Mariana crow (Corvus kubaryi)

Also known as: aga, Guam crow
GenusCorvus (1)
SizeLength: 38 cm (2)

Classified as Critically Endangered (CR) on the IUCN Red List (1).

Similar in appearance to other crow species, the Mariana crow is the only corvid found in Micronesia (3). The plumage of this species is uniform black, with a slight greenish-black gloss on the head and underparts, and a bluish-black gloss on the wings and tail (4). During moulting, the grey feather bases become visible on the body and neck, and the head becomes lighter (5). The Mariana crow’s eyes are brown, and the bill is slender and black, with bristly feathers extending over the nostrils (4) (5). The immature Mariana crow resembles the adult, but has less glossy plumage, with a brown hue on the wings and tail (4).

The Mariana crow inhabits the islands of Guam and Rota in the Northern Marianas Islands of the western Pacific. The population on Guam has suffered a severe decline and today the only remaining individuals are introduced specimens from Rota (2) (5).

The Mariana crow occurs in a variety of forest habitats, including mature, coastal strand and secondary forest (2) (5). This species is most abundant in native limestone forest and is likely to nest exclusively within this habitat (5).

Shyer than most crows species, the Mariana crow is usually well-concealed amongst the foliage of its forest habitat (2). This species forages in the canopy, the understorey and on the ground, and takes a variety of food items, including fruits, seeds, buds, as well as small lizards, immature mammals, bird eggs and invertebrates (2). When searching for live prey, the Mariana crow rustles through the leaf litter and tears at bark to expose insects (5).

The Mariana crow forms family groups consisting of a monogamous pair and one to three offspring, although large social aggregations occasionally form. Breeding is believed to occur throughout the year on Rota, with peak nesting activity occurring from August to February. On Guam, recent observations have only recorded nesting between October and mid-April. Both parent birds contribute to all aspects of breeding, though incubation is primarily carried out by the female (5). The nest is constructed over a period of around one week, after which, a clutch of one to four eggs is laid and incubated for 21 to 23 days (4) (5). The offspring remain in the nest for 36 to 39 days, but continue to receive parental care for a remarkably long period, ranging between three and 18 months (5).

The principal cause of the Mariana crow’s decline on Guam has been predation by the introduced brown tree snake Boiga irregularis. Despite protection of nest sites, this has resulted in the near complete extirpation of the species, with the exception of a small number of introduced specimens (2). On Rota, this species has been seriously affected by a number of threats including: forest clearance for homestead and agricultural development; direct persecution; nest predation by introduced rats and monitor lizards; competition with the introduced bird species Dicrurus macrocercus; and disease (2). In addition, recent typhoons have destroyed large areas of forest habitat on Rota, and records indicate that brown tree snake individuals have been introduced to the island (1). If the snake establishes a population, it is likely to have a catastrophic effect on the Mariana crow’s dwindling population, potentially driving it to extinction (1) (2).

The Mariana crow is listed by the U.S. Fish and Wildlife Service as an endangered species, and several conservation measures are in place to preserve its imperilled island populations (2) (5). At present, one of the main priorities for the preservation of the Mariana crow is to research and reduce the threat posed by the brown treesnake on both islands, including the prevention of further spread to Rota (5). In 1993, a National Wildlife Refuge was established on Guam to preserve the remaining forest, and a 40 hectare snake-free area has been created for the introduction of Mariana crows. On Rota, a predation control experiment is to be initiated, and there are plans to extend the coverage of the island’s protected areas under the Habitat Conservation Plan for Rota's Agricultural Homesteads (1) (2). In addition to the implementation of measures to reduce the brown treesnake population and to protect remaining habitat, it is essential that further research is carried out to assess the population status of the Mariana crow on Rota. This includes detailed research into the relative importance of different factors affecting the species’ survival and reproduction (5).

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  1. IUCN Red List (November, 2011)