|Listed||August 27, 1984|
|Description||Small black crow, slight greenish black gloss in the head; back, wings, and tail with bluish-black gloss.|
|Habitat||Limestone forest, mixed forest, coconut plantations.|
|Food||Insects, lizards, hermit crabs, fruits, seeds, flowers.|
|Reproduction||Nesting has been observed in March, May, June, September through November, and October through February. Clutch and brood sizes are not well known.|
|Threats||Brown tree snake.|
The Corvus kubaryi (Mariana crow) is a small black crow with a slight greenish black gloss on the head. The back, wings, and tail have a bluish-black gloss and underparts have a dull greenish-black gloss. The base of feathers are light grayish in color, and are nearly white on neck, producing a somewhat ragged appearance. Nasal bristles are short but extend over the nostrils and the base of culmen. The Mariana crow has a black bill and feet, and dark brown irises. The female is smaller than the male. Immatures resemble adults, but their feathers have less gloss, and the wings and tail are browner. This characteristic, however, may be of limited use in the field. Fledglings' plumage is "ragged" looking, with white showing through from the base of the feathers at numerous locations on the neck, back, and breast. In addition, the lower mandible, near base, has pink-tan showing.
Little is known of the reproductive biology of the Mariana crow. It apparently nests year-round although it has been suggested that this species does not breed in June, July, or August. Nesting has been observed in March, May, June, September through November, and October through February. Clutch and brood sizes are not well known. One nest was observed on Guam containing one egg in October 1985. On Rota one adult pair was observed with two fledglings and another pair with one fledgling in March 1986. A pair of crows was observed with two fledglings, and a single adult with one fledgling. Both adults participate in all aspects of the construction of the nest. The female does all of the incubating at night and most of the incubation during the day with the male relieving the female for only short periods of five or 10 minutes at a time. While the female does leave the nest during the day to forage, the male will feed the female on the nest several times a day. Incubation time is unknown but during a study of the crow on Guam in the fall-winter of 1985-86, a crow pair incubated for as long as 18 days on two occasions before abandoning the nest presumably due to predation from the brown tree snake.
The breeding biology of the Mariana crow has not been studied extensively. The time required for development of the altricial young is unknown. Both parents participate in the care of the young. Apparently an extensive learning period is necessary for the young, with fledglings closely following their parents, begging for food, and learning the foraging patterns. Both parents defend the nest site, although the male plays a greater role than the female. The duration of the pair bond is unknown.
The Mariana crow apparently does not seasonally migrate although nothing is known concerning possible movement, if any, between Guam and Rota. As many as 68 individuals have been observed at roost sites. The movements of these individuals (apparently not breeding individuals) is not currently understood.
The Mariana crow is an omnivorous, opportunistic feeder that is known to feed on insects, lizards, hermit crabs, fruits, seeds, flowers, and occasionally foliage and bark. The crow has also been observed feeding on leaves and bark, where it forages for insects and lizards. It may also consume worms, mollusks, and amphibians. A study in the fall of 1985 found the crow feeding primarily on animal matter and not on bark or foliage, although much time was spent there by the crow searching for insects. Fledglings on Rota were observed tearing up dead bark and leaves in search of insects. The Mariana crow forages on the ground as well as in the forest canopy and feeds on other birds' eggs.
The Mariana crow is a diurnally active species.
The Mariana crow is still found in the wild on Guam, but is primarily confined to limestone forest and mixed forest areas in extreme northern Guam. It also utilizes old coconut plantations in northern coastal Guam. This species' preference for mature native forest on Guam has been noted several times. One study found the crow confined to coconut plantations and forest areas and only infrequently found it in areas of human habitation. In 1983, during a survey of northern Guam by the Guam Dept. of Aquatic and Wildlife Resources, this latter observation was confirmed by noting the presence of the crow in mature forest on Anderson Air Force Base and its absence in areas of human habitation such as base housing, active airfield areas, and other locations around buildings with high human usage. A two week study on Guam and Rota in July of 1980 listed 13 plant species used by the crow for foraging and other activities. Eleven of these species were native and typical of limestone forest and coastal strand. The two exceptions were Leucaena leucocephala and Mangifera indica. During a study of the crow in the fall of 1985, it was found that the crow restricted itself to mature forest in the Northwest Field and Conventional Weapons Storage Area on Anderson Air Force Base (AAFB), using emergent trees such as Ficus prolixa and Elaeocarpus joga from which to view and defend their territory and in which to build nests. Furthermore, crow nests were reported in Ficus prolixa trees in an area of mature mixed woodland-native forest on Northwest Field, AAFB. The pair unsuccessfully incubated eggs in at least 10 successive nests, in at least six different emergent Elaeocarpus joga trees in native forest at the Conventional Weapons Storage area on AAFB over a six-month period.
