Overview

Brief Summary

Biology

The Seychelles warbler feeds primarily on a variety of insects, including bugs and their eggs, beetles, bees and ants. It also feeds on spiders and, occasionally, small skins and geckos (2). It forages for its prey in trees, particularly Pisonia grandis, Morinda citrifolia (the Indian mulberry) and Ficus (fig) species, plucking insects from the undersides of leaves and twigs (2) (3) (5). Occasionally, aerial insects are plucked from the air whilst in flight and rarely the warbler will descend to the ground to feed (2) (3). Being a monogamous bird, male and female Seychelles warblers form pairs and together defend a territory which they will remain in until one of the pair dies (2) (3). The peak breeding period occurs between June and August, with a smaller peak of breeding attempts occurring between December and February. The timing of breeding throughout the year seems to be dependent on rainfall, however, breeding may occur at any time of the year if insects are sufficiently abundant (3). Seychelles warblers have a cooperative breeding system, meaning that helpers, usually the daughters from previous broods, assist with defending the territory, building the nest, incubating the eggs and feeding the young (6). Normally, a clutch of just one egg is laid each season, which is incubated for around 15 days, and the chick remains in the nest for a further 14 days (3). However, even after leaving the nest the young warbler will continue to solicit food from its parents and helpers for months afterwards (3).
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Description

The Seychelles warbler stands as an example of how, whilst humans may push a species to the verge of extinction, human intervention may also protect a species from this fate. Once believed to have just 26 individuals remaining on this planet, the Seychelles warbler, with its long, rather stout bill and long tail, is now thriving (2) (3). Its plumage is dull, yellowish olive-brown with a greenish wash, which fades to pale, faintly streaked, yellow on the underparts. The wing feathers are darker and browner and the tail feathers are also darker but with white tips (2). A whitish line runs above the eye, while an indistinct stripe extends from the bill to the reddish-brown eye and continues behind it. The Seychelles warbler's legs are grey-blue. Males and females are similar in appearance, while juveniles have grey-brown or grey-blue eyes (2). Seychelles warblers sing a short but rich and melodious song of simple whistled phrases, and they call in alarm with a brisk chatter (2).
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Distribution

Range Description

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).

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Range

Cousin (Seychelles Islands).

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Historic Range:
Indian Ocean_Seychelles Island

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Range

The Seychelles warbler currently occurs on the islands of Cousin, Cousine, Aride and Denis in the Seychelles (2) (3) (4). Historically, this species is believed to have also existed on Marianne, possibly Praslin, and there were also occasional reports from Mahé and Félicité (3).
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Ecology

Habitat

Habitat and Ecology

Habitat and 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).


Systems
  • Terrestrial
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Thick scrub and dense, tall woodland dominated by the tree Pisonia grandis are the Seychelles warbler's preferred habitats (3), but it also occurs in scrub in old coconut plantations, and swamps and mangroves (2).
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Life History and Behavior

Life Expectancy

Lifespan, longevity, and ageing

Maximum longevity: 14 years Observations: Although there are no detailed studies, these animals have been known to live over 14 years (Richardson et al. 2007). Evidence of reproductive senescence has been reported in females over 6 years of age (Komdeur 1996). Annual survival has been reported to be 84% for adults (Brouwer et al. 2006).
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Conservation

Conservation Status

IUCN Red List Assessment


Red List Category
VU
Vulnerable

Red List Criteria
D2

Version
3.1

Year Assessed
2012

Assessor/s
BirdLife International

Reviewer/s
Taylor, J. & Butchart, S.

Contributor/s
Lucking, R., McCulloch, N., Parr, S., Rocamora, G., Shah, N. & Skerrett, A.

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.

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Current Listing Status Summary

Status: Endangered
Date Listed: 06/02/1970
Lead Region: Foreign (Region 10) 
Where Listed: Entire


Population detail:

Population location: Entire
Listing status: E

For most current information and documents related to the conservation status and management of Bebrornis sechellensis , see its USFWS Species Profile

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Status

Classified as Vulnerable (VU) on the IUCN Red List (1).
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Population

Population
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.

Population Trend
Increasing
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Threats

Major 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).

