Justification of Red List Category
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 confined to just one island, Rota, where multiple factors are driving the decline, with the additional risk of potential colonisation of the island by the Brown Tree-snake Boiga irregularis.
In 2013-14, Kroner and Ha (2017) recorded 46 breeding pairs with a potential eight more pairs in unsurveyed areas. They estimated the total population to be only 178 individuals. In 2012, Camp et al. (2015) estimated the population to be 81 individuals (95% CI = 30-202), and Faegre et al. (2016) estimated 50 breeding pairs in the 2015-2016 breeding season. Therefore, the population size is placed here in the range 50-249 mature individuals.
Surveys between 1982 and 2012 have revealed an extremely rapid population decline. The population is estimated to have declined by over 80% over the past three generations (22 years).
The species inhabits Rota in the Northern Mariana Islands (to U.S.A.). On Guam (to U.S.A.), it was formerly common but, since the 1960s, declined in numbers and extent, with an estimated 350 birds in 1981 (Engbring and Pratt 1985), fewer than 40 in 1995 (Fancy et al. 1999), and only seven 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 by 2008 (R. Berry in litt. 2008) and is now extinct on Guam (L. Barnhart Duenas in litt. 2013, 2014). On Rota, the population was thought to be stable at 1,318 birds in 1982 (Engbring et al. 1982), but declined to 592 in 1995 (Fancy et al. 1999), to 234 in 1998 (Morton et al. 1999, Plentovich et al 2005), around 85 pairs in 2008 (Amar et al. 2008) and 81 individuals in 2012 (Camp et al. 2015). Surveys on Rota between 1982 and 2012 indicated a decline of 95% (Camp et al. 2015). 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; and 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). The most recent population count was conducted in the breeding season of 2015-2016, in which 50 breeding pairs were detected (Faegre et al. 2016).
It inhabits mature and second-growth forest and coastal strand vegetation (Michael 1987), although occupancy analyses of the 2012 survey indicated that the species no longer inhabits coastal strand vegetation (Camp et al. 2015). 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 was extirpated (L. Barnhart Duenas in litt. 2013, 2014).
On Rota, typhoons have devastated forest habitat and more 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, Camp et al. 2015, Sussman et al. 2015). Nests in native forest were found to have higher reproductive success than those in more disturbed areas, suggesting that damage to habitat may be limiting nesting success (Ha et al. 2011). Although most of the inhabitants of Rota were found to have positive attitudes to this bird, some shooting and chasing does still occur (Sussman 2015).
Predation is likely a significant cause of nest failure (Zarones et al. 2015), and survival rate analysis has shown that the population decline is driven by high mortality in first-year birds (Ha et al. 2010). A major cause of this, therefore, may be feral cats as they have been shown to be the main cause of death for first-year birds (Ha and Ha 2013). Although predation by introduced rats Rattus spp. was thought to be a contributing factor to the species' decline, a recent study found that crow breeding success was higher in areas where rats were more abundant, suggesting that rats were unlikely to be driving declines of the species (Amar and Esselstyn 2014). 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.
Additional potential threats include nest-predation/disturbance by monitor lizard Varanus indicus, competition with introduced Black Drongo Dicrurus macrocercus, and disease (Morton et al. 1999, Amar et al. 2008). Also, having a distribution on a relatively low-lying island, this species is potentially susceptible to climate change through sea-level rise and shifts in suitable climatic conditions (BirdLife International unpubl. data).
Conservation and Research Actions Underway
A cat control program commenced on Rota in 2012, targeting Mariana Crow breeding areas. Since the cat control program commenced, survival rates of first-year birds has increased, as shown from radio-telemetry and mark-resight analysis (Ha et al. 2014, Faegre et al. 2016) A habitat conservation plan was approved for the Rota Agricultural Homestead Development Area and is being implemented. Life history studies are continuing. A captive rear and release program in which crow eggs and nestlings will be harvested from the wild, reared, and then released at around 1.5-2 years of age commenced in 2016.
Conservation and Research Actions Proposed
Continue stringent measures to prevent the spread of brown tree-snakes B. irregularis from Guam to Rota. Continue research including study of population biology and health of birds and regular censuses using standardised methodology (National Research Council 1997, Morton et al. 1999, G. Wiles in litt. 1999). Continue feral cat control. Conduct a public education programme to reduce persecution (National Research Council 1997, Sussman et al. 2015). Study the causes of nest failure. Revise current land-use regulations and implement a monetary compensation programme for owners of crow nesting habitat to improve landowners’ attitudes and practices (Sussman et al. 2015).
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.
Text account compilers
Khwaja, N., Wright, L, Westrip, J., Mahood, S., O'Brien, A., Calvert, R., Shutes, S., Stattersfield, A., Symes, A., Derhé, M., Taylor, J.
de Cruz, T., Berry, R., Morton, J., Amidon, F., Rodda, G., Berry, L., Saunders, A., Camp, R., Wiles, G., Lepson, J.
BirdLife International (2019) Species factsheet: Corvus kubaryi. Downloaded from http://www.birdlife.org on 24/07/2019. Recommended citation for factsheets for more than one species: BirdLife International (2019) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 24/07/2019.