The aga, or Mariana crow (Corvus kubaryi), is a tropical forest crow that occurs on the island of Guam and adjacent Rota. Its precipitous decline on Guam in the last 20 years has been coincident with the extinction or near extinction of all the other native forest birds and with the population explosion of the introduced brown tree snake (Boiga irregularis). Electric barriers on the trunks of trees in which crows are attempting to nest can lower predation on eggs and nestlings by brown tree snakes and monitor lizards (Varanus indicus ). However, even with such protection, a high percentage of eggs fail to result in fledglings. With only about 20 adult aga remaining in forested areas of northern Guam, and no remaining pairs known to lay viable eggs, it is doubtful that this population can be saved from extinction without the addition of birds from elsewhere. Several hundred aga are present on Rota, but that aga population has declined by at least 50% since the early 1980s. The brown tree snake is not known to be present.
The Guam Department of Agriculture Division of Aquatic and Wildlife Resources (DAWR), the Commonwealth of the Northern Mariana Islands (CNMI) Department of Lands and Natural Resources Division of Fish and Wildlife (DFW), and the US Department of the Interior Fish and Wildlife Services (FWS) and National Biological Service (NBS) are all committed to the preservation of the native fauna of the entire Mariana archipelago. Recognizing the need for a coordinated recovery program for the aga, FWS requested that a committee be appointed by the National Research Council to make appropriate scientific recommendations for its preservation. Thanks to the cooperation of the aforementioned agencies, the committee was able to review and analyze all available scientific information, including published papers, unpublished manuscripts, and
internal reports. It was briefed by biologists from DAWR, DFW, FWS, and NBS, and spoke with other persons with relevant expertise, especially US Department of Agriculture Animal Damage Control (ADC) staff on Guam.
The committee concurs with the conclusions of Savidge and subsequent workers that the major cause of the aga's decline on Guam is predation by the brown tree snake. The probable presence of introduced populations of the brown tree snake on other islands within the Mariana archipelago and elsewhere in the Pacific portends a sequence of severe ecological disruptions; this is a matter potentially far more serious than the preservation of the aga on Guam and Rota. Development and implementation of effective, large-scale methods for controlling the brown tree snake are urgently needed to prevent the further spread of this Australasian species to other islands and to enable the recovery of the aga population on Guam. At present, the ADC program constitutes the only impediment to dispersal of the snake from Guam.
The cause of the high percentage of nonviable eggs of the aga on Guam is less clear. The cause of the decline on the aga on Rota is also poorly understood, although the reduction of forested habitats and the direct killing of crows have obviously been contributing factors. On both islands, a segment of the public considers economic development to have a generally higher priority than preservation of habitat for endangered and threatened species. Public leaders on Rota, however, have demonstrated increasing concern for environmental quality and preservation. Several protected areas have been established recently, and additional areas have been proposed for protection. CNMI, in cooperation with FWS personnel, is preparing a habitat-conservation plan for Rota that will designate areas for conservation and for potential development; it is hoped that this plan will serve as a model for conservation planning on other islands in the Pacific basin. Clearly, a viable aga population on Rota, Guam, or elsewhere cannot be maintained unless adequate habitat is preserved.
There are 12 aga in captivity. Between 1993 and 1995, ten were collected from Rota, of which nine are still alive. Additionally, one young bird, which was produced by a pair housed at the Houston Zoological Gardens in Texas, remains at that facility. Two young hatched in 1996 from eggs taken from nests of wild aga on Guam are now being held in the DAWR facility there. Because substantial numbers of birds still exist on Rota, the committee believes that an intensive captive-breeding program, like the one for the 'alala in Hawaii, is not justified at this time. Similarly, although it is important to maintain two wild populations, the committee believes that management to preserve the apparent low level of genetic differences between the Guam and Rota populations is not justified. Instead, emphasis should be on implementing a coordinated program of research and management that will alleviate the causes of declines in both wild populations, stabilize the Rota population at a size and distribution that are viable in the longterm, and sustain the Guam population and allow it to increase as quickly as possible. However, because the Rota population is the largest and most-secure
aga population, the committee recommends that the highest priority be given to research and management actions necessary for the long-term preservation of the Rota population while attempts to preserve and restore the Guam population proceed.
In attempting to formulate such a coordinated recovery program for these aga populations, the committee has considered various options, including implementation of large-scale snake-control measures in native habitat, intensive avicultural management, interisland translocations, and return of captive crows to the wild. After considering those options and acknowledging the urgent need for both short- and long-term actions, the committee formulated the following eleven recommendations for steps to be taken over the next three years. Chapter 6 presents a complete exposition of these recommendations.
Recommendation 1. Expand and increase research, development, and implementation of methods to control the brown tree snake and to prevent its spread to other islands.
Recommendation 2. Study the behavior, population biology, and health of marked birds on both Guam and Rota.
Recommendation 3. Release all twelve captive aga on Guam within the next two years.
Recommendation 4. Place electric barriers in all nest trees to protect nests from brown tree snakes and monitor lizards, and increase trapping efforts in nest trees and adjacent areas.
Recommendation 5. Assess the feasibility of translocating birds to islands other than Guam or Rota.
Recommendation 6. Conduct annual censuses of the aga population on both islands.
Recommendation 7. Conduct complete postmortem examinations on all recoverable eggs and birds.
Recommendation 8. Facilitate the development of a habitat conservation plan for Rota.
Recommendation 9. Establish a professional-level research manager position within the National Biological Service on Guam.
Recommendation 10. Appoint a recovery team specifically for the aga.
Recommendation 11. Conduct a public education program that specifically addresses issues relevant to aga conservation, including the problems associated with the spread of the brown tree snake.