Justification of Red List Category
This species is listed as Near Threatened. It is found on five islands and its population size is increasing owing to translocations and habitat management. It has a very small range, and nearly meets the requirements for listing as threatened under criterion B.
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. Since 2007 the population has increased strongly with c. 2,800 adults in 2014 (N. Shah in litt. 2014) and c. 3,000 adults in August 2016 (D. S. Richardson in litt. 2016).
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). The species is increasing as a result of habitat management and translocations, with the population recently estimated to be c. 3,000 adults (D. S. Richardson in litt. 2015).
This species 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). Prior to human colonisation it is thought that the species's range covered the inner granitic islands, with a population of approximately 20,000 individuals (Spurgin et al. 2014, D. S. Richardson in litt. 2015). It occurred historically on Marianne and Cousine (A. Skerrett in litt. 1999), and there are unconfirmed reports from Felicité (Komdeur 1996b; Dijkstra 1997). The population on Cousin was thought to have reached an all time low of less than 30 individuals in 1968 (but see Spurgin et al. 2014), but it has recovered following favourable management and conservation policies (Komdeur and Pels 2005; 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) and most recently Frégate in 2011 (Wright et al. 2014b).
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). In 2016 the population was estimated at c. 3,000 adults (D. S. Richardson in litt. 2016). Most recent population estimates for the five islands are: 320 on Cousin (Brouwer et al. 2006); 210 on Cousine (Van de Crommenacker and Richardson 2007); 1,850 on Aride (Orchard 2004); c.500 on Denis (D. S. Richardson in litt. 2016) and c.150 on Frégate (D. S. Richardson in litt. 2016). It is predicted that the populations on Denis and Frégate may eventually exceed 2,000 and 2,500 individuals respectively (Hammers and Richardson 2011; D. S. Richardson in litt. 2015), increasing the overall population to c. >6,500 individuals (D. S. Richardson in litt. 2015).
It appears to prefer dense scrub vegetation or woodland dominated by large trees like Pisonia grandis and Ficus reflexa (Komdeur and Pels 2005; G. Rocamora in litt. 2007; D. Richardson in litt. 2016), and also inhabits swamps and mangrove forests (Drycz and Sharpe 2020). It is insectivorous, gleaning 98% of its insect food from leaves (Komdeur 1991). Males and females form long-term breeding pairs and are highly territorial year-round (Komdeur 1991), with a complex system of cooperative breeding and the ability to bias the sex ratio of offspring (Komdeur et al. 1991; Komdeur 1996a, 2003; 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), although some females may gain direct parentage by laying an egg in the nest of the dominant birds (Richardson et al. 2002). 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) (84% adult, and 61% juvenile survival [Brouwer et al. 2006]; 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 et al. 1997; 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 (Komdeur 1996c; Dijkstra 1997). Breeding success also depends on population density with the number of fledglings produced decreasing with an increasing density, most likely acting through increased competition among individuals for food (Brouwer et al. 2009).
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).
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). One threat identified on Denis island is that of nest predation by Common Myna Acridotheres tristis (D. S. Richardson in litt. 2016). When the nest is threatened by Mynas the female Seychelles warbler sits tight to protect her eggs. The Mynas then attack the female in order to get her off the nest, causing serious wounds and thus increased female mortality. This has resulted in significantly biased adult sex ratios towards males (c. 70% population are males), leading to retarded population growth on Denis overall (J. Van der Woulde to D. S. Richardson in litt. 2015).
Genetic erosion through inbreeding is a possible future threat (Komdeur et al. 1997). The lack of inbreeding avoidance through active mate choice (Richardson et al. 2004) or dispersal patterns (Eikenaar et al. 2008) implies that inbreeding occurs in the Cousin population (Eikenaar et al. 2008), which can have negative effects on individual and offspring fitness (Richardson et al. 2004; Bebbington et al. 2016). 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 (Komdeur 1996c). Avian malaria may affect survival (van Oers et al. 2010), but this pathogen has been (or is in the process of being) lost, probably due to a lack of the correct vectors, from all populations bar Cousin and Cousine (Fairfield et al. 2016). Given its limited genetic variation, and therefore adaptive potential (Spurgin et al. 2014), the main threat is new challenges to which the Seychelles Warbler is naive; for example the introduction of new pathogens to which it may not have any genetic resistance. Fortunately, genetic variation (including immunological variation) has been retained though the successful translocations to maximise the adaptive potential on all islands (Wright et al. 2014a).
Conservation and Research 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 (Richardson 2001; 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 (Komdeur and Pels 2005; 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; Brouwer et al. 2009). 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 (Eikenaar et al. 2007). The most recent translocation took place in 2011 when 59 adult birds were moved from Cousin to Frégate (Wright et al. 2014b). The translocated population was monitored closely and by 2013 had increased to 80 individuals, of which 38 were the original translocated birds and 42 were hatched on Frégate, and in June 2016 the population was estimated at 150 individuals across 64 territories (D. S. Richardson in litt. 2016). The source population on Cousin recovered to carrying capacity within one breeding season. The translocation to Frégate was funded by a Disney Worldwide Conservation Fund grant to Nature Seychelles through the RSPB, Seychelles Warbler Research Group (a collaboration between the University of East Anglia, University of Sheffield and the University of Groningen) and Frégate Island Private (Anon. 2012).
All populations are currently monitored by the Seychelles Warbler Project (Komdeur et al. 1997; Bristol 2005; Richardson et al. 2006), and research is being carried out on a range of evolutionary, ecological and conservation related topics. Key conservation related topics include the loss of genetic variation due to historical bottlenecks or translocations (Spurgin et al. 2014; Wright et al. 2014a), ageing (Barrett et al. 2013, Hammers et al. 2015) and the impact of pathogen infections (Hammers et al. 2016). The population on Cousin has been intensively studied since 1985, whilst those on Aride, Cousine (Komdeur 2003), Denis (Richardson et al. 2006) and Frégate (Wright et al. 2014b) have been studied from establishment. Breeding ecology and behaviour is monitored annually for nearly all breeding attempts on Cousin (Komdeur 2003).
Conservation and Research Actions Proposed
Continue population monitoring, especially of the condition of individuals so that emerging threats can be rapidly detected (Komdeur et al. 1997; D. Richardson in litt. 2016). 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). Ensure the introduced Common Myna is eradicated (D. Richardson in litt. 2016). Screen the Frégate Island population for pathogens to assess whether new pathogens are causing the slower population growth on this island (Johnson et al. 2017).
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.
Text account compilers
Ashpole, J, Brouwer, L., Ekstrom, J., Komdeur, J., Lucking, R., McCulloch, N., Parr, S., Pilgrim, J., Richardson, D.S., Rocamora, G., Shah, N.J., Shutes, S., Skerrett, A., Symes, A., Taylor, J., Van der Woulde, J., Warren, B. & Westrip, J.R.S.
BirdLife International (2023) Species factsheet: Acrocephalus sechellensis. Downloaded from http://www.birdlife.org on 05/02/2023. Recommended citation for factsheets for more than one species: BirdLife International (2023) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 05/02/2023.