The Micronesian Imperial-pigeon (Ducula oceanica) is a forest species occurring in the Micronesian islands of Palau, Federated States of Micronesia, the Marshall Islands, Nauru (Buden 2008, Stirnemann 2013) and Kiribati (M. O’Brien in litt. 2017, eBird 2020), including many offshore islands. In Palau, it occurs across much of the island group, including across Babeldaob and in the Rock Islands. In the Federated States of Micronesia, it occurs on Pohnpei, Chuuk, Ant Atoll (Engbring et al. 1990, Buden 1996a, eBird 2020) and Kosrae, as well as Yap, where it is considered to have been introduced (Engbring et al. 1990). Small populations have also been reported in recent decades on Pakin and Sapwuahfik atolls in Pohnpei State, and Satawan atoll in Chuuk State (Buden 1996a, b, 1999, 2006). In the Marshall Islands, it is known to occur on Majuro (eBird 2020, G. Wiles in litt. 2020), and has been reported to occur on Lae, Lib and Namdrik (National Biodiversity Team 2000 in Stennemann 2006). In Kiribati, there are recent records on Tarawa (M. O’Brien in litt. 2017, eBird 2020). The species previously occurred on Kapingamarangi Atoll in Federated States of Micronesia (Buden 1998), Kuria and Aranuka in Kiribati (VanderWerf 2007), and across the Marshall Islands from Wotje to Jaluit and Mili (Vander Velde 2001, Baptista et al. 2020), but is now probably extinct from much of its former range.
Hunting has long been the main threat to this species on nearly all of the islands in its range, with some subpopulations being overhunted (and extirpated from several atolls) by the early to mid-1900s (Baker 1951, Engbring et al. 1990, Buden 2006, Ketebengang and Gupta 2011, Hayes et al. 2016). Past and ongoing forest clearance for agriculture and development is another concern, particularly on Pohnpei and Chuuk (Buden 2000, Oleiro and Kesler 2015). As a result of these threats, a significant population decline has been recorded in Palau since 1991 (Engbring 1992, VanderWerf 2007, Olsen and Eberdong 2011). Similarly, surveys on Pohnpei between 1983 and 1994 indicated a decline (Engbring et al. 1990, Buden 2000), although the population there has since increased (Oleiro 2014). Declines have been noted on Kosrae and on Ant Atoll (Engbring et al. 1990, Hayes et al. 2016).
Survey results have not always been comparable and the overall population change over the last three generations is uncertain, so the species is currently listed as Near Threatened. However, reanalysis of data in the light of recent data indicates that the species’s Red List Category may warrant revision. Our current information on the species’s conservation status will now be compared to all Red List Criteria:
Criterion A – In Palau, a National Bird Survey was carried out in 1991, following the US Fish and Wildlife Service’s VCP (Variable Circular Plot) method and using DISTANCE analysis. This produced a population estimate of 13,718 individuals (Engbring 1992). Surveys were repeated in 2005 (VanderWerf 2007), but the resulting data were not analysed in the same way as those from the 1991 survey, and so are not comparable. However, relative abundance (birds/station) was compared between the two surveys, showing that abundance decreased by c.40% between 1991 and 2005 (VanderWerf 2005). From 2005-2010, the National Bird Monitoring programme recorded a marked decrease in the number of monitoring stations at which individuals were observed (Olsen and Eberdong 2011), from 20 to 13 stations (Olsen and Eberdong 2011), suggesting a possible rapid decline in the population and/or a possible restriction in range (Ketebengang and Gupta 2011). In 2014, the population on Palau was estimated to be 3,000 individuals (Beouch 2016 in NEPC 2017). In 2019, the trend was unclear; experts considered that the population was still declining (Eberdong and Kitalong per NEPC 2019), but data from eBird suggested a possible recovery from 2015 (NEPC 2019). Eight-minute count data from the 20 monitoring stations showed a fall in the number of birds per station from 3.65 in 2014 to 1.88 in 2016, followed by a rise to 2.80 in 2017 (NEPC 2019). Based on the population estimate from 2014 and the range of available data for 2014-2018, the population size was estimated to be between and 2,143 and 3,265 individuals in 2016, with 2,143 individuals representing the best estimate. Based on the above information and extrapolating to 2020 with an assumed exponential decline, the population on Palau is estimated to have undergone a reduction of 75-86% over the past three generations (18 years; see Bird et al. 2020)* to 2020, with a best estimate of 86%.
