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.