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Seychelles Warbler Acrocephalus sechellensis

Justification
This species is classified as Vulnerable, since it is confined to just four tiny islands and has a very small range overall. Additional translocations may further safeguard the future of this species, once on the brink of extinction, and allow it to be removed from the threatened species list.

Taxonomic source(s)
Dowsett, R. J.; Forbes-Watson, A. D. 1993. Checklist of birds of the Afrotropical and Malagasy regions. Tauraco Press, Li
Sibley, C. G.; Monroe, B. L. 1990. Distribution and taxonomy of birds of the world. Yale University Press, New Haven, USA.

Synonym(s)
Bebrornis sechellensis Sibley and Monroe (1990, 1993), Bebrornis sechellensis sechellensis Sibley and Monroe (1990, 1993)

Identification
14 cm. Medium-sized warbler. Dull olive-and-brown, with pale, buffish-yellow underparts and obscure, buff eyebrow-stripe. Long, horn-coloured bill with flesh-coloured base. Blue-grey legs. Voice Melodious song and brisk chatter. Hints Easily seen on Cousin and Aride.

Distribution and population
Acrocephalus sechellensis was present on several islands in the Seychelles until human disturbance in the 20th century reduced the species to one population on the tiny (0.3 km2) island of Cousin between 1920 and 1988 (Komdeur 2003). The population on Cousin reached an all time low of less than 30 individuals in 1968, but it has recovered following favourable management and conservation policies (Richardson et al. 2006), and the species has since been translocated to the islands of Aride, Cousine and Denis (Bristol 2005; Richardson et al. 2006). In 1997, its population on Cousin was 323 birds and stable, and on Cousine and Aride 137 and 1,600 birds respectively, and increasing (Komdeur et al. 1997). In September 2005, the population on Cousin stood at 371 individuals, including 322 independent birds, and in August 2005 the population on Denis was 75 birds and increasing (Richardson et al. 2006). In 2007, the total population probably numbered over 2,500 birds (G. Rocamora in litt. 2007). It occurred historically on Marianne and Cousine (A. Skerrett in litt. 1999), and there are unconfirmed reports from Felicité (Dijkstra 1997; Komdeur 1996b).

Population justification
In 2007, the population probably numbered over 2,500 birds (G. Rocamora in litt. 2007), roughly equivalent to a minimum of 1,700 mature individuals.

Trend justification
A population of 58 birds translocated to Denis in May and June 2004 had increased to 75 by August 2005 (Richardson et al. 2006), whilst the number of birds on Cousin increased from 323 in 1997 (Komdeur et al. 1997) to 371 in September 2005 (Richardson et al. 2006), thus the species is increasing as a result of habitat management and translocations.

Ecology
It appears to require scrub habitat and tall, scrub-like vegetation dominated by large trees like Pisonia grandis and Ficus reflexa (G. Rocamora in litt. 2007). It is insectivorous, gleaning 98% of its insect food from leaves (Dijkstra 1997). Males and females form long-term breeding pairs and are highly territorial year-round (Dijkstra 1997), with a complex system of cooperative breeding and the ability to bias the sex ratio of offspring (Komdeur 1996a; Lloyd 1998). Although it can breed independently in its first year, some birds, mostly females, remain in their natal territory as subordinates and help to feed the young (Komdeur 2003). This system of cooperative breeding is thought to be a product of habitat saturation (Komdeur 1992). It usually has a one-egg clutch and high adult survival (Komdeur 1996b; Komdeur et al. 1997) (81% adult survival (Komdeur 2003); average life expectancy at fledging 5.5 years (Komdeur and Daan 2005)). Pairs may reside in a territory for up to nine years (Komdeur 2003). The gender of the egg appears to be modified in reaction to territory quality and the need for new helpers (Komdeur 2003). The breeding success of pairs is highly dependent on the quality of territory (Komdeur 1992) as breeding sometimes continues year-round if insects are abundant (Dijkstra 1997; Komdeur 1996c). Most birds on Cousin breed during the South-east monsoon season (April-September), when the quality (foliage cover and insect availability) of most territories peaks, whilst most birds in territories in the south-east of the island breed every six months, both during the South-east monsoon season, when their territories are very poor quality and the North-west monsoon season (October-March) when the quality of their territories increases slightly (Komdeur and Daan 2005). This results in a main breeding season in June-August and a minor breeding season in December-February (Richardson et al. 2006). These patterns suggest that food availability and weather conditions primarily influence the timing of breeding on Cousin, with periodicity being a secondary factor (Komdeur and Daan 2005). Inter-island dispersal by the species is extremely rare, with only two out of 1,924 ringed birds (0.1%) known to have flown between islands, despite the species's physical adaptations for such flights, the saturation of Cousin, and the potential for higher reproductive success elsewhere (Komdeur 2003; Komdeur et al. 2004). This is perhaps because all islands previously occupied by the species were once saturated and such dispersal behaviour was not favoured (Komdeur 2003; Komdeur et al. 2004).

