J Ornithol DOI 10.1007/s10336-011-0798-7 ORIGINAL ARTICLE Annual survival and breeding dispersal of a seabird adapted to a stable environment: implications for conservation Justine Braby • Sigrid J. Braby • Rodney J. Braby • Res Altwegg Received: 9 December 2010 / Revised: 6 December 2011 / Accepted: 9 December 2011 Ó Dt. Ornithologen-Gesellschaft e.V. 2012 Abstract Understanding the spatial dynamics of popu- based on AICc favoured a model that suggests local annual lations is essential for conservation of species at the survival of Damara Terns for the dataset was 0.88 (95% CI, landscape level. Species that have adapted to stable 0.73–0.96) and the annual dispersal probability was 0.06 environments may not move from their breeding areas even (0.03–0.12). High survival and low dispersal probabilities if these have become sub-optimal due to anthropogenic are consistent with other seabirds adapted to stable envi- disturbances. Instead, they may breed unsuccessfully or ronments. These estimates contribute to the first baseline choose not to breed at all. Damara Terns Sternula balae- demographic information for the Damara Tern. Low dis- narum feed off the highly productive Benguela Upwelling persal probabilities indicate that current protection of System. They breed on the coastal desert mainland of breeding sites is an important management approach for Namibia where development and off-road driving is protecting the species. threatening breeding areas. We report annual survival and breeding dispersal probabilities of 214 breeding adult Keywords Breeding dispersal Á Fidelity Á Survival Á Damara Terns through capture–mark–recapture at two col- Conservation Á Sternula balaenarum Á Damara Tern onies for 9 years (2001–2009) in central Namibia. Using multi-state models in program MARK, model selection Zusammenfassung Ja¨hrliche U¨ berlebensrate und Brutortstreuung bei einem an stabile Umweltbedingungen angepassten Communicated by P. H. Becker. Seevogel: Auswirkung auf Schutzmaßnahmen & J. Braby ( ) Á R. Altwegg Um Arten auf Landschaftsebene zu schu¨tzen ist es wichtig, Animal Demography Unit, Department of Zoology, University of Cape Town, Rondebosch 7701, South Africa die ra¨umliche Dynamik der Populationen zu verstehen. Es ist e-mail: [email protected] mo¨glich, dass Arten, die an eine stabile Umwelt angepasst sind, ihre Brutgebiete nicht vera¨ndern ko¨nnen, selbst wenn J. Braby diese infolge anthropogener Vera¨nderungen nicht mehr Marine Research Institute, University of Cape Town, Rondebosch 7701, South Africa optimal sind. Stattdessen ko¨nnten sie dort erfolglos zu bru¨ten versuchen oder gar nicht mehr bru¨ten. Damara-Seeschwal- S. J. Braby ben S. balaenarum erna¨hren sich im hochproduktiven P.O. Box 656, Swakopmund, Namibia Benguela Auftriebsgebiet. Sie bru¨ten an der Ku¨ste der ¨ R. J. Braby Namibwu¨ste, wo Uberbauungen und Fahren im Gela¨nde Namibian Coast Conservation and Management Project, ihre Brutgebiete gefa¨hrdet. Wir pra¨sentieren ja¨hrliche P.O. Box 7018, Swakopmund, Namibia U¨ berlebensraten und Dispersionswahrscheinlichkeiten ba- sierend auf 214 bru¨tenden adulten Damara-Seeschwalben, R. Altwegg South African National Biodiversity Institute, P/Bag X7, ermittelt durch Fang-Wiederfang Methoden u¨ber neun Jahre Claremont 7735, South Africa (2001–2009) in zwei Kolonien Zentralnamibias. Multi-state 123 J Ornithol Modelle in Programm MARK zeigten fu¨r diese Daten, dass has been reported for several seabird species (Austin 1940, die ja¨hrliche lokale U¨ berlebensrate von Damara-See- 1949; Southern and Southern 1979, 1982; Peterson 1980). schwalben 0.88 (95% Vertrauernsbereich 0.73–0.96) war und The level to which species display fidelity to their breeding die ja¨hrliche Dispersionswahrscheinlichkeit zwischen den sites is important in understanding the management and Kolonien betrug 0.06 (0.03–0.12). Die hohe U¨ berlebensrate protection of these species. und niedrige Dispersionswahrscheinlichkeit sind vergleich- Damara Terns Sternula balaenarum breed predomi- bar mit anderen an stabile Umwelten angepassten Seevo¨- nantly on the coastline where the harsh Namib Desert geln. Diese Scha¨tzungen sind ein erster Beitrag zum meets the cold and dynamic Benguela current (Simmons Versta¨ndnis der Demographie von Damara-Seeschwalben. and Braine 1994). Damara Terns feed primarily inshore Die niedere Dispersionswahrscheinlichkeiten legen nahe, where physical fluctuations result in biological patchiness dass der Schutz der Brutgebiete, wie er momentan betrieben and high variability (Branch et al. 1987). Recruitment of wird, eine tatsa¨chlich wichtige Maßnahme zum Schutz die- prey species like Anchovy Engraulis encrasicolus in the ser Art ist. northern Benguela system shows high inter-annual vari- ability (Boyer et al. 