Current view: Data table and detailed info
Taxonomic source(s)
del Hoyo, J., Collar, N.J., Christie, D.A., Elliott, A. and Fishpool, L.D.C. 2014. HBW and BirdLife International Illustrated Checklist of the Birds of the World. Volume 1: Non-passerines. Lynx Edicions BirdLife International, Barcelona, Spain and Cambridge, UK.
Turbott, E.G. 1990. Checklist of the Birds of New Zealand. Ornithological Society of New Zealand, Wellington.
IUCN Red List criteria met and history
Red List criteria met
Red List history
| Migratory status |
full migrant |
Forest dependency |
does not normally occur in forest |
| Land-mass type |
|
Average mass |
- |
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%.
Country/territory distribution
Important Bird and Biodiversity Areas (IBA)
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.
