Justification of Red List Category
This species is listed as Critically Endangered because it is estimated to have an extremely small population that is inferred to be in decline owing to the interacting threats of human-driven habitat loss, habitat damage caused by cyclones, invasive species and non-target mortality caused by hunters. There is now an urgent need for intensive conservation actions to halt the species's perilous slide towards extinction.
The low number of recent records and lack of sightings by local people strongly suggest that the population is now extremely small, thus it is placed in the band 50-249 mature individuals, assumed to equate to c. 70-380 individuals in total.
No new data are available on population trends, but the species is suspected to have undergone a very rapid decline over the past three generations (estimated at 20 years), based on the low number of recent records, and owing to the partially synergistic effects of forest degradation by cyclones and invasive tree species, as well as accidental mortality from hunting and direct loss of habitat through agricultural expansion. Consensus among reliable hunters corroborate this downward trend (Serra et al. 2016, Serra et al. 2017). This assessment of the rate of decline may be conservative given the perceived change in abundance since the 1990s and the potential impacts of Severe Tropical Cyclone Evan in December 2012 (R. Stirnemann in litt. 2012). It is suspected that a very rapid decline will take place over the next three generations if conservation actions are not increased, owing to the expected impacts of intermittent powerful cyclones and the inherent consequences of the species's extremely small population size.
The species is endemic to Samoa, where it is known as Manumea and occurs on both main islands, Upolu and Savai'i. Its total population was estimated at 4,800-7,200 birds in the mid-1980s (Beichle 1987), but in the 1990s the population showed a drastic decline owing to the effects of cyclones such that in 2000 fewer than 2,500 mature individuals were believed to survive. In 1999 and 2000, surveys on Savai'i showed that it had become rare with pairs scattered in suitable habitat. An eleven-month survey in 2005-2006 reported the species from only ten locations, and the population was estimated to number only a few hundred, although the remote and largely intact uplands of Savai'i remained largely unsurveyed (Ministry of Natural Resources and the Environment, Samoa 2006). Small numbers were recorded in a few locations on Upolu in 2009. No definitive records were made during surveys in 2012 in upland Savai'i (Atherton and Jefferies 2012). Surveys for the species conducted by the Ministry of Natural Resources and the Environment on Savai'i in 2013 yielded one record at the edge of Tafua forest, of a juvenile bird that was photographed, this being the first confirmed sighting on Savai'i for almost a decade. An adult bird was also observed in the same area during the survey (Uili et al. 2013). Reliable local hunters reported a steady decline during the past decades, but that it still exists at Uafato and Malololelei forest areas on Upolu island, and Aopo and Taga forest areas on Savai'i island (Serra et al. 2016, Serra et al. 2017). Surveys in 2016 produced only one definite detection of the species in Uafato forest (M. O’Brien in prep. in: Serra et al. 2017).
It occurs in primary forest from sea-level to 1,600 m, also occurring in forest edge, along forest roads and sometimes visiting clearings where native trees remain (Beichle 1987, Blockstein 1987, U. Beichle in litt. 2000). It is specialised to feed on the seeds of Dysoxylum spp. (using its unusual bill to saw through the tough, fibrous pericarp), also feeding on other fleshy fruit (Beichle 1987). Juvenile bills are unsuitable to open seeds, and it is plausible that juveniles feed disproportionately on the ground until their bills develop (Collar 2015). Local hunters report that the species's preferred fruiting trees are the Ma’ali and Maota out of more than 20 species of fruiting trees used as food, and that the species sometimes feeds on the ground (Serra et al. 2016). Nests have been reported (none since 1987) 5-25 m up in dense foliage, often associated with banyan (Ficus sp.) trees and epiphytes (Beichle 1982, 1987, Collar 2015).
Hunting remains a significant threat, primarily when the commoner Pacific Imperial-Pigeon Ducula pacifica is the main target (Collar 2015, Serra et al. 2017, Stirnemann et al. 2018). Beichle (1982) estimated that approximately 400–500 individuals were being killed annually, and this rate might have continued through the 1980s (Beichle and Baumann 2003). Of 30 hunters interviewed in 2015-2016, 30% reported having accidentally killed at least one Tooth-billed pigeon, including two instances in 2016 (Stirnemann et al. 2018). Depredation by feral cats and rats (Black Rats Rattus rattus and Pacific Rats Rattus exulans) may now represent a serious threat, especially if their numbers increase following cyclones (Collar 2015, Serra et al. 2017). Anthropogenic habitat loss has been significant and continues, especially on Savai'i (Ministry of Natural Resources and Environment 2006). Natural habitat loss from cyclones, such as the 2012 Cyclone Evan, reduce forest quality including loss of fruiting trees, and encourage exotic plants that compromise the recovery of native trees (Ministry of Natural Resources and Environment 2006).
