University of Birmingham Long-term population trends of Sooty Terns Onychoprion fuscatus: Hughes, Bernard; Martin, Graham; Giles, Anthony; Reynolds, Silas DOI: 10.1007/s10144-017-0588-z License: Creative Commons: Attribution (CC BY) Document Version Publisher's PDF, also known as Version of record Citation for published version (Harvard): Hughes, B, Martin, G, Giles, A & Reynolds, S 2017, 'Long-term population trends of Sooty Terns Onychoprion fuscatus: implications for conservation status', Population Ecology, vol. 59, no. 3, pp. 213-224. https://doi.org/10.1007/s10144-017-0588-z Link to publication on Research at Birmingham portal Publisher Rights Statement: The final publication is available at Springer via http://doi.org/10.1007/s10144-017-0588-z General rights Unless a licence is specified above, all rights (including copyright and moral rights) in this document are retained by the authors and/or the copyright holders. 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The most abundant sea- Red List · Tropical pelagic seabirds · United Kingdom birds of tropical oceans are Sooty Terns Onychoprion fus- Overseas Territories catus and they have an IUCN Red List category of ‘Least Concern’. Ascension Island has the largest colony of Sooty Terns in the Atlantic Ocean and censuses between 1990 Introduction and 2013 have shown that its population size is static. In this study we have used historical data and recent censuses Determining the conservation status of species according to describe the population status of Sooty Terns on Ascen- to a formal set of criteria is now widely regarded as essen- sion Island over a century. We show that the breeding pop- tial for conservation action. Over recent decades the Red ulation contained over 2 million individuals in the 1870s List of Threatened Species produced by the International and remained at this level for at least 70 years. However, Union for Conservation of Nature (hereafter referred to as the population declined from >2 million birds in 1942 to the ‘IUCN Red List’) has been adopted internationally as 350,000 birds by 1990. The population trend spanning a the main framework within which the conservation status period equivalent to three generations of the species (63 of animal and plant taxa are assessed (see review by Rod- years; 1942–2005) showed an approximate 84% decline rigues et al. 2006). A major criterion among birds is based in population size. Using IUCN criteria this suggests that upon population trend over 10 years, or three generations, Sooty Terns on Ascension could be considered ‘Critically whichever is longer. Endangered’. We conclude that a re-evaluation of Sooty Compared with many other avian taxa, seabirds pre- Tern conservation status is necessary at the local level and sent challenges when assessing their conservation status possibly globally. Our study highlights that for long-lived because they are typically long-lived, migratory and widely species historic demographic data should be considered distributed, and individuals may not breed annually. Never- when determining conservation status. theless, seabirds are the subject of many long-term moni- toring programmes which have revealed that, compared with other taxa, seabird populations have declined rapidly over recent decades (Croxall et al. 2012), with the com- bined global population size of all seabirds having declined * S. James Reynolds by nearly 70% between 1950 and 2010 (Paleczny et al. [email protected] 2015). 1 Army Ornithological Society (AOS), c/o Prince Consort Library, Knollys Road, Aldershot, Hampshire GU11 1PS, Sources of data for establishing population trends UK Ornithology has a rich history of scientific monitoring 2 Centre for Ornithology, School of Biosciences, College of Life and Environmental Sciences, University programmes that have quantified changes in distribution of Birmingham, Edgbaston, Birmingham B15 2TT, UK and size of bird populations at different spatial scales, Vol.:(0123456789)1 3 214 Popul Ecol (2017) 59:213–224 often over many decades (e.g., Hagemeijer and Blair and Tristan da Cunha, and is isolated in the tropical South 1997; Balmer et al. 2013). Data from these monitoring Atlantic Ocean midway between South America and Africa programmes have provided essential reference points for (Hughes et al. 2010). It lies some 800 km south of the equa- estimating population trends over multiple generations in tor, 2000 km from continental land masses and its nearest many short-lived birds. Among long-lived species, how- neighbour is St Helena which is 1100 km to the south- ever, such multi-generational demographic data are scarce east. Wideawake Fair is the site of the Sooty Tern breed- (Newton 2013). In some species historical data have been ing colony on Ascension Island and is an Important Bird used for setting population baselines and from these pop- Area (IBA reference number SH009; BirdLife International ulation trends have been developed (e.g., Sweetnam et al. 2016). 1999; Igl and Johnson 2005; McClenachan et al. 2012). Such retrospective investigation of population trends has Study species and fieldwork been applied, for example, to populations of Passeriformes (Gooch et al. 1991) and Anseriformes (Cooch et al. 2001). Sooty Tern breeding colonies are distributed pan-equato- rially (Schreiber et al. 2002), and the population breeding Conservation status of Sooty Terns on Ascension Island in the South Atlantic may be the most closely monitored. Ascension Island holds approximately Sooty Terns Onychoprion fuscatus are the most numer- 2% of the global, and 40% of the Atlantic Ocean, popula- ous of seabirds in the three tropical ocean systems tion of Sooty Terns (Schreiber et al. 2002). There are mul- (IUCN 2015). They are the only seabird species with a tiple sources of long-term demographic data for breeding large global population [estimated at 23–25 million birds Sooty Terns on this island from which it has been possible (Schreiber et al. 2002)], in which the trend in population to estimate their distribution and number as early as 1877. size is unknown. Despite a lack of information on popula- Sooty Terns generally breed in large colonies composed tion trend they have been assigned a conservation status of of smaller sub-colonies that contain birds at equivalent ‘Least Concern’ (IUCN 2015), presumably because they reproductive stages. In total, colonies can number in excess are regarded as sufficiently numerous. However, the total of 1 million birds (Schreiber et al. 2002) and this makes population size of Sooty Terns is reported to be falling counting all individuals or incubated eggs impractical. In (Schreiber et al. 2002) most likely because of the combined the Indian Ocean, Le Corre and Jaquemet (2005) estimated threats of over-exploitation of pelagic fish species (upon the population size of Sooty Terns at colonies during incu- which they are dependent for successful foraging) (Danck- bation by mapping the colonies and then determining their werts et al. 2014), climate change (Foden et al. 2013), and area of coverage. Counts of eggs in quadrats were used in egg harvesting (Feare et al. 2007). the estimation of breeding density which was multiplied In this study we determine general changes in the size by the area to estimate the number of Sooty Tern breeding of the population of breeding Sooty Terns on Ascension pairs. This census method is appropriate for Sooty Terns on Island since the nineteenth century. We also determine pop- Ascension Island as the edge of each sub-colony (defined ulation change over the period of three recent generations. as spatially separate areas occupied by breeding birds) is Generation length is defined as the average age of parent well-defined, enabling sub-colony area to be surveyed reli- birds in a current cohort (IUCN 2012). In Sooty Terns the ably with clutches easily found on substrate devoid of vege- average age of adults is 21 years (Schreiber et al. 2002) and tation (Fig. 1). Sooty Terns do not build a nest and clutches accordingly we have constructed a population trend over are laid in different areas of the breeding colony each sea- a recent 63-year period. This is a similar timeframe to an son.