Justification of Red List Category
This species is classified as Near Threatened because although it has a very restricted range and its population may be moderately small, the population is estimated to be stable despite the presence of several invasive species and there is not thought to be any plausible threat which is likely to cause rapid future declines.
The most recent population estimate was of 1,000-10,000 individuals (Beeton et al. 2010), from which c5,000 mature individuals was considered a reasonable estimate by Garnett et al. (2011). However, it has been described as widespread and considered very common on the island (the species was recorded in 98% of 123 sites surveyed by James and Retallick ), and the total population may exceed this estimate by at least an order of magnitude (D. James in litt. 2007).
Numbers have apparently fluctuated, and perhaps declined, between the time of the island's settlement in 1888 and the mid-1970s, but no hard data are available. Nonetheless, the species adapts well to secondary habitats and is thus now suspected to have a relatively stable population.
Ducula whartoni is endemic to Christmas Island (to Australia), in the Indian Ocean. Numbers have apparently fluctuated, and perhaps declined, between the time of the island's settlement in 1888 and the mid-1970s, but no good data are available. In 1975, the population was estimated at 10-100 breeding pairs. The breeding population was estimated at c.1,000 mature individuals in 2000, and at 5,000 mature individuals byt Garnett et al. (2011). However, it is widespread and considered very common on the island, and the total population probably exceeds the 2000 estimate by at least an order of magnitude (D. James in litt. 2007). This is supported by surveys in 2005 and 2006, in which the species was recorded during 92% of point counts and at 98% of surveyed sites (James and Retallick 2007). At the end of the 19th century, it was unsuccessfully introduced to Cocos-Keeling Islands (to Australia). It occupies a range of no more than 137 km2 (D. James in litt. 2007).
It is found in rainforest and, to some extent, in secondary regrowth dominated by the introduced Japanese cherry Muntingia calabura. Surveys in 2005 and 2006 found the species to be more common in evergreen forest than in either semi-deciduous forest or cleared areas (James and Retallick 2007). It nests in the top of rainforest trees and other dense vegetation, and feeds on native and exotic fruits, as well as buds and leaves.
About one third of the species's previously favoured plateau forest was cleared for phosphate mining before clearance ceased in 1987. This loss has been partly offset by the introduction of M. calabura, which flourishes on many former mine fields and other disturbed areas, and provides a rich food source for much of the year. Future habitat loss is possible through clearance for mining (S. Garnett in litt. 2003), and in 2007 significant patches of mature secondary forest were cleared for mining (D. James in litt. 2007). Also in 2007, a new application to mine a 250 ha area of rainforest (P. Green in litt. 2007) was turned down (J. Hennicke in litt. 2007). Illegal hunting continued after prohibition in 1977, but is now less prevalent. The failure of the introduction to Cocos-Keeling Islands has been attributed to hunting and/or lack of suitable food-trees. The introduced yellow crazy ant Anoplolepis gracilipes, which formed super-colonies during the 1990s and spread rapidly to cover about 25% of the island or about 3,400 ha, but was controlled over about 2,900 ha in September 2002, was thought to be a potential threat. Once controlled, however, the ants are re-establishing supercolonies (D. James in litt. 2007). In 2006, the ants were regarded as widespread and patchily common (T. Low in litt. 2006). If allowed to spread uncontrolled, ant super-colonies may prey directly on nestlings, although there is no evidence that this is a threat to the overall population, and alter island ecology by killing the dominant life-form, the red crab Gecaroidea natalis, which otherwise inhibits understorey plant growth and the spread of weeds by eating the seeds and seedlings of both native and invasive species (P. Green and D. O'Dowd in litt. 2003, S. Garnett in litt. 2003, D. James in litt. 2007). The ants also farm scale insects, causing canopy die-back, which in turn promotes weed growth and further alters forest structure (D. James in litt. 2007). The scale at which these processes occur is uncertain (D. James in litt. 2007), but fears that the pigeon would be affected by the ants appear to have been unfounded, perhaps due control of the ants and because there are fewer ants in the canopy where the pigeons feed (Garnett et al. 2011). Black rats Rattus rattus may reduce nesting success and feral cats Felis catus are present but neither are currently thought to be causing declines (Garnett et al. 2011).
Conservation Actions Underway
The Christmas Island National Park was established in 1980, and has since been extended to cover more than 60% of the island (D. James in litt. 2007). A control programme for A. gracilipes has been successfully initiated since 2000 and effectively eliminated the ant from 2,800 ha of forest (95% of its former extent) (P. Green and D. O'Dowd in litt. 2003). Monitoring of the problem continues and hand-baiting measures have been ongoing. Aerial baiting was carried out in September 2002 and proved to be successful against major colonies (Olsen 2005), with 98% of ants controlled (D. James in litt. 2007). However, the ants have since recovered (D. James in litt. 2007). The ants remained persistent in 2006, and perpetual baiting may be the only means of controlling them (T. Low in litt. 2006). The bait used so far is known to be toxic to invertebrates, including crabs, and although alternatives have been trialled, an effective replacement has not been found (D. James in litt. 2007). Control of the scale bugs that the ants tend for their sugar secretions has been suggested, in order to reduce this food supply (T. Low in litt. 2006, D. James in litt. 2007). Plans were being developed in 2000 for the establishment of a captive breeding population on mainland Australia (Garnett & Crowley 2000) and control of the ants may have allowed the species's population to stabilise (S. Garnett in litt. 2005).
39 cm. Unmistakable, large, dark grey pigeon. Sexes similar. Purplish, greenish or reddish-brown iridescent sheen on upperparts and breast, depending on viewing angle. Yellow or orange iris. Red-brown undertail-coverts. Juvenile more dull, more brown on underside, paler on lores, chin and forehead, with brown iris. Legs and feet purplish-red in adults and grey-brown in juveniles. Black bill. Voice Purring coo, rising then falling in pitch, of about one second duration. Also deep booming whoo. Hints Confiding when feeding.
Text account compilers
Benstead, P., Garnett, S., McClellan, R., Pilgrim, J., Symes, A. & Taylor, J.
Blyth, J., Garnett, S., Green, P., Hennicke, J., James, D., Low, T. & O'Dowd, D.
BirdLife International (2019) Species factsheet: Ducula whartoni. Downloaded from http://www.birdlife.org on 18/10/2019. Recommended citation for factsheets for more than one species: BirdLife International (2019) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 18/10/2019.