Conserving the Seychelles warbler Acrocephalus sechellensis by translocation: a transfer from Cousin Island to Aride Island

Published source details Komdeur J., Bullock I.D. & Rands M.R.W. (1991) Conserving the Seychelles warbler Acrocephalus sechellensis by translocation: a transfer from Cousin Island to Aride Island. Bird Conservation International, 1, 177-185.
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Summary

The Seychelles warbler Acrocephalus sechellensis is a small, insectivorous passerine endemic to the Seychelles. In 1959, the population numbered just 26 individuals, confined to the 29 ha island of Cousin (4º20' S, 55º40' E). Following long-term habitat management, the species made a dramatic recovery, with the population growing to nearly 320 birds (with c.115 breeding territories) by 1982. Since then, however, the number of birds and territories on Cousin has remained around this level, suggesting that the island has reached carrying capacity. In order to give the species the security of an additional breeding population, the suitability of other islands within the Seychelles for a translocation was investigated, and Aride (4º13' S, 55º40' E), 9 km north of Cousin, was selected. This study describes the translocation of Seychelles warblers to Aride in September 1988, and documents the breeding activity observed following the transfer.

During mid-September 1988, all Seychelles warbler territories on Cousin were checked for breeding birds and those undergoing tail-feather moult, in order to select individuals for the translocation. Between 23-29 September, 29 adult warblers (16 males and 13 females) were captured in mist-nets, weighed and put in individual 15 × 15 × 20 cm cardboard transfer cages, which were well-ventilated but dark inside to keep birds inactive (thus reducing stress and energy loss). The birds were taken to the laboratory, ringed, weighed again and sexed (through a combination of weight and wing length). They were then transferred to Aride by motorboat, with the cardboard cages kept secure in a wooden carrying cage, and placed on a thick foam mattress to absorb any shocks from rough sea. Immediately upon arrival on Aride, the warblers were taken to their release sites in pre-determined insect- and vegetation-rich areas of the island. Before release, birds were weighed again, and water was provided by throwing bucketfuls over the vegetation immediately surrounding the release cages. On average, individuals spent just over three hours in captivity.

All 29 warblers were released before they reached the minimum known survival weight (14.6 g for males and 12.6 g for females) and survived the transfer to Aride. Males lost on average 0.9 g and females 1.0 g (5.7% and 7.1% of their body weight respectively). Following release, birds displayed a dramatic burst of breeding activity, with nesting commencing within three days in one territory. At least 64% of translocated pairs bred during October 1988, and possibly all pairs bred during January-March 1989, compared with just 0-31% of pairs on Cousin between October 1988 and March 1989. Reproductive output on Aride during the first six months following the translocation was high, with average clutch size (1.8 vs 1.0), number of days to fledging (14.1 vs 19.1) and annual number of young per pair reaching one year of age (3.1 vs 0.2) significantly better than on Cousin. This success was probably at least partly a consequence of the initial low density of warblers and high food availability on Aride. Six months after the transfer, all translocated birds were still alive and 26 young had fledged successfully, bringing the total Aride population to 45 birds. The spaces vacated in territories on Cousin by the removal of translocated individuals were filled by non-territorial birds within six hours.

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