Black-billed Gull (Larus bulleri): revise global status?

This discussion was first published as part of the 2018 Red List update. At the time a decision regarding its status was pended, but to enable potential reassessment of this species as part of the 2020 Red List update this post remains open and the date of posting has been updated.

Black-billed Gull (Larus bulleri) is endemic to New Zealand, with the majority of the population (c.70%) in Southland (McClellan 2009). It mainly breeds along braided river systems, though it can use other habitat types such as lake margins, sand-spits, riverflats and shellbanks (Higgins and Davies 1996, Taylor 2000); and it will roost and feed in a variety of other habitats, e.g. urban areas and farmland (Higgins and Davies 1996).

The species is currently listed as Endangered because it is thought to have undergone, and still be undergoing a rapid decline (see BirdLife International 2017). The key threats that may be causing this decline are the impact of invasive predators and habitat loss/degradation. Brown rats Rattus rattus will predate nests on North Island, while on South Island the main invasive predators appear to be feral cats and mustelids (Biswell 2006). Hedgehogs are also a potential threat to eggs. Development of river systems, either through gravel extraction or to build hydroelectric plants, is impacting the species’ habitat, while suitable nesting habitat may be reduced by the spread of weeds (Maloney 1999, Taylor 2000). Recreational activities in rivers and coastal areas also increase the level of disturbance for this species (Taylor 2000).

Further surveys using aerial colony counts (McClellan and Smith 2015) have been conducted, and these suggest that the overall rate of decline may be even worse than that currently used in the Red List assessment. Therefore, after incorporating this new information, the species has been reassessed here against all criteria.

Criterion A – The species is currently listed as Nationally Critical in New Zealand (Robertson et al. 2017). However, it is listed as such because of a predicted decline of >70% over 3 generations (see Townsend et al. 2008), while the threshold for listing as Critically Endangered on the global Red List based on rate of decline is either 90% (criterion A1) or 80% (criteria A2, A3 and A4) (IUCN 2001, 2012). Therefore, the information from Robertson et al. (2017) does not immediately mean it qualifies as globally Critically Endangered.

Modelling work by McClellan and Smith (2015) predicted an overall decline of 77.7% between 2014 and 2044 on South Island. Assuming that the very small % of the population that is found on North Island is declining at a similar rate would place the overall rate of decline at 82.2% over 3 generations (34.5 years).

McClellan and Smith (2015) also present historical and the most recent count data at various sites. Analysis of these data imply a 91.2% reduction in population size over the past 3 generations on South Island. However, these data may not show the whole picture about overall trends as the species is highly mobile with significant fluctuations at a given site, and certain areas such as large rivers are difficult to survey (McClellan and Smith 2015). Therefore, the actual past rate of decline is likely less than this 91.2% value. Despite this, while we cannot give an exact past reduction figure, the data does suggest that it is likely that McClellan and Smith’s predicted future declines of >80% over 3 generations may have already been occurring for some time. Rates of decline of this level meet the threshold for Critically Endangered under criteria A2, A3 and A4, and so the species likely warrants listing as Critically Endangered under criteria A2abce+3bce+4abce.

Criterion B – The Extent of Occurrence of this species (394,000 km2) is considerably larger than the threshold for Vulnerable under criterion B1 (20,000km2). The species’ Area of Occupancy has not been calculated, but it is not thought to approach the threshold for Vulnerable under criterion B2 (2,000km2).

Criterion C – The total population size was estimated by McClellan (2009) as 90,000 mature individuals, based on fairly complete surveys of Southland populations. However, they were not complete censuses and some individuals may have been absent from the colony (McClellan and Smith 2015), and so the true figure could be higher. Whether it is higher or not, this population size figure is too large to warrant listing under this criterion.

Criterion D – The population size and range are too large to warrant listing under this criterion.

Criterion E – To the best of our knowledge no quantitative analysis of extinction risk has been carried out for this species. Therefore, it cannot be assessed against this criterion.

Therefore, it is proposed that Black-billed Gull be uplisted to Critically Endangered under criteria A2abce+3bce+4abce. Comments are welcome about this proposed uplisting.

Please note that this topic is not designed to be a general discussion about the ecology of the species, rather a discussion of the species’ Red List status. Therefore, please make sure your comments are about the proposed listing.

BirdLife International. 2017. Species factsheet: Larus bulleri. Downloaded from on 21/11/2017.

Biswell, S. 2006. Ferals filmed killing endangered chicks. Forest and Bird 319.

Heather, B. D.; Robertson, H. A. 1996. The field guide to the birds of New Zealand. Oxford University Press, Oxford, UK.

