University of Groningen Conserving the seychelles warbler Acrocephalus sechellensis by translocation from Cousin Island to the islands of Aride and Cousine Komdeur, Jan Published in: Biological Conservation DOI: 10.1016/0006-3207(94)90360-3 IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below. Document Version Publisher's PDF, also known as Version of record Publication date: 1994 Link to publication in University of Groningen/UMCG research database Citation for published version (APA): Komdeur, J. (1994). Conserving the seychelles warbler Acrocephalus sechellensis by translocation from Cousin Island to the islands of Aride and Cousine. 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Download date: 25-09-2021 Biological Conservation 67 (1994) 143-152 CONSERVING THE SEYCHELLES WARBLER Acrocephalus sechellensis BY TRANSLOCATION FROM COUSIN ISLAND TO THE ISLANDS OF ARIDE AND COUSINE Jan Korndeur* Department of Zoology, University of Cambridge, Cambridge, UK, CB2 3EJ and BirdLife International Wellbrook Court, Girton Road, Cambridge, UK, CB3 0NA (Received 24 November 1992; revised version received 16 March 1993; accepted 26 March 1993) Abstract birds. In September 1988 29 warblers were transferred The Seychelles warbler Acrocephalus sechellensis was to Aride Island (68 ha) and in June 1990 29 warblers once a highly threatened single-island endemic species to Cousine Island (26 ha). This paper describes the with a population of 26 individuals confined to Cousin ecological studies carried out prior to, during and after Island in the inner Seychelles. Following long-term the translocations and documents the subsequent estab- management of Cousin, the population steadily recovered lishment of the new populations. to around 300-360 birds. Given the vulnerability of one small island in the Indian Ocean, the possibility of estab- Past and present distribution of the Seychelles warbler lishing the species on additional islands had been The Seychelles warbler is known only from the proposed as a priority conservation measure, in order to Seychelles Islands, where in 1870 it was recorded on give the species the security of additional breeding popu- Marianne and Cousine (Oustalet, 1878; Fig. 1). Be- lations, lest some ecological disaster shouM befall the tween 1910 and 1920 both islands were planted with parent population. Successful translocations of warblers coconuts Cocos nucifera. Coconut palm are vulnerable to the islands of Aride and Cousine took place in to root competition, so to maintain reasonable yields September 1988 and June 1990 respectively. Given the plantations had to be kept clear of other vegetation by presence now of three healthy breeding populations, it is regular cutting. This policy caused the removal of most considered that the Seychelles warbler will soon no indigenous vegetation, which resulted in the disappear- longer be a globally threatened species. It is not often ance of the species from both islands. Odd records that people are allowed to pull a species so dramatically have been reported from Mah6 and F61icit6 and there back from the brink of extinction. is some confusion as to whether the original Cousine record may have been in error for Cousin (Collar & Keywords: Seychelles warbler, island quality, niche Stuart, 1985). The species was certainly present on overlap, translocation, breeding activity. Cousin in 1938 but was by then considered rare (Collar & Stuart, 1985). No records exist of the composition of the original vegetation of Cousin but in the list of the INTRODUCTION flora of the Seychelles only one species, Pisonia grandis, The oceanic islands of the western Indian Ocean sup- is mentioned from Cousin (Summerhayes, 1931). How- port 26 globally threatened bird taxa that are listed in ever, it has been suggested that Cousin Island was the ICBP/IUCN Bird Red Data Book and include mainly covered by mature Pisonia forest (Fosberg, some of the smallest and most threatened bird popula- 1970). Between 1910 and 1920 Cousin was planted with tions in the world (Collar & Stuart, 1985). In the central coconuts, leaving very little suitable natural habitat group of the Seychelles eight taxa are threatened. One for the warblers. Fortunately alien predators, such as of these is the Seychelles warbler Acrocephalus sechel- cats, rats and mice, have never found their way onto lensis (Oustalet, 1878), which at one time was reduced the island. The introduction of these species would to a population of 26 individuals, entirely confined create by far the greatest threat to the future of the to Cousin Island (29 ha) (Crook, 1960). Since Cousin warblers. No other self-sustaining population