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Translocation of the Seychelles Warbler Acrocephalus Sechellensis to Establish a New Population on Frégate Island, Seychelles

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Translocation of the Seychelles Warbler Acrocephalus Sechellensis to Establish a New Population on Frégate Island, Seychelles D.J. Wright, N.J. Shah & D.S. Richardson / Conservation Evidence (2014) 11, 20-24 Translocation of the Seychelles warbler Acrocephalus sechellensis to establish a new population on Frégate Island, Seychelles David J. Wright1, Nirmal J. Shah2 & David S. Richardson1,2* 1 School of Biological Sciences, University of East Anglia, Norwich Research Park, Norwich, Norfolk, NR4 7TJ, UK 2 Nature Seychelles, Roche Caiman, PO Box 1310, Mahé, Republic of Seychelles SUMMARY In December 2011, 59 adult Seychelles warblers Acrocephalus sechellensis were translocated between two islands in the Seychelles. Birds were captured on Cousin Island and translocated to Frégate Island using a hard release method, with minimum time in captivity. Frégate had been previously identified as a suitable host for a substantial population of Seychelles warblers, although the presence of the species had never been confirmed on this island. It was estimated that Frégate currently has the potential to support about 500 Seychelles warblers, rising to over 2,000 after habitat regeneration. All birds survived the translocation and were released unharmed at the new site within 24 hours of capture. Close monitoring of both the new and source population was undertaken over a period of 18 months. By June 2013, the Frégate population had increased to 80 individuals, which included 38 of the original translocated birds and 42 birds which had hatched on Frégate. There was also evidence that multiple generations had already hatched on the island. This shows that the Seychelles warbler responded well to a hard release translocation, with observed population growth on Frégate comparable to previous warbler translocations. The source population on Cousin recovered to carrying capacity within a single breeding season. This is the fourth translocation of this species, fulfilling the species action plan requirement of five populations of this endemic island passerine. BACKGROUND downgrade in its conservation status from Vulnerable to Near Threatened (Richardson 2001). It was agreed that twice the The Seychelles warbler Acrocephalus sechellensis is a number of individuals should be moved in future small, insectivorous passerine endemic to the granitic islands translocations, to ensure increased representation of the genetic of the Republic of Seychelles. Due to habitat destruction for variation present on Cousin within the new populations. intensive coconut Cocos nucifera cultivation and the Subsequently, a third translocation of 58 individuals from introduction of invasive predators (e.g. rats Rattus spp.), the Cousin to Denis Island (3°48′S, 55°40′E, 1.42 km²) was Seychelles warbler was extirpated from all islands except undertaken (Richardson et al. 2006). The present work details Cousin Island (4°20′S, 55°40′E, 0.29 km²) where the the fourth translocation undertaken, from Cousin to Frégate population may have fallen to as low as 26 individuals by the Island (4°35′S, 55°56′E, 2.19 km²), to fulfil the conservation mid 20th century (Crook 1960, Spurgin et al. 2014). The action plan. warbler has since been the focus of intensive conservation action that commenced with the purchase of Cousin in 1968 by a consortium led by the International Committee for Bird ACTION Protection (now BirdLife International). Since then the island has been managed solely for conservation and research. Study site assessment: From 1999 to 2002, 10 islands in the Careful habitat management, including the regeneration of Seychelles were surveyed by Nature Seychelles to assess their native forest, enabled the population to recover and the suitability for sustaining translocated populations of population of Seychelles warblers has remained stable at about endangered endemic bird species, including the Seychelles 320 individuals since 1982 (Brouwer et al. 2006). warbler (Hill 2002). Three islands were selected as potentially Seychelles warbler has not, so far, naturally re-colonised suitable for Seychelles warbler, and were surveyed in more the other islands with suitable habitat (Komdeur et al. 2004). detail (D'Arros: Van de Crommenacker et al. 2009; North: To increase the species range, translocations of 29 individuals Richardson & van der Woude 2010; Frégate: Richardson & were undertaken from Cousin to Aride Island (4°12′S, Hammers 2011). Whole island surveys were conducted during 55°40’E, 0.68 km²) in 1988 and from Cousin to Cousine Island which native and exotic vegetation was identified and its (4°21′S, 55°39′E, 0.25 km²) in 1990 (Komdeur 1994). In 2001, distribution mapped. Seychelles warbler prey availability was Nature Seychelles commissioned a species conservation action determined using standardised insect counts following the plan for the Seychelles warbler. This plan outlined the protocols outlined by Komdeur (1992) and Brouwer et al. objective of increasing the species range to five populations, (2009). These data were used to estimate the quantity and with a total exceeding 3,000 individuals, to further enhance quality of warbler habitat across each island. The absence of long term survival prospects and potentially allow a introduced predator species and the presence of ongoing measures to minimise the possibility of their accidental introduction, such as culling and island landing controls, were * To whom correspondence should be addressed: [email protected] also confirmed. 