Threats
Habitat destruction and predation by introduced predators are thought to be the main reasons for its very limited range and extremely small population in the past. Seychelles Fody Foudia sechellarum and skinks take warbler eggs but it is unlikely that this is having a major impact on population levels (Komdeur et al. 1997; R. Lucking in litt. 1999). Genetic erosion through inbreeding is a possible future threat (Komdeur et al. 1997), and the lack of inbreeding avoidance through active mate choice implies that inbreeding must occur in the Cousin population (Eikenaar et al. 2008). Fewer breeding attempts occurred on Cousin in 2004 compared to 2003, probably owing, at least in part, to the scarcity of rain and the resulting decrease in foliage and insect abundance (Richardson et al. 2006), indicating that the species is vulnerable to relative drought. Avian malaria may affect survival (van Oers et al. 2010).


Conservation Actions Underway
The species's action plan aimed to increase its range to five islands and its population to over 3,000 individuals by 2006 (Bristol 2005). The spectacular recovery of this species has followed management of Cousin as a nature reserve, including the regeneration of Pisonia woodland, and cessation of intensive management of coconut Cocos nucifera plantations (Richardson et al. 2006). This resulted in territories reaching a saturation level of c.115 in 1981 and the population reaching a carrying capacity of c.320 birds by 1982 (Komdeur 2003). A new management plan for Cousin has been drawn up, which continues to place a high priority on habitat management (Shah et al. 1999). Aride is also managed as a nature reserve. In 1988 and 1990 respectively, new populations were established by moving 29 birds to both Aride (9 km from Cousin) and Cousine (1.6 km from Cousin) (Komdeur 2003). By 2006, the populations on Aride and Cousine were close to their carrying capacities (Richardson et al. 2006). In May and June 2004, one month before the main breeding season, 58 subordinate individuals (27 females and 31 males) were moved from Cousin to Denis to establish a breeding population, following successful predator eradication and habitat management (Bristol 2005; Richardson et al. 2006). They were observed nest-building within three days of release (Richardson et al. 2006), and they have since bred successfully (Bristol 2005; Richardson et al. 2006). By August 2005, the population had increased to 75 birds (Richardson et al. 2006). The translocation left 35 vacant territories on Cousin, and all but three of these were occupied in an average of 5.4 days (range 1-20 days) by subordinate birds (Richardson et al. 2006). All populations are currently monitored (Komdeur et al. 1997; Bristol 2005; Richardson et al. 2006), and research is being carried out into genetic variation, parentage analyses, egg-predation and sex ratio bias (Komdeur et al. 1997). The population on Cousin has been intensively studied since 1985, whilst those on Aride, Cousine (Komdeur 2003) and Denis (Richardson et al. 2006) have been studied from establishment. Breeding ecology and behaviour is monitored annually for nearly all breeding attempts on Cousin (Komdeur 2003). Conservation Actions Proposed
Continue population monitoring (Komdeur et al. 1997). Continue to carry out research (Komdeur et al. 1997), including further investigations into the species's cooperative breeding behaviour (Komdeur 2003). Continue appropriate management and habitat conservation (Komdeur et al. 1997). Consider additional translocation to other islands, free from introduced predators (Komdeur et al. 1997).

References
Bristol, R. 2005. Conservation introductions of Seychelles Fody and Warbler to Denis Island, Seychelles. Re-introduction News 24: 35-36.

Collar, N. J.; Stuart, S. N. 1985. Threatened birds of Africa and related islands: the ICBP/IUCN Red Data Book. International Council for Bird Preservation, and International Union for Conservation of Nature and Natural Resources, Cambridge, U.K.