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The reasons behind the Seychelles warblers past decline is a story that is echoed across many islands throughout the world. Following human colonisation, the natural habitat of the Seychelles was extensively modified, particularly between 1910 and 1920 when most of the islands were planted with coconut palms (Cocos nucifera). This left very little suitable natural habitat for the Seychelles warbler and as a result, and in combination with the effects of introduced predators, this species subsequently disappeared from all islands except for Cousin. Since that time, until around 1968, the global population of Seychelles warblers was restricted to just a one hectare patch of mangroves and numbered around only 26 individuals (3) (5). Thankfully, 1968 saw Cousin being purchased and managed as a nature reserve, and the regeneration of native vegetation allowed the warbler population to recover (3) (5). Subsequent human intervention also established new populations on Aride, Cousine and Denis (3). Whilst this is a remarkable story of how the warbler's extinction was successfully averted, a number of threats remain to this vulnerable bird. Cousin Island has so far remained free of introduced predator species, such as cats and rats, and introduced competitors, but the accidental or intentional introduction of any of these species to Cousin undoubtedly poses a great threat to the future of the warbler (5). While Cousin Island is now a nature reserve, and thus the protection of its habitat remains secure, Cousine Island is privately owned and hence the future of the island remains uncertain (3). The decline of the warbler to such a tiny number in the past probably left the population with severely limited genetic variation and the negative effects of inbreeding poses a potential long-term threat (3). In addition, if predictions of global warming and sea level rise are correct, this may result in the loss significant areas of the warbler's habitat, which is found at sea-level (3).
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Management

Conservation Actions

Conservation Actions
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).

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Conservation

The turning point for the Seychelles warbler, when it was on the brink of extinction, came with the purchase of Cousin Island in 1968 and its management as a nature reserve (3). Intense efforts to clear coconut trees allowed native vegetation to regenerate and by 1982 most of the island was covered with tropical, primarily native, forest (5). By this point, Cousin Island supported the largest Seychelles warbler population that was possible, and so new populations were established on Aride and Cousine Island (3), and later to Denis Island (4). Both Cousin and Aride are protected as Nature Reserves under Seychelles Law and management of the habitat continues, with efforts also aimed at keeping the islands rat-free (3). The establishment of five separate populations on five islands has been deemed necessary to improve the conservation status of this species and with four breeding populations already established (3) (4), it may not be long before this goal is achieved.
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Wikipedia

Seychelles warbler

The Seychelles warbler (Acrocephalus sechellensis), also known as Seychelles brush warbler,[2] is a small songbird found on five granitic and corraline islands in the Seychelles. It is a greenish-brown bird with long legs and a long slender bill. It is primarily found in forested areas on the islands. The Seychelles warbler is a rarity in that it exhibits cooperative breeding, or alloparenting; which means that the monogamous pair is assisted by nonbreeding female helpers.

A few decades ago the Seychelles warbler was on the verge of extinction, with only 26 birds surviving on Cousin Island in 1968. Due to conservation efforts there are more than 2500 of the species alive today with viable populations on Denis, Cousine and Aride Islands, as well as Cousin Island. In December 2011, 59 Seychelles warblers were translocated to Frégate Island in order to establish a fifth population.

Taxonomy and systematics[edit]

The Seychelles warbler is closely related to the Rodrigues warbler (Acrocephalus rodericanus) and the two species have sometimes been placed in their own genus, Bebrornis. The two species have also been considered allied to the Malagasy genus Nesillas. A 1997 study confirmed however that the two species were part of a clade of Afrotropical warblers within Acrocephalus that also includes the Madagascan swamp warbler, the greater swamp warbler, the lesser swamp warbler and the Cape Verde warbler.[3][4][5]

Description[edit]

The Seychelles warbler is a small, plain Acrocephalus warbler, between 13–14 cm (5.1–5.5 in) in length and with a wingspan of 17 cm (6.7 in).[6] It has long grey-blue legs, a long horn coloured bill, and a reddish eye. Adults show no sexual dimorphism in their plumage, the back, wings, flanks and head are greenish brown and the belly and breast are dirty white. The throat is a stronger white and there is a pale supercilium in front of the eye. Juvenile birds are darker with a more bluish eye.

The voice of the Seychelles warbler is described as rich and melodious,[6] similar to a human whistle. Its structure is simple and is composed of short song sequences delivered at a low frequency range.[7] The lack of a wide frequency range sets it apart from other species in its genus, such as the reed warbler, its song is similar to its closest relatives in Africa such as the greater swamp warbler.

Behaviour[edit]

The Seychelles warbler naturally occur in dense shrubland and in tall forests of Pisonia grandis. It is almost exclusively an insectivore (99.8% of its diet is insects), and obtains 98% of its prey by gleaning small insects from the undersides of leaves. It does occasionally catch insects on the wing as well.[8] Most of the foraging occurs on Pisonia, Ficus lutea[verification needed] and Morinda citrifolia.[9] Studies of the foraging behaviour found that Seychelles Warblers favour Morinda and spend more time foraging there than in other trees and shrubs, the same study found that insect abundance is highest under the leaves of that shrub.[10] The planting of Morinda on Cousin, and the associated improved foraging for the warbler, was an important part of the recovery of the species.

