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

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.

References
BirdLife International. 2017. Species factsheet: Larus bulleri. Downloaded from http://www.birdlife.org 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: www.iucnredlist.org/technical-documents/categories-and-criteria.

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 http://hdl.handle.net/10523/6181.

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.

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