The Seychelles warbler Acrocephalus sechellensis was once highly threatened with a population of only 26 individuals confined to one island, Cousin (29 ha) in the Seychelles archipelago. Following reserve designation in 1968 and subsequent management, the population increased to 300-360 birds. This was considered the island’s carrying capacity, with many young birds unable to establish new territories. Given this and the vulnerability of a single population on one small island, introductions to two nearby islands were undertaken in an attempt to secure additional breeding populations.
Several islands were investigated for suitability. Suitable habitat, including adequate insect food, and absence of feral cats Felis catus and rats Rattus spp. were considered prerequisites, and a conservation management commitment highly desirable. Three islands fulfilled these criteria (Aride, Cousine and Frégate). Aride (68 ha; 9 km north of Cousin), a Royal Society for Nature Conservation nature reserve, and Cousine (26 ha; 1.6 km southwest of Cousin) a privately owned island, were selected.
In September 1988, 29 warblers were transferred to Aride and in June 1990, 29 to Cousine. All groups of warblers were studied: since January 1986, 115-123 groups (330-360 birds) on Cousin; and subsequently from September 1988 to June 1991, 13-36 groups (29-165 birds) on Aride; and June 1990 to June 1991, 12-16 groups (29-43 birds) on Cousine.
Monitoring was undertaken (weekly or fortnightly) to record breeding activity and territory formation.
Native woodland recovered quickly in response to management. There was rapid and successful nesting following release. The first young on Aride and Cousine hatched four and three weeks respectively after transfer. Vacant territories created on Cousin were quickly filled (some within hours) by previous helping and/or 1 year-old birds.
In June 1991, 33 months after release on Aride and 12 months after release on Cousine, all transferred birds were still alive and 150 young (136 and 14, respectively) had fledged.
In one year, numbers increased on Aride from 29 to 59 and on Cousine from 29 to 43.
There was still suitable habitat remaining on Aride (40.8 ha) and Cousine (18.0 ha) for young to establish territories.
As a consequence of the introductions, two new breeding Seychelles warbler populations have become established.
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