In Pohnpei, the population size was estimated at 822 individuals in 1983 (Engbring et al. 1990). The survey took place during a severe drought, which may have enhanced detectability of the species (Buden 2000). Repeat surveys took place in 1994 and a decline in relative abundance was detected, estimated at approximately 73% since 1983 (Buden 2000). If this decline is applied to the 1983 population estimate, then the estimated population on Pohnpei in 1994 is c. 222 individuals. Surveys in 2012 resulted in a population estimated of 5,025 individuals based on habitat models linked to density functions (Oleiro and Kesler 2015). Oleiro (2014) re-analysed detection rates from 1994 and reported an increase of 220% from 1994 to 2012. However, the report by Oleiro (2014) also presents relative detection rates by elevation band for each survey (1984, 1994 and 2012). If the relative detection rates are applied to the reported 2012 population densities and scaled by the relative areas of the island that fall within each elevation band, the population in 1994 can be calculated as 2,746 individuals. This suggests that the population size increased by 83% from 1994 to 2012. Based on the rate of increase from 1994-2012 reported by Oleiro (2014) and assuming exponential change, the population on Pohnpei can be estimated to have increased by 226% over the past three generations to 2020. Conversely, based on the rate of change since 1994 derived from Oleiro’s (2014) population densities and relative detection rates, here considered the best estimate, the population is estimated to have increased by 85% over the past three generations.
On Kosrae, Engbring et al. (1990) estimated a population of 7,474 individuals in 1983. Hayes et al. (2016) reported hearing only a few individuals during several long hikes in the mountains of the island in 2013 and 2014. The population is therefore suspected to have declined, but less severely than the decline observed on Pohnpei. In the last three generations to 2020, the population on Kosrae is suspected to have declined by 15-50%, with a best estimate of 30%.
On Chuuk, a survey in 1983 estimated 51 individuals (Engbring et al. 1990), but this may have been an underestimate (O’Brien in litt. 2017). This population is not thought to have increased greatly since 1983 (G. Wiles in litt. 2020). Over the past three generation to 2020, the population on Chuuk is suspected to have declined by 0-50%, with a best estimate of no change.
A small population of c.25-30 individuals has been reported on Ant Atoll (Buden 1996a). Engbring et al. (1990) reported a density of 27 individuals per km2 on Kikalap Aru in 1983. Surveys in 1994-1995 reported a density of 14.9 individuals per km2 (Buden 1996a). Extrapolating this rate of change with assumed exponential decline produces an estimated reduction of 63% over the past three generations. The reduction over the past three generations to 2020 is suspected to be between 25% and 63%, with a best estimate of 25%.
There were thought to be c. 50 individuals on Sapwuahfik Atoll (Buden 1999) and small numbers have been reported on Pakin Atoll (70 individuals/km2 in 1994; Buden 1996b) and on Satawan Atoll (Buden 2006). It is not known whether the populations on these atolls are still extant, and the changes over the past three generations to 2020 are not known, but a best estimate may be a 50% decline in each.
The population on Nauru was roughly estimated at 75-100 individuals in 2006-2007 (Buden 2008). Micronesian Imperial-pigeon is thought to have declined on the island, and local people reported that they did not see it as often as they did previously (Buden 2008). The species is now thought to be restricted to the Pinnacles area in the uplands, with more recent surveys suggesting a population of 50-150 individuals (Stirnemann 2015). Based on the figures from 2006-2007 and 2015 and extrapolating to 2020, the population change on Nauru over the last three generations could range from an 80% reduction to a 388% increase, with a best estimate placed at a 33% increase.
On Majuro Atoll in the Marshall Islands, a recovery programme has led to an increase in numbers to c. 80 individuals across 27 islets with a combined area of 5.2 km2 (M. O’Brien in litt. 2011). The rate of population increase is not known however. Over the past three generations to 2020, the population is suspected to have increased by between 0 and 242% (based on an increase of 50-80 from 2004 to 2011), with a best estimate of a 51% increase.
The population on Tarawa is assumed to have undergone a population change ranging from a 50% reduction to a 50% increase over the past three generations, with a best estimate of no change. The populations on Lae, Lib and Namdrik are assumed to have undergone a reduction of 0-100% over the past three generations.
