Justification of Red List Category
Although intensive conservation action has now reversed declines and has resulted in a population increase, the population size remains very small, and only small numbers of adults breed. The species's range is extremely small, within which habitat quality is thought to be declining as a result of the invasive Little Fire Ant, and the species's entire range could quickly be affected by a threat such as predation by Black Rats, should rat control cease. For these reasons, the species is listed as Critically Endangered.
In 2014, there were an estimated 43 territorial adults (20 in Maruapo, 13 in Papehue and 10 in Tiapa), plus at least ten more non-territorial birds (Blanvillain et al. 2015), fledging 12 young (LPO 2014). In 2015, 53 adults (but only 13 breeding pairs) fledged 14 young (Blanvillain et al. 2018). The population size has continued to rise, and in 2018 the population size was estimated at 79 adults (Blanvillain 2019, Blanvillain et al. 2020), followed by an estimated 91 adults in 2019 (C. Blanvillain in litt. 2020). The species has a low reproductive rate; a study found that only 56% of pairs attempt to lay an egg in any year. (Blanvillain et al. 2018). Between 2015 and 2019, the number of breeding individuals was 26, 28, 28, 34 and 48 (C. Blanvillain in litt. 2020). Therefore, the number of mature individuals is placed in the band 25-100 mature individuals.
The distances between the range valleys are small, and an individual transferred from Maruapo to Tiapa in 2009 was found to have moved back to its original Maruapo territory in 2012 (Blanvillain et al. 2013). However, differences have been observed between the calls of individuals in each valley, and ringed individuals have not been confirmed to move between the valleys spontaneously (C. Blanvillain in litt. 2020). The species is therefore thought to have 1-3 subpopulations.
It was apparently rare throughout the 20th century and, during the period 1986-1991, was noted in only four valleys (several pairs at each locality) out of 39 visited (Monnet et al. 1993). In 1998, 25 birds were located in four valleys containing isolated populations of 5–7 birds each (Blanvillain et al. 2003). Following the initiation of a recovery programme in 1998, the population size began to increase slowly. Twenty-eight birds were recorded in 2000 (Blanvillain et al. 2003). In 2001, a further population of 33 birds was discovered upstream beyond several waterfalls, in one of the species's range valleys (Blanvillain et al. 2018). The total population in 2006 was estimated at 40-45 individuals (P. Raust in litt. 2005, Gouni et al. 2007). In the mid-2000s, the population size declined slightly, and by 2009, the population size was estimated at 33 individuals (Ghestemme 2009). From 2009 onwards, the population size has continued to increase. There were 36 known mature birds in 2010 (Blanvillain et al. 2013), 40 individuals (including seven fledglings) in 2011 (Ghestemme et al. 2011) and 43-44 individuals in 2012 (Blanvillain 2012). In 2014, there were an estimated 43 territorial adults (20 in Maruapo, 13 in Papehue and 10 in Tiapa), plus at least ten more non-territorial birds (Blanvillain et al. 2015). In 2015, 53 adults (but only 13 breeding pairs) fledged 14 young (Blanvillain et al. 2018). The rate of population increase has accelerated, and in 2018 the population size was estimated at 79 adults (Blanvillain 2019, Blanvillain et al. 2020), followed by an estimated 91 adults in 2019 (C. Blanvillain in litt. 2020).
The rate of population increase has been estimated at 1% per year from 1998-2012, 11% per year from 2012 onwards, and 17% per year from 2016 (SOP-Manu 2018, Blanvillain et al. 2020).
Pomarea nigra is endemic to Tahiti in the Society Islands, French Polynesia. It was previously common throughout the island, but was apparently rare throughout the 20th century and, during the period 1986-1991, was noted in only four valleys out of 39 visited (Monnet et al. 1993). By 1998, the species had disappeared from two of the valleys where it had been recorded during 1986-91, but it was found in two additional valleys, and was then known to occur in Papehua, Orofero, Tiapa and Maruapo valleys, in the Paea and Punaauia districts in the south-west of the mainland (Gaze and Blanvillain 1998, Blanvillain et al. 2003). In 2001, a further population was discovered upstream in the Maruapo valley, beyond several waterfalls (Blanvillain et al. 2018). By 2010, the population in the Orofero valley had disappeared (Blanvillain et al. 2013, 2018). In 2015, the total area used as territories by the species was estimated at 0.5 km2 (Blanvillain et al. 2018).