IUCN. 2001. IUCN Red List Categories and Criteria: Version 3.1. IUCN Species Survival Commission. IUCN, Gland, Switzerland and Cambridge, U.K.

IUCN. 2012. IUCN Red List Categories and Criteria: Version 3.1. Second edition. IUCN Species Survival Commission. IUCN, Gland, Switzerland and Cambridge, U.K. Available at:

Maloney, R. F. 1999. Bird populations in nine braided rivers of the Upper Waitaki Basin, South Island, New Zealand: changes after 30 years. Notornis 46: 243-256.

McClellan, R. K. 2009. The ecology and management of Southland’s Black-billed Gulls. PhD Thesis. University of Otago Retrieved from

McClellan, R.; Smith, D. 2015. Population trends of Black-billed Gulls (Larus bulleri) on South Island rivers 1962-2014. Contract Report no. 3442 prepared for Department of Conservation, Invercargill and Christchurch offices.

Robertson, H. A.; Baird, K.; Dowding, J. E.; Elliott, G. P.; Hitchmough, R. A.; Miskelly, C. M.; McArthur, N.; O’Donnell, C. F. J.; Sagar, P. M.; Scofield, R. P.; Taylor, G. A. 2017. Conservation status of New Zealand birds, 2016. Department of Conservation, Wellington.

Taylor, G. A. 2000. Action plan for seabird conservation in New Zealand. Department of Conservation, Wellington.

Townsend, A. J.; de Lange, P. J.; Duffy, C. A. J.; Miskelly, C. M.; Molloy, J.; Norton, D. A. 2008. New Zealand Threat Classification System manual. Department of Conservation, Wellington.

This entry was posted in Pacific, Seabirds and tagged , . Bookmark the permalink.

6 Responses to Black-billed Gull (Larus bulleri): revise global status?

  1. Hugh Robertson says:

    In the 2016 assessment of bird conservation status in New Zealand, Robertson et al. (2017) retained the designation of Threatened – Nationally Critical by using criterion C (predicted decline of >70% in three generations) that had been first used in 2012 (Robertson et al 2013). The assessment panel noted on our database at the time that “Population model based on long-term trend predicts 77.7% decline per 3 generations (Nationally Critical C). Some suggestion decline may have slowed (would move it to Nationally Vulnerable E), but this requires confirmation by South Island-wide censusing and trend monitoring. Suggestion that increasing proportion of the population is in Canterbury rather than majority in Southland as previously.”
    Since this assessment was carried out, there is new evidence that historical data likely overestimated numbers of breeding birds and inconsistencies in previous surveys make trend analyses difficult (Mischler 2018). This paper also reports on nationwide attempts in 2014/15 to 2016/17 to re-estimate the breeding population using aerial surveys to locate photograph and count breeding colonies, with ground counts at 16 colonies to provide correction factors. The 2016/17 survey was the most complete of the three annual counts and revealed 60,256 nests (=120,500 mature individuals), which was higher than most people had expected. In light of this new evidence, it is quite likely that the New Zealand assessment of the conservation status of black-billed gulls will be downgraded from Nationally Critical.
    As a note, the assumption that the small percentage of the population that breeds in the North Island (c. 1.6%) is declining at a similar rate to what was thought to be happening in the South Island, is not supported by data; in fact, it seems more likely that that numbers in the North Island are stable or increasing.
    Overall, it seems very likely that the historical and/or predicted rates of decline do NOT meet the ≥80% threshold for Critically Endangered under Criteria A2, A3 and A4, and so I do not support the uplisting of Black-billed Gull to Critically Endangered at the present time.

    Mischler, C.P. 2018. Estimating the breeding population of black-billed gulls Larus bulleri in New Zealand, and methods for future count surveys. Notornis 65: 67-83.

    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.

    Hugh Robertson
    Chair, NZ Bird Conservation Status Assessment Panel

  2. Rob Martin (BirdLife International) says:

    Preliminary proposals

    Based on available information, our preliminary proposal for the 2018 Red List would be to close this topic and retain the species as Endangered under criteria A2abce +A3bce + A4abce for now, pending any further information regarding current trends.

    There is now a period for further comments until the final deadline in mid-July, after which the recommended categorisations will be put forward to IUCN.

    Please note that we will then only post final recommended categorisations on forum discussions where these differ from the initial proposal.

    The final 2018 Red List categories will be published on the BirdLife and IUCN websites in November, following further checking of information relevant to the assessments by both BirdLife and IUCN.