Habitat usage of the Mariana crow on Rota has been insufficiently studied, but found the crow on Rota to be similar to those observed on Guam. On Rota, the Mariana crow was observed only in limestone (native) forest and coastal strand vegetation.
A study in the fall and winter of 1985 observed both the male and female C. kubaryi defending nesting-foraging territory. The 2-3 acre (0.8-1.2 hectare) territory often contains several emergent trees, typically Elaeocarpus joga or Ficus spp. from which the crows view the territory and call to warn intruders, both individuals and groups, before actively chasing and sometimes physically attacking them. There are often several old nests in a territory in addition to an active one. It is not known if there is seasonal variation in the size or shape of territories.
The Mariana crow is known only from Guam and Rota in the Mariana Islands and it is presently widely distributed on Rota. Presently, crows are found on Guam in forest areas from Ritidian Point to Anao along the northern cliff line forests and in the adjacent Northwest Field and Conventional Weapons Storage Area of Anderson Air Force Base. In addition to U.S. Department of Defense lands, the Mariana crow may possibly occur on Federal Aviation Administration land in the Finegayen, Dededo area. The species could also possibly occur on Government of Guam land at Anao Point and Falcona Beach, and on private land at Urunao and Jinapson in northern coastal Guam.
Historically, the Mariana crow inhabited all of the island of Guam and Rota.
Current limiting factors affecting the Mariana crow on Guam and Rota are unknown with the exception of the predation by the brown tree snake reducing the population on Guam. On Guam, the reproductive rate appears to be very low or non-existent based on the lack of fledglings found on Guam and the lack of nest success found during a study of the crow on Guam in the fall-winter of 1985-86.
The population on Guam and Rota declined from an estimated 350 birds in 1981 to fewer than 50 in 1993, probably because of predation by the introduced brown tree snake. The Rota population was thought to have remained stable since 1945 and estimated at 1,300 birds in 1982, although a more recent estimate is 500 birds. There is concern that the brown tree snake may be making its way to Rota.
On Guam this species was a common, endemic insectivore sought by bird watchers. The species was also important as a dispersal vector for seeds of native forest trees. In the past hunting may have added an additional stress on the population, but presently there is no evidence to suggest illegal hunting is responsible for the recent declines.
Predation of eggs, young, and adults by the introduced nocturnal brown tree snake (Boiga irregularis ) is thought to be responsible for the present decline of the Mariana crow and other forest birds on Guam. The Mariana crow is apparently not declining on Rota.
Introduced diseases were considered as possible causes for both the range reductions and extinctions noted for most native forest birds on Guam, but no infectious organisms have been isolated on Guam that could account for the decline. Feral dogs, cats, and rats which are found on both Guam and Rota are not thought to be a major factor in the decline of the crow.
There were major changes in Guam's vegetation before, during, and after the Second World War. However, substantial native habitat still remains on Guam and habitat degradation is not thought to be a major factor. If present development patterns continue to reduce and segment the dwindling available mature forest, recovery of the Mariana crow may be hindered because of its apparent requirement of mature forest with little human usage.
While pesticides may have been a factor in the past, an intensive pesticide survey conducted in 1981 on Guam concluded that pesticides were not a significant factor.
While agonistic interactions between the Mariana crow and the black drongo (Dicrurus macrocerus ) have been noted, competition seems to be an unlikely factor in the decline on Guam. Hunting may have stressed the crow on both Guam and Rota but there is no evidence to suggest it was responsible for the recent decline of the crow on Guam.
Conservation and Recovery
The recent, drastic decline of forest bird populations on Guam has been one of the most alarming and challenging endangered species problems of modern times. Evidence points to an efficient predator of small birds, the brown tree snake, as the culprit. The U.S. Fish and Wildlife Service has implemented a comprehensive plan to control the brown tree snake, which will greatly improve the chances of all Guam bird species to repopulate their numbers. Once thought to be relatively resistent to snake predation, the Mariana crow population on Guam has dwindled to just a few birds. In contrast, the population on Rota remains healthy. The species is included in the Endangered Native Forest Birds.
U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
Eastside Federal Building
911 N. E. 11th Ave.
Portland, Oregon 97232-4181
Telephone: (503) 231-6121
U. S. Fish and Wildlife Service. 18 Jan. 2000. "Mariana Crow / Corvus kubaryi / Aga." Threatened and Endangered Animals in the Pacific Islands. http://pacific.fws.gov/pacific/wesa/crowmariaindex.html.