It is only found in dense forest between 80 m and 400 m in altitude at the bottom of valleys, close to permanent or temporary streams (Thibault et al. 1999, Portier 2010). Nowadays, its forest habitat is dominated by the introduced African Tulip tree Spathodae campanulata (Blanvillain et al. 2013), and the tree "mara" Neonauclea forsteri is the last native common feature of the valleys where it survives (Thibault et al. 1999). It is highly territorial, foraging both in the canopy and the undergrowth for insects (Pratt et al. 1987). Its nest is constructed in a fork of mara or Tulip Tree (Blanvillain et al. 2013). It lays eggs year-round, with more laying occurring between August and January, peaking in November (Blanvillain et al. 2018). It has a low reproductive productivity (Blanvillain et al. 2018).
Predation by the Black Rat (Rattus rattus) is thought to have been the main cause of the monarch's historical decline, and still poses a significant threat (Thibault et al. 2002). The monarch population in the upper Maruapo valley declined from 33 to 12 individuals over 2002-2009 in the absence of rat control (Blanvillain et al. 2013, 2020). Rat control measures have been effective in substantially reducing the level of predation by rats (Blanvillain et al. 2013).
The introduced Red-vented Bulbul (Pycnonotus cafer) and Common Myna (Acridotheres tristis) have also been identified as serious ongoing threats (Thibault et al. 2002, Blanvillain et al. 2003, Ghestemme 2011). Colonisation by Red-ventel Bulbul was implicated in a recent decline of the monarch population (Thibault et al., 1999, 2002). A three-year study showed that significantly more Red-vented Bulbul and Common Myna were present in territories that experienced nest failure or early fledgling death (Blanvillain et al. 2003). Common Myna has been implicated in the predation of eggs and chicks while in the nest and probably following fledging, and may cause injuries to adults (Blanvillain et al. 2003, C. Blanvillain in litt. 2018). Red-vented Bulbul has been implicated in reducing chick survival by disrupting the reproduction of paired Monarchs and competing for food and territories (Thibault et al. 2002, Blanvillain et al. 2003, Ghestemme 2011). Following extensive control measures, in 2018 it was noted that the Common Myna had disappeared from the range of the monarch, and the Red-vented Bulbul had become less common for the third consecutive year (SOP-Manu 2018). Control of invasive birds was shown to have resulted in increased reproductive success for the monarch (Blanvillain et al. 2020).
A more recent threat is the spread of Little Fire Ants (Wasmannia auropunctata) (M. O'Brien and C. Blanvillain in litt. 2015). This invasive species is able to advance 50 to 100 metres per year in colonies covering tens of hectares of land (Blanvillain 2019). It predates bird nests and competes with native species for food, and Tahiti monarchs have been observed to have deserted several territories following their invasion by Little Fire Ants (Bousseyroux et al. 2019). From 2012, three supercolonies eventually covering a total of 70 ha became established at the entrances of the valleys where the monarch is found (SOP-Manu 2018). Following control measures, the area of the colonies was reduced by 85% by the end of 2018 (SOP-Manu 2018, Blanvillain 2019, Blanvillain et al. 2020), but treatments have ceased and the Little Fire Ant has once again continued to expand its range (C. Blanvillain in litt. 2020).
Other potential predators include cats and Swamp Harrier (Circus approximans) (Ghestemme 2009). Tahiti Kingfisher (Todiramphus veneratus) is an aggressive native competitor which has been observed driving several monarchs off their territory (Ghestemme 2009) and preventing colonisation of suitable sites by young monarchs (Ghestemme 2011).
Invasive plants are a further likely threat, as forest on Tahiti is now largely composed of introduced invasive species such as Miconia calvescens and the African tulip tree (Spathodea campanulata), which have substantially modified the forest structure and ecology (Blanvillain et al. 2013). As these plants invade forests, they reduce food availability for the monarch and facilitate the penetration of the forest by introduced birds and the Tahiti Kingfisher (Ghestemme 2009). Goat grazing has also led to habitat degradation and monarch population declines in the Maruapo valley (Ghestemme 2009, Blanvillain et al. 2013).