  3. Mike Bell says:

    I agree that the current proposal to list black-billed gull as endangered is not warranted, in fact the data from the national population estimate (Mischler 2018) and other counts from Canterbury and Marlborough (personal data) suggests that the population is not in decline.
    As mentioned differences in methodology cause some issues with detecting trends, but on balance it appears that the population has likely been stable for the past 20-30 years, although likely did decline prior to this time, the population has no stabilised.

  4. Rob Martin (BirdLife International) says:

    If the population is indeed stable, and has been for some time, then the subject of this topic should turn to the revision and potential downlisting of the species, rather than simply rejecting the proposal to uplist to CR. Given that we are now at the deadline for this year, it would appear the best approach is to pend this discussion for a final decision in 2019, while retaining the current status for the 2018 Red List.

    Clearly further information is needed on the recent, current and projected trends if they differ dramatically from those presented by McClelland and Smith (2015), and downlisting in the New Zealand assessment of bird conservation status will be taken into account by BirdLife.

  5. Graeme Taylor says:

    I think the evidence published in Claudia’s paper (Mischler 2018) is that the black-billed gull population is significantly larger than anyone realised. Vastly improved survey techniques (aerial surveys to locate breeding colonies and then immediate ground-based census methods to correct for nesting pairs) have given a much more accurate estimate of the population than we had in the past. The McClelland and Smith (2015) review took it on good faith that that the early river bird nesting surveys had been done more accurately than seems the case now and that a large decline had occurred. However the later analysis by Claudia is that these past surveys were probably not that accurate. Nest density estimates were variable, areas of colonies were not clearly defined. Time delays between checking nests and ratios of birds to nests as seen on aerial photos probably underestimated the bird counts in the 1990s Therefore the current estimate of breeding pairs indicates that the population is stabile or increasing. How birds responded to past pressures such as rivers being dammed for hydroelectricity and the lack of predator management on braided rivers will remain unknown. Today there is greater awareness of the plight of this species and while many threats remain (nest predation, weed encroachment, gravel extraction, climate change related storm frequency and changing agricultural practises) the reality is that black-billed gulls have proven to be a highly adaptable and resilient species that can still produce a lot of offspring at colonies each year. By contrast the much smaller black-fronted tern that uses the same habitats is in a much more dire state as nest predation can wipe out entire colonies on a regular basis. Therefore downgrading black-billed gull to a lower threat status needs appraisal by Birdlife.

    Here is the abstract from the Mischler (2018) paper

    Abstract A national census of breeding black-billed gulls (Larus bulleri) conducted across New Zealand in 1995‒98 estimated 48,000 nests, however the methodology used was unclear. In 2013, the New Zealand threat status for the endemic black-billed gulls was changed to Nationally Critical, based on estimates of recruitment failure causing population decline. To inform future threat classification, the breeding population was re-estimated using aerial surveys to locate, photograph, and count breeding black-billed gulls across New Zealand in 2014‒2016. Large spatial gaps in nest count data during 2014/15 and 2015/16 did not allow for annual variability to be taken into account across the 3 seasons, but the 2016/17 survey successfully covered the entire country. Ground counts of nests were conducted at 16 colonies to determine a correction factor of 0.90 to apply to aerial photograph counts of apparently occupied nests. A total of 60,256 nests were found, with 33,703 nests in Southland and 20,675 nests in Canterbury. The North Island was surveyed on the ground and had 992 nests. Historical survey methods were reviewed, highlighting the inaccuracies of using nest densities or applying factors of gulls/nest to total bird counts based on photographs, as well as only counting individual birds on aerial photographs. Historical data likely overestimated numbers of breeding birds, and the inconsistencies of previous surveys make trend analyses difficult. Key recommendations for future counts include: (i) carrying out ground surveys before flights to determine the breeding stage of birds and hence the optimal time to fly; (ii) taking high resolution and zoomed in photos; (iii) carrying out ground nest counts immediately after flights to determine a correction factor; and (iv) using the same observers for all counts to maintain consistency.

  6. Hannah Wheatley (BirdLife) says:

    Many thanks for the detailed and helpful comments on this topic. As part of next year’s comprehensive Red List update, we will be undertaking a revision of species’s generation lengths. Given that generation length is fundamental to assessing species under Criterion A, our proposal for the 2019 Red List is to pend the decision on this species and keep the discussion open until 2020, while leaving the current Red List category unchanged in the 2019 update.

    Final 2019 Red List categories will be published on the BirdLife and IUCN websites in December, following further checking of information relevant to the assessments by both BirdLife and IUCN.

Leave a Reply

You have to agree to the comment policy.

All comments must follow the rules of usage.

This site uses Akismet to reduce spam. Learn how your comment data is processed.