The population on Yap is not considered here because it is believed to have been introduced (Engbring et al. 1990).
Combining the pessimistic estimates (above) of the population change over the past three generations (18 years) to 2020, and weighting by population size, gives a suspected reduction of 74%. Combining the optimistic estimates of the population change over the past three generations, and weighting by population size, above gives a suspected reduction of 10%. Combining the best estimates of the population change over the past three generations, and weighting by population size, gives a best estimate of a suspected 37% reduction. The species therefore could qualify for Least Concern, Vulnerable or Endangered under Criterion A2, with the reduction estimate considered to be most realistic qualifying the species as Vulnerable under Criterion A2cd.
The population is suspected to continue to decline for at least several years in the future, so the species would likely qualify for Vulnerable under Criterion A4cd as well. However, trends over the next three generations may differ due to the population increase seen on Pohnpei, and are highly uncertain, so the species may qualify as Least Concern or Near Threatened under Criterion A3cd.
Criterion B – Based on the area of a minimum convex polygon around the species’s mapped extant native range, the Extent of Occurrence (EOO) is estimated at 2,240,000 km2. This does not approach the threshold for listing the species as threatened under Criterion B1, and Micronesian Imperial-pigeon is assessed as Least Concern under this criterion.
The Area of Occupancy (AOO) has not been quantified, but based on a 4km2 grid placed over the area of mapped range, must be smaller than 2,732 km2. The true AOO is expected to be smaller than this, but given that the species tends to be distributed widely within its range islands, it is considered likely to be greater than 2,000 km2. This approaches the threshold for Vulnerable under Criterion B2, so the species may qualify as Near Threatened, depending on whether one or two further conditions are met or nearly met.
The species is not severely fragmented sensu IUCN, since it does not meet the condition that ‘most (>50%) of its total area of occupancy is in habitat patches that are (1) smaller than would be required to support a viable population’ (IUCN 2019). The species occurs on a large number of islands and the number of locations, based on the threat of hunting, is likely to be considerably greater than 10. Condition a is not met. Based on the evidence presented under Criterion A, the species is inferred to have a continuing decline in the number of mature individuals. Declines in the AOO and area of habitat are also estimated or inferred from quantitative data on habitat trends in Pohnpei (Trustrum 1996, Merlin and Raynor 2005, Department of Resources & Development, FSM 2019) and from observations on other range islands (VanderWerf 2007, Department of Resources & Development, FSM 2019). Condition b(ii,iii,v) is met. There is no evidence that the species’s population or range size are undergoing extreme fluctuations. Condition c is not met.
In sum, the AOO approaches the threshold for listing as Vulnerable and condition b is met. Micronesian Imperial-pigeon therefore qualifies as Near Threatened, approaching the threshold for listing as threatened under Criterion B2b(ii,iii,v).
Criterion C – The population size in Palau in 2020 is placed in the band 1,300-3,300 individuals.The population size on Pohnpei in 2020 is highly uncertain, but is here placed in the band 500 – 8,500 individuals.The population on Kosrae is thought to number 1,700-5,400 individuals.Assuming the population on Chuuk is stable, the population in 2020 is placed in the band 50-300 individuals.The population on Ant Atoll is here placed in the band 6-20 individuals.The population estimate for 2020 for Sapwuahfik Atoll, Pakin Atoll and Satawan Atoll is placed in the band 0-300 individuals. The population on Nauru in 2020 is placed in the band 30-240 individuals. On Majuro Atoll in the Marshall Islands, the 2020 population is placed in the band 80-150 individuals. The populations on Lae, Lib and Namdrik, if still extant, must be small, as must the population on Tarawa; each is assumed to have a populations of no more than 50 individuals.Since the population on Yap is considered to be introduced (Engbring et al. 1990), the population is not included in this assessment.
Overall, the population size is suspected to fall in the band 3,800-18,300 individuals. This roughly equates to 2,500-12,200 mature individuals, with the best estimate placed at 7,000 mature individuals. This range of estimates could meet or approach the threshold for listing the species as Vulnerable under Criterion C, with the best estimate falling beneath the threshold for Vulnerable. To list the species as threatened under Criterion C, further conditions must also be met.