Dijkstra, S. 1997. Population dynamics and space use of the Seychelles Warbler (Acrocephalus sechellensis) on the plateau area of Aride Island, a saturated environment.

Eikenaar, C.; Komdeur, J.; Richardson, D. S. 2008. Natal dispersal patterns are not associated with inbreeding avoidance in the Seychelles Warbler. Journal of Evolutionary Biology 21: 1106-1116.

Komdeur, J. 1992. Importance of habitat saturation and territory quality for evolution of cooperative breeding in the Seychelles Warbler. Nature 358: 493-495.

Komdeur, J. 1996. Facultative sex ratio bias in the offspring of Seychelles Warblers. Proceedings of the Royal Society of London Series B 263: 661.

Komdeur, J. 1996. Influence of helping and breeding experience on reproductive performance in the Seychelles warbler: a translocation experiment. Behavioral Ecology: 326-333.

Komdeur, J. 1996. Seasonal timing of reproduction in a tropical bird, the Seychelles Warbler: a field experiment using translocation. Journal of Biological Rhythms 11: 333-346.

Komdeur, J. 2003. Adaptations and maladaptations to island living in the Seychelles Warbler. Ornithological Science: 79-88.

Komdeur, J.; Blaakmeer, K.; Richardson, D. 1997. The Seychelles Warbler Acrocephalus seychellensis. In: Rocamora, G. (ed.), Rare and threatened species, sites and habitats monitoring programme in Seychelles. Project G1 EMPS, pp. 185-196. Ministry of Environment/Government of Seychelles/World Bank and Global Environment Facility, Mahé, Seychelles.

Komdeur, J.; Daan, S. 2005. Breeding in the monsoon: semi-annual reproduction in the Seychelles Warbler (Acrocephalus sechellensis). Journal of Ornithology 146: 305-313.

Komdeur, J.; Piersma, T.; Kraaijeveld, K.; Kraaijeveld-Smit, F.; Richardson, D. 2004. Why Seychelles Warblers fail to recolonise nearby islands: unwilling or unable to fly there? Ibis 146: 298-302.

Lloyd, P. 1998. Sex ratios in Seychelles Warblers. Africa - Birds & Birding 3(3): 16.

Richardson, D. S.; Bristol, R.; Shah, N. J. 2006. Translocation of the Seychelles warbler Acrocephalus sechellensis to establish a new population on Denis Island, Seychelles. Conservation Evidence 3: 54-57.

Shah, N. J.; Pickup, T.; Parr, S. 1999. Cousin Island Special Reserve Site Management Plan 1999-2003.

van Oers, K.; Richardson, D. S.; Sæther, S. A.; Komdeur, J. 2010. Reduced blood parasite prevalence with age in the Seychelles Warbler: selective mortality or suppression of infection? Journal of Ornithology 151(1): 69-77.

Text account compilers
Ekstrom, J., Pilgrim, J., Shutes, S., Symes, A., Taylor, J., Warren, B.

Contributors
Lucking, R., McCulloch, N., Parr, S., Rocamora, G., Shah, N., Skerrett, A.

IUCN Red List evaluators
Butchart, S., Taylor, J.

Recommended citation
BirdLife International (2014) Species factsheet: Acrocephalus sechellensis. Downloaded from http://www.birdlife.org on 19/04/2014. Recommended citation for factsheets for more than one species: BirdLife International (2014) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 19/04/2014.

This information is based upon, and updates, the information published in BirdLife International (2000) Threatened birds of the world. Barcelona and Cambridge, UK: Lynx Edicions and BirdLife International, BirdLife International (2004) Threatened birds of the world 2004 CD-ROM and BirdLife International (2008) Threatened birds of the world 2008 CD-ROM. These sources provide the information for species accounts for the birds on the IUCN Red List.

To provide new information to update this factsheet or to correct any errors, please email BirdLife

To contribute to discussions on the evaluation of the IUCN Red List status of Globally Threatened Birds, please visit BirdLife's Globally Threatened Bird Forums.

Additional resources for this species

ARKive species - Seychelles warbler (Acrocephalus sechellensis) 0

Key facts
Current IUCN Red List category Vulnerable
Family Sylviidae (Old World warblers)
Species name author (Oustalet, 1877)
Population size 1700 mature individuals
Population trend Increasing
Distribution size (breeding/resident) 3 km2
Country endemic? Yes
Links to further information
- Additional Information on this species