding birds for the period 1995-2006 was less clear, with a non statistically significant decline of 2.6% per year (McClellan 2009), which would equate to a reduction of 46% over three generations (23.55 years). The number of breeding colonies on the four rivers was found to have declined by almost 70%, and the mean number of breeding birds per colony also declined over this period (McClellan 2009). By combining the results of this analysis with the decline found at the Ashurton River and assuming equivalent declines in the rest of the South Island population, the entire New Zealand population was estimated to have declined at a rate equivalent to 78% in 30 years (McClellan 2009), which would be equivalent to a rate of 70% over three generations (23.55 years). The total population size in New Zealand was estimated at approximately 90,000 mature individuals (McClellan 2009). It was noted that the results should be treated with caution, due to inconsistencies in, and lack of documentation of the methods used to estimate population size, and possible bias in the historical estimates (McClellan 2009). Additionally, it has been suggested that gaps between the timing of some of the ground counts and aerial photographs, together with the application of inaccurate conversion factors for nests and individuals, could have led to inaccuracies in the population estimates in this analysis, resulting in overestimates (Mischler 2018a,b).
An analysis of counts from 30 South Island rivers over 52 years from 1962-2014 found that the best model predicted an overall 77% decline over 30 years, which would equate to a reduction of 68% over three generations (23.55 years). For counts from Southland, the best model predicted a 90% reduction over 30 years (Smith and McClennan 2016).
A further nationwide census took place from 2014-2017. Aerial surveys were used to locate, photograph and count nests, and ground surveys were used to determine a correction factor to apply to the nest counts. The maximum number of nests recorded in a season was 60,256 in 2016/17 (Mischler 2018a,b). The methods and results of historical surveys were reviewed, and the methods were found to have been inaccurate and inconsistent, and likely to have produced over-estimates of the population size. Additionally, there was a large annual variability of the number of breeding pairs within regions (15-79%, with a mean of 42%: Mischler 2018a,b). As a result, analysis of trends was difficult, but overall, the data did not indicate a severe population decline since 1995-1998, and the study concluded that the population is likely to be slightly declining or stable (Mischler 2018a,b).
Following the conclusions of Mischler (2018a,b), the population trend is unknown, but is likely to be slightly declining or stable. It is not known whether there has been a population reduction over the past three generations, but any reduction is most likely to have been small (and less than 30% over three generations). However, based on the trend for the period 1995-2006 in Southland (a decline of 2.6% per year) found by McClellan (2009), combined with the decline found in the Ashburton River, Canterbury (3.6% per year; C. O'Donnell unpubl. data, in McClellan 2009) and assuming that the latter rate is representative of the rest of the South Island population, assuming the North Island population is stable, and weighting declines by the proportion of the total population found by Mischler (2018a, b), there could have been an overall reduction of up to 50% over the past three generations. The estimated past reduction is therefore placed in the band 0-50%.
Larus bulleri is endemic to New Zealand. It is mainly found in the South Island, with a small percentage of the population inhabiting the North Island (Mischler 2018a,b). The region of Southland supports the majority of the population (Powlesland 1998, Taylor 2000, Mischler 2018a,b), with 55.9% of the total population in 2016/7 (Mischler 2018a,b). Other regions with breeding populations include Canterbury (34.3%), Otago (4.6%), West Coast (2.6%), North Island (1.6%), Marlborough (0.6%) and Tasman (0.3%; Mischler 2018a,b). The species has expanded its breeding range in the North Island in the last few decades, with the largest breeding populations found in the Auckland region, the Bay of Plenty and Hawke's Bay (Mischler 2018a,b). South Island rivers with large populations include the Ashburton in Canterbury, and the Aparima, Eyre Creek, Mataura and Oreti in Southland (Mischler 2018a,b).
It breeds in colonies, mainly on sparsely-vegetated gravels on riverbeds, often in braided river systems (Higgins and Davies 1996, Taylor 2000). It appears to favour rivers adjacent to high-producing pasture (McClellan and Smith 2015). It will also nest on sand-spits, shellbanks, lake margins, swamps, harbours and sandy coasts (Higgins and Davies 1996, Taylor 2000). It often roosts and feeds on farmland, and scavenges in urban areas where refuse is available (Higgins and Davies 1996). Some birds remain at colonies throughout the year, but most move from inland breeding sites to the coasts (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).
The main threat is predation by introduced mammals. Brown Rats Rattus norvegicus take eggs and chicks in the North Island. Remote video cameras have shown that Stoats (Mustela erminea), Ferrets (Mustela furo) and feral cats (Felis catus) are major predators on South Island colonies, often taking hundreds of chicks in a season (Biswell 2006). Hedgehogs (Erinaceus europaeus) may also take eggs. Increased populations of the native Swamp Harrier (Circus approximans) and Kelp Gull (Larus dominicanus) also exert significant predation (Mischler 2018a). The recreational use of riverbeds and coastal areas is increasing, causing greater disturbance of nesting colonies (Taylor 2000). There have been incidents of vehicles being driven through colonies and large numbers of birds being shot (McClellanan and Habraken 2019). River modification, including hydroelectric development, channelisation and water and gravel extraction, also has a significant impact. The spread of invasive alien plant species such as Russell Lupin (Lupinus polyphyllus) and Broom (Cytisus scoparius) is a major threat, reducing suitable nesting habitat on riverbeds, providing cover for predators and forcing birds to nest closer to the water where they are more vulnerable to flooding (Maloney 1999, Taylor 2000). Intensification of agricultural practices, such as the increased use of pesticides, may reduce the availability of invertebrates. Extreme weather events such as droughts and heavy snowfall can kill thousands of individuals (McClellanan and Habraken 2019). Although the species is adapted to flooding at nesting colonies, an increased frequency of flooding may impact on productivity (McClellan 2009).
Conservation and Research Actions Underway
It is legally protected in New Zealand.
Studies of population size, survey methods, breeding biology, genetics, habitat, threats, movements and dispersal have been undertaken.
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). In Southland, invasive plant species and ferrets are extensively controlled in the main riverbeds. Other invasive mammal species are occasionally trapped. Invasive mammals have also been trapped at the Ashburton Ricer and Hawke's Bay (Mischler 2018a). Kelp Gulls have been controlled in the lower Waimakariri River, and information boards have been installed to alert the public to their presence (Thierry et al. 2016).
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.
Text account compilers
Wheatley, H.
Contributors
Bell, M., Benstead, P., Harding, M., Hitchmough, R., Khwaja, N., Mahood, S., McClellan, R., Robertson, H.A., Symes, A., Taylor, G.A. & Taylor, J.
Recommended citation
BirdLife International (2024) Species factsheet: Black-billed Gull Larus bulleri. Downloaded from
https://datazone.birdlife.org/species/factsheet/black-billed-gull-larus-bulleri on 22/12/2024.
Recommended citation for factsheets for more than one species: BirdLife International (2024) IUCN Red List for birds. Downloaded from
https://datazone.birdlife.org/species/search on 22/12/2024.