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This species qualifies as Critically Endangered because surveys have shown that it is declining extremely rapidly. It now has an extremely small population and is effectively confined to just one island where multiple factors are driving the decline. Urgent conservation measures are required to ensure that this species does not become extinct.
Sibley, C. G.; Monroe, B. L. 1990. Distribution and taxonomy of birds of the world. Yale University Press, New Haven, USA.
38 cm. Medium-sized crow. All black, with slight gloss. Similar spp. Black Drongo Dicrurus macrocercus much smaller and slimmer with red eyes and forked tail. Micronesian Starling Aplonis opaca similarly shaped, but much smaller with golden-yellow eyes. Voice Loud scream kaaa-ah. Hints Shyer than most crows, often hiding in the forest.
Distribution and population
Corvus kubaryi inhabits Guam (to U.S.A.) and Rota in the Northern Mariana Islands (to U.S.A.). On Guam, it was formerly common but, since the 1960s, declined in numbers and area inhabited, with an estimated 350 birds in 1981 (Engbring and Pratt 1985), fewer than 40 in 1995 (Fancy et al. 1999), and 7 in 1999. Following introduction of birds from Rota the population rose to 16 in 2001 (G. Wiles in litt. 1999), but had declined again to two (both male) by 2008 (R. Berry in litt. 2008). The most recent sighting on the island was in 2011 and the species may now be extinct on Guam (F. A. Amidon in litt. 2012). On Rota, the population was thought to be stable at 1,318 birds in 1982 (Engbring et al. 1982), but has since declined to 592 in 1995 (Fancy et al. 1999), and then to 234 in 1998 (Morton et al. 1999, Plentovich et al 2005); in 2007 there were c.50 confirmed pairs and a few more suspected (G. Rodda in litt. 2007), and the population in 2008 stood at around 85 pairs (Amar et al. 2008). One bird remains in captivity (J. Morton in litt. 2000, F. A. Amidon in litt. 2012). Surveys on Rota between 1982 and 2004 indicated a decline of 93% (Amar et al. 2008). Apparent survival analysis of birds ringed between 1990 and 2010 revealed that the rate of first-year survival fell from 70% to 40% over that period, roughly equivalent to a doubling in the rate of mortality; this was accompanied by a slight decrease in adult survival over the same period. Population modelling using the most recent estimate for apparent survival in first-year birds predicts extinction of the species in 75 years, with models that incorporate the removal of birds for captive breeding and the impact of catastrophic events projecting more rapid declines (Ha et al. 2010).
In 2008 only two males remained on Guam (Berry et al. in litt. 2008). On Rota, the population was estimated at 234 mature indiviuals in 1998 by Plentovich (2005) and there were only 60 confirmed pairs recorded in 2008 (Berry et al. in litt. 2008), hence the band 50-249 mature individuals seems appropriate. This equates to 75-374 individuals in total, rounded here to 70-400 individuals.
Surveys between 1982 and 2004 have revealed an extremely rapid population decline. The records of only 60 pairs in 2009 indicates that this decline has continued (Berry et al. in litt. 2008), though potentially at a slower rate. It is estimated to have declined by over 80% over the past three generations (22 years) owing to habitat loss, persecution and predation by invasive species (particularly brown tree snake on Guam).
It inhabits mature and second-growth forest and coastal strand vegetation (Michael 1987). Little is known about preferred nest sites but the species appears to favour primary and secondary limestone forest (Plentovich et al. 2005). It forages in the forest canopy and understorey, and occasionally on the forest floor, for seeds, fruit, arthropods and lizards (Michael 1987).
On Guam, its decline was due to predation by the introduced brown tree snake Boiga irregularis and, despite protection of nest-sites by electrical tree barriers, the population is now down to only two individuals (R. Berry in litt. 2008). On Rota, typhoons have devastated forest habitat and forest has been cleared for homestead development, resort and golf-course construction and agricultural settlement; actions which are often accompanied by direct persecution (National Research Council 1997, Plentovich et al. 2005, J. Lepson in litt. 1999, G. Wiles in litt. 1999). Nests in native forest were found to have higher reproductive success than those in more disturbed areas, suggestng that damage to habitat may be limiting nesting success (Ha et al. 2011). Additional threats include nest-predation/disturbance by introduced rats Rattus spp. and monitor lizard Varanus indicus, competition with introduced Black Drongo Dicrurus macrocercus, and disease (Morton et al. 1999, Amar et al. 2008). Feral cats, amongst other predators, have been implicated in the falling apparent survival rate of first-year birds (Ha et al. 2010). The brown tree snake is not yet established on Rota, but if a snake population does invade then an even more serious decline is likely. Having a distribution on relatively low-lying islands, this species is potentially susceptible to climate change through sea-level rise and shifts in suitable climatic conditions (BirdLife International unpubl. data).
