Endangered Species in the Pacific Islands
Mariana Crow / Corvus kubaryi / Aga
||The Mariana Crow is a small black crow with a slight greenish-black gloss on its head, back, underparts, and wings. Its tail has a bluish-black gloss. Females are smaller than the males.
|Mariana crow chicks - Photo credit Mike Lusk/USFWS
Habitat & Behavior:
The Mariana crow is also known as the aga in Chamorro, one of the native languages of the Mariana Islands. This crow prefers native limestone forest for breeding and foraging, but it will forage in other habitats such as beach strand vegetation and coconut groves. This species is an omnivorous, opportunistic feeder that is known to feed on insects, lizards, bird eggs, hermit crabs, fruits, and seeds.
Peak nesting activity occurs from August through February, but timing can vary considerably depending on typhoon activity. Its nest consists of relatively large-diameter sticks woven into a platform, small sticks that form the cup, and leaf fibers that line the cup. Clutch size varies from 1-4 eggs, although only 1-2 chicks will fledge. Both adults participate in incubation of eggs, brooding of chicks, and care of juveniles after they fledge. Parents will continue to care for their offspring long after fledging; periods of parental care are known to range from 5 to 18 months. Juveniles may take as long as 3 years before entering into the adult breeding cohort.
Past & Present:
Endemic to Guam and Rota, the Mariana Crow is the only corvid in Micronesia. On Guam, crows have historically been found throughout forested areas and were considered common, even into the early 1960s.
A U.S. Fish and Wildlife Service survey, however, estimated only 357 crows in 1981, mostly in the northern cliffline forests. The last born Guam crow was observed in 2000. Currently 2 crows translocated from Rota as eggs and/or chicks, are found in Guam. Although predation by introduced brown tree snakes (Boiga irregularis) is now widely accepted as being responsible for this dramatic decline, other factors such as infertility, predation by rats (Rattus spp.) and monitor lizards (Varanus indicus), and mobbing by introduced drongos (Dicrurus macroercus) may cumulatively be preventing recovery.
On Rota, crows were widely distributed throughout the island in both mature and secondary forests, but the population has been declining. Service surveys estimated 1,348 crows in 1982 but only 592 crows in 1995, a 56% decline. In 1999, 110 breeding pairs were estimated to occur on Rota but recent estimates indicate only 60 breeding pairs.
The Mariana crow was listed as endangered by Guam in 1979, in 1984 by the United States, and in 1991 by the Commonwealth of the Northern Mariana Islands. A recovery plan for the crow and other endemic avifauna was prepared by the Service in 1990. The National Research Council’s Committee on Scientific Bases for Preservation of the Mariana crow independently reviewed the existing data on the crow and published their findings in 1997. The Mariana Crow Recovery Team also met for the first time in October 1997. A recovery team for the species was formed in 1997 and a draft revised recovery plan was published in 2005.
Crow habitat has been protected by the Guam National Wildlife Refuge since 1992. On Rota, crow habitat has been protected since 1994 by the Sabana Conservation Area and the I Chenchon Bird Sanctuary, both established by Rota Local Law No. 9-1. Critical habitat for the species was designated on Guam and Rota in 2004.
The Guam Division of Aquatic and Wildlife Resources has actively pursued recovery techniques for the Mariana crow since 1989, including development of an electrical barrier to protect nests from brown tree snakes, development of facilities for artificial incubation, and exploration of dummy egg and hacking techniques. Guam DAWR has released 31 Mariana crows on Guam since 1997 as part of the translocation and captive recovery and release program. The Marianas Archipelago Rescue and Survey team captured and maintained 10 crows at two U.S. zoos in the early 1990s to develop protocols for captive breeding; six of these wild-caught birds were released on Guam in 1997.
Currently, the CNMI is conducting a multi-year study of Mariana crows on Rota. Initiated in November 1996, this study is focused on determining the factors that affect breeding success and the dynamics of juvenile dispersal. The Guam DAWR is also expected to implement a program to translocate crows from Rota to Guam in the near future.
Research on the Rota population of Mariana crows has also been undertaken since 1996 by the Service and CNMI. This research has focused on breeding success, life history, and population dynamic. Research on sources of mortality in adult and juvenile Mariana crows is expected to be undertaken in 2009.