Research Actions Underway
An extensive survey using sound recorders was conducted across Samoa, alongside research into the species's habitat preferences and breeding biology (Darwin Initiative 2017a). Interviews have been carried out with hunters to assess the species's distribution and levels of hunting (Serra et al. 2017, Darwin Initiative 2017a, Stirnemann et al. 2018).
Carry out further research on tooth-billed pigeon distribution, populations, life-history, habitat requirements and threats, including a long-term monitoring program and a detailed study of rat and cat infestation levels and optimal control measures (Collar 2015). Research methods of identifying the species's call, and particularly how to distinguish it from the similar call of the much more common Pacific Imperial-pigeon Ducula pacifica, to enable effective population detection and monitoring (M. O'Brien pers. comm.). Test birds for avian diseases (K. Swinnerton in litt. 2016). Continue the use of sound recorders to identify key sites (K. Swinnerton in litt. 2016).
Conservation Actions Underway
It is fully protected by law and hunting is banned, although this has not been well enforced. It occurs in some proposed and a few existing protected areas, but these have suffered cyclone damage. To create offshore island refuges for several Samoan species threatened by invasive pests, in 2009 rat eradication from Nu’ulua was successful but failed on Nu’utele (Tye and Butler 2013). A species recovery plan for 2006-2016 was published by the Samoan Ministry of Natural Resources and Environment, and components of the plan have been implemented with the Samoan Conservation Society, a local NGO formed in 2015, although large portions of the plan were not implemented (Serra 2017). Communities were visited in 2013-2014 to survey for sightings by local people and raise awareness of the species, potentially reducing hunting and increasing the chances of future sightings (Ministry of Natural Resources and the Environment, Samoa in litt. 2013). Materials have been produced to increase awareness of the species and a consultation was held in December 2013 on Savai'i to promote the conservation of the species (Uili et al. 2013). Rat control has been initiated in the Malololelei forest area (Darwin Initiative 2017b), with limited success (M. O'Brien pers. comm. 2019). Further rat control is being carried out by Auckland Zoo (M. O'Brien pers. comm. 2019). Tree planting has been carried out in several locations on Upolu and Savai'i (Darwin Initiative 2017b).
Conservation Actions Proposed
Manage key forest areas where the species survives, in particular the targeted removal of rats and cats (Collar 2015). Enforce hunting bans in Community Conservation Areas (M. O'Brien pers. comm. 2019). Reforest degraded areas to increase resilience to cyclones and allow recolonisation by the species (Collar 2015). Establish reserves, including private/Samoan reserves promoted by government tourist authorities, and the cat-free island of Nuulua (Collar 2015). Upgrade the Malololelei forest reserve of 12 ha into a larger and functional protected area (Serra et al. 2017). Encourage crop plantations on previously used seaward land rather than expanding landward (Collar 2015). Carry out public education on the value of forests, produce a field guide to promote knowledge of birds and increase governmental encouragement of the study and conservation of wildlife by Samoans (Collar 2015). Reliable hunters should be supported by an awareness and capacity building programme in order to raise their status within their villages as the guardians of their ancestral forests and local champions of the species's conservation (Serra et al. 2017). Outlaw the cutting of Dysoxylum, control ammunition and regulate hunting of all pigeons to avoid mistaken killing (Collar 2015, Stirnemann et al. 2018). Establish new populations on rat-free islands and possibly on Tau in American Samoa, new mainland sites and in captivity (Collar 2015). Predation is the issue that most urgently requires attention; invasive predators should be eradicated from offshore islands to create refuges for the species, as well as from areas where the species survives (Collar 2015; K. Swinnerton in litt. 2016).
31 cm. Chunky, dark pigeon with heavy, hooked bill. Lower mandible has two "teeth" that overlap the upper. Mostly greenish-black with chestnut upperparts. Bill red at base, then yellow. Red eye-ring. Similar spp. Metallic Pigeon Columba vitiensis and Pacific Imperial-pigeon Ducula pacifica slimmer and longer tailed with no chestnut on upperparts. Voice Uniform territorial coo, higher-pitched in female. Irregular, frequent repetitions (Beichle 1991). Hints Prefers to stand on thick branches in dark interior of tree-crowns, often 15-20 m high, but up to 40 m (Beichle 1982).
Text account compilers
Dutson, G., Shutes, S., Stattersfield, A., Symes, A., Taylor, J., Wheatley, H., Ashpole, J, Mahood, S., Benstead, P., O'Brien, A.
Atherton, J., Beichle, U., Collar, N., Freifeld, H., Hobcroft, D., O'Brien, M., Scott, D. & Stirnemann, R.
BirdLife International (2020) Species factsheet: Didunculus strigirostris. Downloaded from http://www.birdlife.org on 08/08/2020. Recommended citation for factsheets for more than one species: BirdLife International (2020) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 08/08/2020.