Population status and genetic diversity of the endemic black-billed gull Larus bulleri of New Zealand

Abstract

The endangered black-billed gull (Larus bulleri) is endemic to New Zealand, and is the world’s most threatened gull species due to suspected large population declines. These gulls primarily rely on open gravel beds in braided rivers of the South Island for breeding but will also nest along lakeshores and on beaches at river mouths. The majority of black-billed gulls are found in the Southland and Canterbury regions of the South Island, with about 1.6% of the population inhabiting the North Island. In 2013, the New Zealand threat status for the black-billed gulls was changed from ‘Threatened, Nationally Endangered’ to ‘Threatened, Nationally Critical’ based on estimates of recruitment failure causing population decline. A national count of breeding black-billed gulls across New Zealand was last conducted in 1995-98 and estimated 48,000 nests; however, the methodology used was unclear. Although management of this species is increasing, there are very little data published about their movements, demographics, and population status. Therefore, determining an up-to-date population estimate and understanding dispersal and genetic structure across the country are high priorities in the conservation of black-billed gulls. 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 fully 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 over-estimated 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. Both mitochondrial DNA (mtDNA control region, domain I) and single nucleotide polymorphisms (SNPs) of the nuclear DNA were examined to gain an understanding of dispersal and population structure across the country to inform management decisions. Blood samples were collected from chicks in colonies across New Zealand. The results from mtDNA and nuclear DNA were discordant; mtDNA showed no population structure with high haplotype and low nucleotide diversity, and analyses highlighted introgression with the closely related red-billed gulls (L. novaehollandiae scopulinus). Nuclear DNA analyses were indicative of two groups, with Rotorua birds in the North Island being distinct from the rest of New Zealand, and isolation by distance being evident in South Island populations. There is lower connectivity between the North Island and the South Island, and between the Rotorua and Wairarapa populations, than there is among the South Island populations. Gene flow primarily occurs between nearby colonies with a stepwise movement across the landscape. A decision on whether the North Island populations should be managed as a separate unit from the South Island needs to be considered. Since the North Island holds only 1.6% of the total population, the importance of these birds from a genetic perspective needs to be evaluated. South Island management should focus on maintaining at least several large populations within each region (i.e. Canterbury, Marlborough, West Coast, Otago, Southland) and across catchments to maintain dispersal and gene flow rather than focusing on protecting every single colony. Future studies are needed to investigate the genetic structure of Auckland populations at the northern limit of the gulls’ range, as well as identifying the mechanisms behind, and extent of, the hybridisation between red-billed and black-billed gulls.