Based on the information presented under Criterion A, the population size is inferred to be undergoing a continuing decline. Although data suggest that the population size may start to increase over the next decades if current trends continue, this is highly uncertain, and a decline is projected to continue for at least several more years. Although there is data from which to suspect a rate of decline, there is a lot of uncertainty around trends and the trends in some supopulations are not known, so a rate of decline cannot be directly estimated. The species cannot be assessed under subcriterion C1.
Although the species is known to fly among islands within island groups (Baptista et al. 1997), there are likely to be at least seven subpopulations. Based on the population estimates above, the largest possible subpopulation has between 1,700 – 8,600 individuals (assuming Pohnpei, Ant Atoll and Pakin Atoll may represent one subpopulation), roughly equating to 1,100-5,800 mature individuals. This does not meet subcriterion 2a(i), although it may approach the threshold. The maximum percentage of the total population that is found in a single subpopulation is around 72%. The species does not meet subcriterion 2a(ii). There is no evidence that the population size is undergoing extreme fluctuations, and so the species doesn’t meet subcriterion 2b.
Although the population size falls below the threshold for listing the species as Vulnerable under Criterion C and the population is declining, none of subcriteria 2a(i), 2a(ii) or 2b are met, although subcriterion 2a(i) may nearly be met. Depending on the most likely size of the largest subpopulation, the species may therefore qualify as Least Concern or as Near Threatened, approaching the threshold for listing as threatened under Criterion C2a(i).
Criterion D – Based on the estimate described above, the population size does not meet or approach the threshold for Vulnerable under Criterion D1. Furthermore, the species does not have a restricted AOO or number of locations, such that deforestation could drive the species to Critically Endangered or Extinct within one generation. The species does not qualify for listing as Vulnerable under Criterion D2. Micronesian Imperial-pigeon is therefore assessed as Least Concern under Criterion D.
Criterion E – To the best of our knowledge no quantitative assessment of the probability of extinction has been conducted for this species, and so it cannot be assessed against this criterion.
Based on the above assessment, it is proposed to list the Micronesian Imperial-pigeon (Ducula oceanica) as Vulnerable under Criterion A2cd+4cd. However, should further evidence indicate that the population is unlikely to have undergone a reduction of at least 30% over the past 18 years, the species may be retained as Near Threatened.We welcome any comments to the proposed listing. Information is particularly requested on the population size, trends and current occurrence within the range.
Please note that this topic is not designed to be a general discussion about the ecology of the species, rather a discussion of the species’s Red List status. Therefore, please make sure your comments are about the proposed listing. By submitting a comment, you confirm that you agree to the Comment Policy.
*Bird generation lengths are estimated using the methodology of Bird et al. (2020), as applied to parameter values updated for use in each IUCN Red List for birds reassessment cycle. Values used for the current assessment are available on request. We encourage people to contact us with additional or improved values for the following parameters; adult survival (true survival accounting for dispersal derived from an apparently stable population); mean age at first breeding; and maximum longevity (i.e. the biological maximum, hence values from captive individuals are acceptable).
An information booklet on the Red List Categories and Criteria can be downloaded here and the Red List Criteria Summary Sheet can be downloaded here. Detailed guidance on IUCN Red List terms and definitions and the application of the Red List Categories and Criteria can be downloaded here.
Baker, R. H. 1951. The avifauna of Micronesia: its origin, evolution and distribution. University of Kansas, Lawrence.
Baptista, L. F., Trail, P. W., Horblit, H. M., Boesman, P. & Garcia, E. F. J. 2020. Micronesian Imperial-pigeon (Ducula oceanica). Barcelona Available at: https://www.hbw.com/species/micronesian-imperial-pigeon-ducula-oceanica. (Accessed: 30 April 2020).
Baptista, L.F., Trail, P.W. and Horblit, H.M. 1997. Micronesian Imperial-pigeon (Ducula oceanica). In: del Hoyo, J., Elliott, A., Sargatal, J., Christie, D.A. and de Juana, E. (eds), Handbook of the Birds of the World Alive, Lynx Edicions, Barcelona.
Beouch, J. 2016. Belau Watershed Alliance (BWA) Leadership Meeting Report: Renewing MCS Commitment after 10 Years of Partnership. BWA, Ngarchelong.
Bird, J. P.; Martin, R.; Akçakaya, H. R.; Gilroy, J.; Burfield, I. J.; Garnett, S.; Symes, A.; Taylor, J.; Šekercioğlu, Ç.; Butchart, S. H. M. 2020. Generation lengths of the world’s birds and their implications for extinction risk. Conservation Biology online first view.