Conservation Actions Underway
On Guam, a national wildlife refuge was established in 1993 to preserve remaining forest (Wiles et al. 1995), and birds are being translocated from Rota in an effort to maintain the wild population on the island. A 40 ha snake-free area created on North-west Field is available for the introduction of crows, and an adjacent larger area is being readied for translocation (Beck and Savidge 2000). A predation control experiment will soon be starting on Rota, which may include demographic work in the future. A grant has been obtained for a habitat conservation plan for Rota's agricultural homesteads, which will increase the amount of land in protected areas (T. de Cruz in litt. 2003). Biological control for the brown tree snake is also being investigated (Beck and Savidge 2000). On Rota, life history studies are currently being conducted and there are proposals to protect vital tracts of forest under a habitat conservation plan (National Research Council 1997, G. Wiles in litt. 1999). Research is being conducted into the fate of nests through the use of remote cameras and the fate of juveniles by using radio-tags (Ha et al. 2010). Conservation Actions Proposed
Implement stringent measures to prevent the spread of B. irregularis from Guam to Rota (National Research Council 1997, G. Wiles in litt. 1999). Continue research including study of population biology and health of marked birds and conduct annual censuses using standardised methodology (National Research Council 1997, Morton et al. 1999, G. Wiles in litt. 1999). Continue nest protection and increase trapping of Rattus spp. and Varanus indicus in adjacent areas. Begin control of Dicrurus macrocercus and feral cats. Facilitate the enactment of the proposed habitat conservation plan (National Research Council 1997, Morton et al. 1999, G. Wiles in litt. 1999). Conduct a public education programme to reduce persecution (National Research Council 1997). Introduce more stringent controls for construction projects. Study the causes of nest failure and first-year mortality. Assess the feasibility of a captive-breeding programme (Ha et al. 2010).
Related state of the world's birds case studies
Amar, A.; Amidon, F.; Arroyo, B.; Esselstyn, J. A.; Marshall, A. P. 2008. Population trends of the forest bird community on the Pacific island of Rota, Mariana Islands. Condor 110(3): 421-427.
Beck, R. E. Jnr; Savidge, J. A. 2000. Mariana Crow. In: Reading, R.P.; Miller, B. (ed.), Endangered animals: a reference guide to conflicting issues, pp. 191-195. Greenwood Press, London.
Engbring, J.; Pratt, H. D. 1985. Endangered birds in Micronesia: their history, status and future prospects. In: Temple, S. A. (ed.), Bird Conservation, pp. 71-105. University of Wisconsin Press, Madison, U.S.A.
Engbring, J.; Ramsey, F. L.; Wildman, V. J. 1982. Micronesian forest bird survey, 1982: Saipan, Tinian, Agiguan, and Rota. U.S. Fish and Wildlife Service, Honolulu.
Fancy, S. G.; Lusk, M. R.; Grout, D. J. 1999. Status of the Mariana Crow population on Rota. Micronesica 32: 3-10.
Ha, James C., Butler, Alyssa., HA, Renee R. 2010. Reduction of first-year survival threatens the viability of the Mariana Crow Corvus kubaryi population on Rota, CNMI. Bird Conservation International 20(4): 335-342.
Ha, R. R.; Morton, J. M.; Ha, J. C.; Berry, L.; Plentovich, S. 2011. Nest site selection and consequences for reproductive success of the endangered Mariana Crow (Corvus kubaryi). The Wilson Journal of Ornithology 123(2): 236-242.
Michael, G. A. 1987. Notes on the breeding biology and ecology of the Mariana or Guam Crow. Avicultural Magazine 93: 73-82.
Morton, J. M.; Plentovich, S.; Sharp, T. 1999. Reproduction and juvenile dispersal of Mariana Crows (Corvus kubaryi) on Rota 1996-1999.
National Research Council. 1997. The scientific bases for preservation of the Mariana crow. National Academy Press, Washington, DC.
Plentovich, S.; Morton, J.M.; Bart, J.; Camp, R.J.; Lusk, M.; Johnson, N.; Vanderwerf, E. 2005. Population trends of Mariana Crow Corvus kubaryi on Rota, Commonwealth of the Northern Mariana Islands. Bird Conservation International 15: 211-224.
Wiles, G. J.; Aguon, C. F.; Davis, G. W. 1995. The status and distribution of endangered animals and plants in northern Guam. Micronesica 28: 31-49.
Text account compilers
Calvert, R., Derhé, M., Mahood, S., O'Brien, A., Shutes, S., Stattersfield, A., Symes, A., Taylor, J. & Khwaja, N.
Camp, R., Lepson, J., Morton, J., Rodda, G., Saunders, A., Wiles, G., de Cruz, T., Berry, R. & Amidon, F.
BirdLife International (2014) Species factsheet: Corvus kubaryi. Downloaded from http://www.birdlife.org on 24/07/2014. Recommended citation for factsheets for more than one species: BirdLife International (2014) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 24/07/2014.
This information is based upon, and updates, the information published in BirdLife International (2000) Threatened birds of the world. Barcelona and Cambridge, UK: Lynx Edicions and BirdLife International, BirdLife International (2004) Threatened birds of the world 2004 CD-ROM and BirdLife International (2008) Threatened birds of the world 2008 CD-ROM. These sources provide the information for species accounts for the birds on the IUCN Red List.
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Additional resources for this species
|Current IUCN Red List category||Critically Endangered|
|Family||Corvidae (Crows and jays)|
|Species name author||Reichenow, 1885|
|Population size||50-249 mature individuals|
|Distribution size (breeding/resident)||90 km2|
|Links to further information|
|- Additional Information on this species|