Justification of Red List Category
This species is listed as Near Threatened because it has a very small population, which occupies a small range. The species is threatened by introduced and native predators. However, there is no evidence of a decline in the population at present.
In 1983, the total population was estimated to number c.1,500 individuals, equivalent to c.1,000 mature individuals.
Point counts along trails conducted between 2002 and 2013 showed a stable population trend for the species (van de Crommenacker et al. 2015).
Dicrurus aldabranus is found on all four main islands of Aldabra Atoll and neighbouring islets, Seychelles (Penny 1974, G. Rocamora in litt. 2008, van de Crommenacker et al. 2015), and also on the larger lagoon islands of Ile Esprit (where breeding has been reported), Ile Moustique, Gros Ilot, Ile Michel and Ile aux Cèdres (Rocamora and Yeatman-Berthelot 2009, Rocamora 2013, A. Skerrett in litt. 1999). Its extent of occurrence is less than 150 km2 (Stoddart and Westoll 1979). In 1983, the total population was estimated at c.1,500 individuals. It is widely distributed but generally uncommon (Stoddart and Westoll 1979).
This species frequents mixed scrub, mangroves and Casuarina coastal forest, but prefers to breed in the latter two habitats, perhaps because the taller trees provide better nesting sites (Penny 1974, Stoddart and Westoll 1979, Threadgold and Johnson 1999, Rocamora and Yeatman-Berthelot 2009, Rocamora 2013). Territory size ranges from 2.25 ha in Casuarina woodland to 4.5 ha in mixed scrub (Collar and Stuart 1985, Prys-Jones and Prys-Jones undated). It feeds mainly on a variety of insects and small vertebrates such as geckos, but also has been known to prey on bird eggs, seabird chicks and to cannibalise its own dead offspring (Seychelles Islands Foundation [SIF], unpublished data). Breeding has been recorded between October-November and March-May, and is timed to coincide with the rainy season from December to March (Rocamora and Yeatman-Berthelot 2009). Its nest, in which 1-3 eggs are laid, is a perfectly round cup made from intertwined fine plant fibres bound with spider webs, situated in a fork near the end of a thin horizontal branch, usually 2-8 m above the ground. The species usually nests in trees such as Casuarina equisetifolia, Mystroxylon aethiopicum, Clerodendrum glabrum, Sideroxylon inerme, Cordia subcordata, Euphorbia pyrifolia and mangroves (e.g. Rhizophora mucronata), but also in Ficus spp. in scrub (SIF unpublished data). The incubation period is 16-18 days, followed by a fledging period of 15-19 days. Juvenile birds are dependent for at least several weeks after fledging, and then remain with their parents until at least the following breeding season (François and Fanchette 2003, Rocamora 2013).
It has a high rate of nest failure, probably largely due to introduced native predators (Rocamora and Yeatman-Berthelot 2009, Rocamora 2013, SIF unpublished data). Up to 8/10 studied nests are predated at the egg stage, mainly by crows Corvus spp., rats, coucals and bulbuls (Pycnonotidae), as observed or suggested by nest monitoring (SIF unpublished data) and artificial nest experiments (Rocamora and Jules 2000). There are no known threats to its habitat, but its small island range and population put it at risk from catastrophic events, such as disease or cyclones (Rocamora and Berthelot 2009, Rocamora 2013).
Conservation Actions Underway
Aldabra is protected as a Special Reserve under Seychelles legislation (G. Rocamora in litt. 2008), inscribed as a UNESCO World Heritage Site and designated a Ramsar Wetland Site of International Importance(N. Bunbury and A. Burt in litt. 2016). The population has been monitored since 1999 (N. Bunbury and A. Burt in litt. 2016).
Text account compilers
O'Brien, A., Pilgrim, J., Robertson, P., Symes, A., Taylor, J. & Westrip, J.
Bunbury, N., Burt, A., Rocamora, G. & Skerrett, A.
BirdLife International (2021) Species factsheet: Dicrurus aldabranus. Downloaded from http://www.birdlife.org on 19/10/2021. Recommended citation for factsheets for more than one species: BirdLife International (2021) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 19/10/2021.