existed Island has been managed as a nature reserve, the warbler until September 1988. population reached carrying capacity of around 320 In 1959 a survey revealed a population of 26 birds * Present address: National Environmental Research Insti- only (Crook, 1960). Since the species was believed to be tute, Department of Wildlife Ecology, Kalo, Grenhvej 12, on the verge of extinction, Cousin was purchased for DK-8410 Ronde, Denmark the International Council for Bird Preservation (ICBP) Biological Conservation 0006-3207/94/$07.00 © 1994 Elsevier in 1968, which has since managed the island as a Science Ltd, England. Printed in Great Britain nature reserve. Management has been directed towards 143 144 J. Komdeur c:~ Aride Cousin° Cousine ~ • (~ Marianne Fregate l~ ;Seychelles 0 ,8000m Fig. 1. Map showing the Seychelles islands under discussion. increasing suitable habitat for the warbler. In order to digenous vegetation has quickly regenerated and is now allow the original vegetation to regenerate, the regular tending towards predominantly Pisonia grandis forest. clearing of bush growing up under the coconut palms This has led to a spectacular recovery of the warbler, was stopped. In addition the coconut crop continued to numbers rising to nearly 320 birds in 1982 (Fig. 2). be harvested so as to prevent regeneration of coconut This recovery is partially documented in a Collar & palms which, if allowed to grow unchecked, results in Stuart (1985), who record a somewhat slower recovery an impenetrable thicket of young palms, probably than shown in Fig. 2. The data in Fig. 2 are based on a crowding out everything else. As a consequence the in- re-examination of the original territory maps of Bathe 400 Birds - 200 .... 0 .... Territories 350 - 160 300 tO .e.4 In 250 • 120 O °oO -0- o 4J .r-.I .,-I 200 -80 150 iO0 40 50 0 .... t .... i .... i .... i .... i .... i .... i .... i . , , 50 55 60 65 70 75 80 85 90 Year Fig. 2. Increase in Seychelles warbler numbers and territories since 1959. Seychelles warbler conservation by translocation 145 and Bathe (1982) and N. J. Phillips (pers. comm.) as Similarity in feeding ecology among iandbird species well as more recent total counts. Since 1982 the Between January 1987 and March 1988 the feeding population has fluctuated around that level, suggesting ecology of all six landbird species inhabiting Cousin that this is the carrying capacity of the island. The in- Island was studied: the barred ground dove Geopelia crease in number of territories showed the same trend, striata, the turtle dove Streptopelia picturata, the but reached its saturation level of c. 115 territories Madagascar fody Foudia madagascariensis, the Seychelles earlier, in 1973. sunbird Nectarinia dussumieri, the Seychelles fody Foudia sechellarum, and the Seychelles warbler. The Justification for a Seychelles warbler translocation first three and the first five mentioned were at The Seychelles warbler has a clutch-size of one which, the time of this study the only breeding landbird when combined with low breeding success, leads to a species occurring on Aride and Cousine respectively, low reproductive rate (on average only 0.32 young per the islands selected for the establishment of new warbler warbler pair per year become adults). This need not be populations. a severe problem if the pressures placed on reproduc- Twice each week a fixed transect (1.5 km) was tion are only temporary, since warblers are long-lived walked, once in the morning (0600 h~0800 h) and once (mean annual adult survivorship~the probability of in the afternoon (1630 h-1900 h), stopping for 2-min surviving to the next year, starting at age one year old-- periods at 30 points, each 50 m apart. The transect is 83-5% and mean adult life expectancy is 3.9 years) passed through all the main habitats. All instances of and can therefore tolerate periods of low recruitment. feeding movements by landbirds that appeared to in- They are particularly vulnerable to factors which lead volve prey-capture were recorded; searching behaviour to increased adult mortality and/or prolonged reduc- was not included. For each half-minute (with no more tion in recruitment. than four observations per bird) the following observa- As the Seychelles warbler population on Cousin had tions were made (n = number of categories): reached carrying capacity, many young birds were unable to establish new territories (Komdeur, 1992). (1) The feeding height. All foraging was recorded as Thus in order to give the species the security of more occurring in one of the following bands: 0 m (ground), breeding populations, lest some ecological disaster 04 m, 4-8 m, >8 m (n --- 4).