20 ISSN 1758-2067 D.J. Wright, N.J. Shah & D.S. Richardson / Conservation Evidence (2014) 11, 20-24 As a result, Frégate was selected to host the fifth Seychelles previous Seychelles warbler translocations, Cousin was used as warbler population. The island is outside the confirmed historic the single source population. This was because: 1) over 97% of range of the species, although during the last glacial maximum adult birds on Cousin are ringed and of known age and sex the inner Seychelles islands would have been a single land (Richardson et al. 2001), 2) the population was at carrying mass. Furthermore a reconstruction of the species historic capacity and so many territories contained surplus, subordinate effective population size, estimated at 2,500, suggests that individuals (Brouwer et al. 2006), and 3) as the last remaining Seychelles warbler was formerly abundant and widespread natural population, Cousin is the most genetically diverse, and across these inner islands (Spurgin et al. 2014). has not suffered any loss of variation as a result of earlier Frégate currently possesses 0.37 km2 of suitable habitat and translocations (Wright et al. 2014). this is predicted to increase to 0.42 km2 by 2016 as a result of ongoing regeneration work. Prey availability within this habitat Capture: Based on previous experience and logistical was higher than on Cousin (Richardson & Hammers 2011). feasibility, the translocation was split into two transfers; the Importantly, after the successful eradication of common myna first took place on 7 December 2011 and the second on 14 Acridotheres tristis, Frégate was free of non-native predators December 2011. For each transfer, four teams of 2-3 people (Millet & Shah 2001, Canning 2011). The survey estimated were distributed around the island in allocated catching zones, that Frégate could currently sustain about 500 Seychelles where they were already familiar with the territory structure warblers, with an eventual population exceeding 2,000 once and the status of the occupying birds. These teams then relayed the regeneration of habitat is completed (Richardson & captured birds to a central processing team. Birds were caught Hammers 2011). An area of native pisonia Pisonia grandis using mist nets (5-6 nets of 9-12 m/team) and audio lures woodland was identified as a suitable release site based on its between 15:00 h and 18:00 h on the evening before, and from close similarity to the species’ preferred habitat on Cousin. 06:30 h until 13:00 h on the day of transfer; at 13:00 h they were translocated. This timing was designed to allow the birds Pre-translocation census: As part of the ongoing research time (daylight hours) to feed both before capture, to ensure undertaken by the Seychelles Warbler Study Group, there is they were in the best condition for translocation, and after detailed information on nearly all individuals in the Cousin release on Frégate, to allow quick subsequent recovery of body Seychelles warbler population (Komdeur 1992, Richardson et weight. Individuals caught in the evening prior to the al. 2005, Barrett et al. 2013). Prior to the translocation, a translocation were kept overnight in individual cardboard complete island census was conducted from 25 November boxes (approximately 35 x 25 x 25 cm) furnished with a perch, 2011 to 7 December 2011, to locate and check the suitability of pisonia leaves sprinkled with water and fresh termite individuals for translocation. The translocation was planned for Nasutitermes spp. eggs for food (Figure 1). December, prior to the minor breeding season (December- We attempted to catch individuals from across the entire February, Komdeur 1991), when individuals are in peak island with only two restrictions: 1) avoid possible breeding condition (Richardson et al. 2006). A small percentage of pairs territories, and 2) avoid taking both dominant and may breed at any time of year and so the census enabled subadults/subordinates/recent fledglings from the same identification of individuals exhibiting breeding behaviour that territory where they may be first order relatives. Subordinate were subsequently excluded from catching effort. As with all individuals were particularly targeted as they were surplus, Figure 1. Individual Seychelles warbler translocation boxes. Note small air hole punctures and pisonia leaf lining. Image: D. Gilroy. 21 ISSN 1758-2067 D.J. Wright, N.J. Shah & D.S. Richardson / Conservation Evidence (2014) 11, 20-24 adult birds that were normally relatively young (but see were recorded. The birds had colonised many areas across the Richardson et al. 2007), but often had breeding experience island, with individuals sighted on the opposite side of the from helping the dominants in their natal territory (Komdeur island to the release site (Wright & Richardson 2012).
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