Cooperative breeding habits[edit]

Seychelles warblers demonstrate cooperative breeding, a reproductive system in which adult male and female helpers assist the parents in providing care and feeding the young. The helpers may also aid in territory defense, predator mobbing, nest building, and incubation (females only).[11] Breeding pairs with helpers have increased reproductive success and produced more offspring that survived per year than breeding pairs with the helpers removed.[12] Helpers only feed the young of their parents or close relatives and do not feed unrelated young. This is evidence for the kin-selected adaptation of providing food for the young. The indirect fitness benefits gained by helping close kin are greater than the direct fitness benefits gained as a breeder. This could be evidence for the kin-selected adaptation of providing food for the young.

On high-quality territories where there is more insect prey available, young birds were more likely to stay as helpers rather than moving to low-quality territories as breeders.[13] On low quality territories, having a helper is unfavorable because of increased resource competition. Females are more likely to become helpers,[14] which may explain the adaptive sex ratio bias seen in the Seychelles warblers. On high quality territories, females produce 90% daughters; on low quality territories, they produce 80% sons. Clutch sex ratio is skewed towards daughters overall.[15] When females are moved to higher quality territories, they produce two eggs in a clutch instead of a single egg, with both eggs skewed towards the production of females. This change suggests that Seychelles warblers may have pre-ovulation control of offspring sex ratio, although the exact mechanism is unknown.

References[edit]

  1. ^ BirdLife International (2012). "Acrocephalus sechellensis". IUCN Red List of Threatened Species. Version 2013.2. International Union for Conservation of Nature. Retrieved 26 November 2013. 
  2. ^ a b Penny, M. (1974): The Birds of Seychelles and the Outlying Islands
  3. ^ Leisler, Bernd; Petra Heidrich, Karl Schulze-Hagen and Michael Wink (1997). "Taxonomy and phylogeny of reed warblers (genus Acrocephalus) based on mtDNA sequences and morphology". Journal of Ornithology 138 (4): 469–496. doi:10.1007/BF01651381. 
  4. ^ Helbig, Andreas; Ingrid Seibold (1999). "Molecular Phylogeny of Palearctic–African Acrocephalus and Hippolais Warblers (Aves: Sylviidae". Molecular Phylogenetics and Evolution 11 (2): 246–260. doi:10.1006/mpev.1998.0571. PMID 10191069. 
  5. ^ Bairlein, Franz; Alström, Per; Aymí, Raül; Clement, Peter; Dyrcz, Andrzej; Gargallo, Gabriel; Hawkins, Frank; Madge, Steve; Pearson, David; Svensson, Lars (2006), "Family Sylviidae (Old World Warblers)", in del Hoyo, Josep; Elliott, Andrew; Christie, David, Handbook of the Birds of the World. Volume 11, Old World Flycatchers to Old World Warblers, Barcelona: Lynx Edicions, p. 635, ISBN 84-96553-06-X 
  6. ^ a b Skerrett A, Bullock I & Disley T (2001) Birds of Seychelles. Helm Field Guides ISBN 0-7136-3973-3
  7. ^ Dowset-Lemaire, Francoise (1994). "The song of the Seychelles Warbler Acrocephalus sechellensis and its African relative". Ibis 136 (4): 489–491. doi:10.1111/j.1474-919X.1994.tb01127.x. 
  8. ^ Richardson D. (2001) Species Conservation Assessment and Action Plan, Seychelles Warbler. [1] Nature Seychelles.
  9. ^ BirdLife International (2007) Species factsheet: Acrocephalus sechellensis. Downloaded from http://www.birdlife.org on 20/6/2007.
  10. ^ Komdeur J & Pels M. (2005) "Rescue of the Seychelles warbler on Cousin Island, Seychelles: The role of habitat restoration" Biological Conservation 124 15-26
  11. ^ Komdeur, J. (1994). "The Effect of Kinship on Helping in the Cooperative Breeding Seychelles Warbler". Proceedings of the Royal Society 256 (1345): 47–52. doi:10.1098/rspb.1994.0047. 
  12. ^ Komdeur, J. (1992). "Experimental Evidence for helping and hindering by previous offspring in the cooperative-breeding Seychelles warbler Acrocephalus sechellensis". Behavioral Ecology and Sociology 34: 175–186. 
  13. ^ Komdeur, J. (1992). "Importance of habitat saturation and territory quality for evolution of cooperative breeding in the Seychelles warbler". Letters to Nature 358: 493–495. doi:10.1038/358493a0. 
  14. ^ Davies, N. B., Krebs J. R., West, S. A. (2012). An Introduction to Behavioural Ecology Fourth Edition. West Sussex, UK: Wiley-Blackwell.
  15. ^ Komdeur, J.; Magrath, M. J.; Krackow, S (2002). "Pre-ovulation control of hatchling sex ratio in the Seychelles warbler". Proceedings of the Royal Society 269 (1495): 1067–1072. doi:10.1098/rspb.2002.1965. 
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