Buden, D. W. 1996a. Reptiles, birds, and mammals of Ant Atoll, Eastern Caroline Islands. Micronesica 29: 21-36.
Buden, D. W. 1996b. Reptiles, birds, and mammals of Pakin Atoll, eastern Caroline Islands. Micronesica 29: 37-48.
Buden, D. W. 1998. The birds of Kapingamarangi Atoll, including first record of the Shining Cuckoo (Chysococcyx lucidus) from Micronesia. Notornis 45: 141-153.
Buden, D. W. 2006. The birds of Satawan Atoll and the Mortlock Islands, Chuuk, including the first record of Tree Martin Hirundo nigricans in Micronesia. Bulletin of the British Ornithologists’ Club 126(2): 137.
Buden, D.W. 1999. The birds of Sapwuahfik Atoll, with first record of the Grey Wagtail Motacila cinerea from the Federated States of Micronesis. Bulletin of the British Ornithologists’ Club 119: 261–270.
Buden, D.W. 2000. A comparison of 1983 and 1994 bird surveys of Pohnpei, Federated States of Micronesia. Wilson Bulletin 112: 403-410.
Buden, D.W. 2008. The birds of Nauru. Notornis 55(1): 8-19.
Department of Resources & Development, FSM. 2019. FSM State-Wide Assessment and Resource Strategy (SWARS) 2010 – 2015+. Federated States of Micronesia.
eBird. 2020. eBird: An online database of bird distribution and abundance [web application]. Ithaca, NY, USA Available at: http://www.ebird.org.
Engbring, J. 1992. A 1991 survey of the forest birds of the Republic of Palau. U.S. Fish and Wildlife Service, Honolulu.
Engbring, J.; Ramsey, F. L.; Wildman, V. J. 1990. Micronesian forest bird surveys, the Federated States: Pohnpei, Kosrae, Chuuk, and Yap. U.S. Fish and Wildlife Service, Honolulu.
Hayes, F. E., Pratt, H. D. and Cianchini, C. J. 2016. The Avifauna of Kosrae, Micronesia: History, Status, and Taxonomy. Pacific Science 70(1): 91-127.
IUCN Standards and Petitions Committee. 2019. Guidelines for Using the IUCN Red List Categories and Criteria. Version 14. Prepared by the Standards and Petitions Committee. Downloadable from http://www.iucnredlist.org/documents/RedListGuidelines.pdf.
Ketebengang, H. and Gupta, A. 2011. State of Palau’s Birds 2010: A conservation guide for communities and policymakers. Palau Conservation Society.
Merlin, M. and Raynor, W. 2005. Kava cultivation, native species conservation, and integrated watershed resource management on Pohnpei Island. Pacific Science 59: 241-260.
National Biodiversity Team. 2000. The Marshall Islands, Living Atolls amidst the Living Sea; the National Biodiversity Report of the Republic of the Marshall Islands. National Biodiversity Team, Majuro, Republic of the Marshall Islands.
National Environmental Protection Council (NEPC). 2017. 2017 State of the Environment Report, Republic of Palau.
National Environmental Protection Council (NEPC). 2019. 2019 State of the Environment Report, Republic of Palau.
Oleiro, P. C. 2014. Avian population responses to anthropogenic landscape changes in Pohnpei, Federated States of Micronesia. Unpublished Masters Thesis. University of Missouri.
Oleiro, P. C. and Kesler, D. C. 2015. Effects of landuse change and factors likely to influence future avifauna population change on Pohnpei Island. Pacific Islands Climate Change Cooperative, Honolulu.
Olsen A. R. and Eberdong M. 2011. State of Palau’s birds, 2010. Belau National Museum, Koror, Palau.
Stirnemann, R. 2015. Bird Fauna of Nauru. In: McKenna, S. A., Butler, D. J. and Wheatley, A. (eds), Rapid biodiversity assessment of Republic of Nauru, pp. 75-83. SPREP, Apia, Samoa.
Trustrum, N. A. 1996. Pohnpei’s watershed spatial plan and management guidelines.
Vander Velde, N. 2001. Birds and their threats in the Marshall Islands.
VanderWerf, E. A. 2007. 2005 bird surveys in the Republic of Palau: final report. U.S. Fish and Wildlife Service and Palau Conservation Society, Honolulu, Hawaii.