2001). However, the Benguela system is not as variable as, say, the Humboldt system, and seabirds here are generally adapted to a more stable Introduction supply of food (Crawford et al. 2006). We would expect that Damara Terns have adapted to this stable food supply When habitat becomes unsuitable for a species, it is of by displaying high annual survival and fidelity to their great conservation interest to know if and how far the breeding sites. In addition, although there is ample individuals will move to resettle in other suitable areas. available undisturbed habitat along the coastline of Knowledge of dispersal is therefore vital when considering Namibia, Damara Terns are found breeding in specific wildlife management at the landscape scale. The level to areas (Clinning 1978). However, predation rates are high which a species displays fidelity or dispersal depends on and predation has been considered the major driving force the environmental conditions it has evolved in (McPeek in the evolution of the unique one-egg clutch trait of this and Holt 1992; Clobert et al. 2001). We expect such a life- tern (Frost and Shaughnessy 1976; Clinning 1978). High history strategy is dependent on various environmental or fluctuating predation has resulted in lower fidelity in conditions, such as food availability, predation rates, and some other tern species (e.g. Spendelow et al. 2002), and how much habitat is available. Generally seabirds are long- even caused mass periodic dispersal in other species of lived and faithful to their breeding areas (Beadell et al. gulls (e.g. Oro et al. 1999). 2003), but species living in highly variable environments The Damara Tern is globally near-threatened (IUCN are nomadic and may suffer higher rates of mortality 2009) and it is the only small tern that predominantly lays (Renken and Smith 1995a, b; Robinson and Oring 1997). just one egg (Simmons and Braine 1994; de Villiers and The tendency for individuals to return to areas where they Simmons 1997). During the non-breeding season (May– were hatched (natal philopatry), and/or previously bred September), Damara Terns migrate to West African (breeding philopatry), has been well documented for a countries such as Gabon, Cameroon and Nigeria (Williams variety of seabirds, including gulls (Coulson and White et al. 2004). The age at first breeding is estimated at 1958; Southern 1977; Southern and Southern 1980; 3 years, and immature survival is estimated at 0.59 (Braby Blockpoel and Courtney 1980; Stenhouse and Robertson et al. 2011). The Benguela Upwelling System is currently 2005) and terns (Austin 1940, 1949; Atwood and Massey undergoing changes due to over-fishing and climate change 1988; Becker and Wendeln 1997; Spendelow et al. 1995, (Clark 2006). In addition, Damara Tern breeding grounds 2008, 2010; Becker et al. 2001; Lebreton et al. 2003; Ezard are threatened by off-road driving and coastal development et al. 2006; Devlin et al. 2008). Because prior knowledge (Braby et al. 2001, 2009; Braby, unpublished data). It is exists on available nesting sites, food resources and pre- thus becoming increasingly important to understand the dation pressures, it is often considered a safer strategy for life-history adopted by the species. Few demographic birds to return to a previous breeding site or natal territory parameters exist for the Damara Tern and these are nec- than to move to a new area with unknown resources and/or essary to test whether the current management approach of risks (Renken and Smith 1995b). Factors such as predation, protecting specific breeding sites is appropriate. The low reproductive success and changes in habitat have been objective of this paper is to provide estimates of (1) adult implicated in causing breakdowns in fidelity (McNicholl survival and (2) dispersal probabilities of Damara Terns 1975; Erwin 1977; Conover and Miller 1978; Oro et al. based on 9 years of capture–mark–recapture data of adult 1999). However, in spite of the obvious negative impacts Damara Terns breeding at two close-proximity colonies of these factors, continued re-use of adversely affected sites along the central coastline of Namibia. 123 J Ornithol Methods gravel ridge through the centre (Braby et al. 2001). The next closest colonies of similar size were 280 km away. Study area Both Caution Reef and Horses Graves are fenced off from the public to ensure minimal disturbance from off- The Horses Graves colony contained 30–60 pairs, was road driving. These two areas are protected from coastal located 4 km south of Swakopmund, on the central Nam- development. There were three additional colonies within a ibian coast (22°420S, 14°330E; Fig. 1), and comprised 30-km radius of Horses Graves and Caution Reef. One approximately 3.5 km2. Over the course of