Trend justification
Population trends have not been empirically assessed, but the species has been extirpated from one island (Tabueran) in the 1970s, and threats within its extant range include introduced predators and habitat degradation driven by a variety of processes. However, on Kiritimati, monitoring has been ongoing since 2007, but no declines have been detected (R. Pierce in litt. 2016). The species is precautionarily assessed as having a small ongoing decline. Should the species be found not to be declining overall, it would warrant downlisting to a lower threat category.

Acrocephalus aequinoctialis is endemic to Kiritimati (Christmas Island) and Teraina (Washington) islands in the Northern Line Islands, Kiribati. It remained common on Teraina in 1980 (A. Gupta 2007) and still is in the early 2010s (R. Bebe in litt. 2016). It formerly occurred on Tabueran (Fanning), but was apparently extirpated in 1972. It was described as 'common' on Kiritimati in 1966 (Pratt et al. 1987), but just 400 individuals were estimated there in 1972 and it was 'scarce in the vicinity of settlement' by 1980 (Sherley 2001). Surveys on Kiritimati in 2007-2011 indicate that the species is widespread and locally common on the mainland and individuals occasionally visit small motu in the lagoons (R. Pierce in litt. 2012). The distribution on Kiritimati is patchy, e.g. there are large areas in the south-east of the island where the warbler is absent due to unsuitable habitat. Densities of up to 50 birds/km² have been reported on Kiritimati (Pierce et al. 2007).

It occurs in vegetated areas with scattered dominant Tournefortia argentea trees (growing 4-6 m) and Cassytha filiformis, mixed with dense Scaevola and many other indigenous shrubs, and sometimes with introduced coconut palms (Cocos nucifera), but not in areas with dense coconuts palms. Recent research found the species to be more abundant in areas with taller Heliotropium trees and taller Scaevola shrubs, less abundant in areas with increased Suriana shrubs, bare ground, and grass (VanderWerf et al. 2016) and absent from cleared or recently burnt areas that have been cleared or recently burned (R. Pierce in litt. 2012). Birds will cross relatively open grassy areas or sandflats and cross up to 300 m to access motu that have recently been cleared of Rattus exulans (R. Pierce in litt. 2012). The species has also been sighted in gardens close to human habitation (T. Mark in litt. 2010). It feeds on insects, including flies and dragonflies; and small lizards. It spends much of its time foraging on the ground, in low ground cover, and in dead or low branches close to ground level. It is sedentary and a weak flier. Breeding occurs from at least February-July. It is reportedly monogamous; females incubating 2-4 eggs in nests placed just below the canopy of Tournefortia argentea. The territory size is reportedly 1.8-2.3 ha. Maximum densities recorded in 2007 were 0.5 birds/ha (R. Pierce in litt. 2012).

The land areas of Kiritimati and Teraina are 322 km² and 14 km² respectively, and both support introduced rats Rattus spp. and cats Felis catus (A. Wegmann in litt. 2008), although the spread of Black Rats R. rattus on Kiritimati appears to have slowed or stalled, with black rats being relatively rare beyond the inhabited areas of the island and outer beaches (Pierce et al. 2015, R. Pierce in litt. 2016). Although the rate of spread of R. rattus has been unusually slow, it does need ongoing monitoring. The successful management of T. argentea trees is likely to be a significant factor for the species (T. Marks in litt. 2010). Hunting of the species by children with slingshots may also pose a threat (A. Wegmann in litt. 2008), but only locally. Kiritimati Atoll is also suffering from poorly planned immigration leading to degradation of remaining habitats (J. Millet in litt. 2008); principal threats include habitat loss from fires, clearing for coconuts, development and habitat modification (e.g. proliferation of the weed Pluchea indica following fire, and potentially R. rattus impacts [Pierce et al. 2007]).

Conservation Actions Underway

A survey for the species gathered information on baseline population density, habitat preference and threats in 2007. This survey has made a number of recommendations for further conservation work (Pierce et al. 2007). Subsequently, the population has been monitored annually or 2-yearly at several sites which to date show no signs of decline (Pierce et al. 2015). Pacific rats were eradicated from many motus in the central lagoons in 2009 (Pierce and Brown 2009) and this was followed by colonisation and breeding on some islands (Pierce et al. 2015). Black rats are being monitored in core part of habitat on Kiritimati and rat-free motus are also being monitored at least annually to check for reinvasion. Contingency plans are in place for re-invaded motus. Habitat parameters are better understood (VanderWerf et al. 2016). An assessment of suitability of Palmyra and Malden was made, with the former being considered to provide some suitable habitat (VanderWerf 2014).   

Conservation Actions Proposed
Continue with management actions currently underway as above. Step up biosecurity at Kiritimati and Kiribati generally. Continue with annual or biannual monitoring in May-June to determine whether the population is in decline and if so, where and what are the causes, and what contingency plan is best implemented (Pierce et al. 2007, 2015). Identify suitable habitat that supports the species. Advocate for protection of key habitats from fire, development etc, e.g. by engaging with key landowners, community and government to identify risks, opportunities, solutions, etc and gaining better community buy-in (Pierce et al. 2007, Pierce 2010). Continue with surveillance of rodent eradication success from Kiritimati motus and monitor outcomes for seabirds and warbler (Pierce et al. 2007, 2015). Use external advice to address biosecurity issues, e.g. spread of Pluchea and the potential arrival of invasive ants. Implement rat trapping at important sites. Investigate the feasibility of emergency translocations to another island, e.g Orona in Phoenix Islands (Pierce et al. 2007).