Study species and system
The Seychelles warbler is a small (ca 15 g) insectivorous passerine
endemic to the Seychelles. The species was distributed across the
archipelago prior to human colonisation (Spurgin et al., 2014), but
underwent a severe population reduction in the 1900s due to
anthropogenic effects, with just ca 29 individuals remaining on Cousin
Island (4°20’S, 55°40’E; 0.29 km2) by the 1960s
(Crook, 1960). After intensive conservation, the population recovered to
carrying capacity on Cousin (ca 320 adults present in ca 110
territories) by the 1980s (Brouwer et al., 2009; Komdeur, 1992).
Additional populations were established by translocations to four nearby
islands: Aride (29 birds in 1988), Cousine (29 birds in 1990), Denis (58
birds in 2004), and Frégate (59 birds in 2011) (Komdeur, 1994;
Richardson et al., 2006; Wright et al., 2014). Founder individuals (all
from Cousin) were selected based on sex, age, body condition, and
breeding experience but without reference to genetic characteristics
(Wright et al., 2014). Translocations to Aride and Cousine were
undertaken before blood sampling became routine, whereas sampling of all
the founders of the Denis and Frégate populations was undertaken (Wright
et al., 2014). Of the translocated populations, two are now at carrying
capacity (Aride: ca 1,850 individuals; Cousine: ca 210 individuals
(Wright et al., 2014)), while the populations on the other islands are
still increasing (Denis: ca 424 birds in 2015 (Doblas & McClelland,
2015); Frégate: ca 141 birds in 2016 (Johnson, Brown, Richardson, &
Dugdale, 2018)).
The Seychelles warbler on Cousin island has been monitored since 1986
(Komdeur, 1992; Hammers et al., 2019). A comprehensive population census
has taken place every year during the major breeding season
(June–September), and – since 1997 – also during the minor breeding
season (November–March) except in 2000–2002 and in 2006 (Brouwer et
al., 2010). Individuals were recorded as present if caught, or observed,
during the field season. The other populations have not been censused
regularly and only sporadic census data are available.
The rate of annual resighting of individuals on Cousin is high (0.98,
Brouwer et al., 2010) and there is virtually no inter-island dispersal
(0.1%, Komdeur et al., 2004), thus enabling accurate survival estimates
(Brouwer, Richardson, Eikenaar, & Komdeur, 2006). Individuals can be
confidently presumed dead if not seen for two consecutive breeding
seasons; the date of death is assigned as the end of the last season in
which a bird was observed (Hammers, Richardson, Burke, & Komdeur,
2013). Ages were rounded to the nearest 0.5 years. Adult annual survival
is high (84%), with mortality being greatest in first-year birds
(Brouwer et al., 2006). Median lifespan is 5.5 years post-fledging, and
maximum lifespan is 19 years (Hammers & Brouwer, 2017).
Females typically lay single-egg clutches (Richardson et al. 2001) and
only occasionally two or three eggs (Komdeur 1991). They are
facultatively cooperative breeders, with a socially monogamous dominant
breeder pair defending strict territories year-round (Komdeur, 1992).
Some adult birds delay independent breeding and become subordinates
(Kingma, Bebbington, Hammers, Richardson, & Komdeur, 2016), and may
help raise offspring (Komdeur, 1992, Hammers et al. 2019). Although 44%
of female subordinates gain reproductive success by co-breeding, male
subordinates rarely gain paternity (Richardson et al., 2002; Raj Pant,
Komdeur, Burke, Dugdale, & Richardson, 2019). Extra-pair paternity is
frequent in this species (Richardson et al., 2001), with 41% of
offspring fathered outside the natal territory (Raj Pant et al., 2019).
Individuals are caught either by mist-net, or as nestlings, and are aged
based on hatch date, behaviour, and eye colour at first catch (for
details see Komdeur, 1992; Wright, 2014). Each bird is given a metal
British Trust for Ornithology (BTO) ring and a unique combination of
three colour rings (Richardson et al., 2001). Routine blood sampling
began in 1993. Since 1997, >96% of the Cousin population
has been ringed and blood sampled (Raj Pant et al., 2019). Samples (ca
25 µl) are collected by brachial venipuncture and stored in 0.8 ml of
absolute ethanol at 4°C.