The Scientific Bases for Preservation of the Mariana Crow (1997) / Chapter Skim
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2 History
2 History
Pages 11-38
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From page 11... ...
To place the current status of the Guam and Rota populations of the aga within a historical framework, the initial sections of this chapter describe the physical and climatic characteristics of both islands, provide an overview of the ecological changes that have occurred from human colonization to the present, and summarize recent avian population trends and extinctions on each island. Later sections focus specifically on the aga, providing an overview of its natural history, probable causes of decline, population demography, and past and present conservation measures.
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From page 12... ...
. Guam is the most southerly and populous of the Mariana Islands and is administered as a US territory.
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From page 13... ...
Source: Adapted from Jenkins 1983. Mariana Islands (CNMI)
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From page 14... ...
. The Mariana Islands are warm and humid with only moderate seasonal or daily variation in temperature.
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From page 15... ...
. Recent paleontologic excavations on Rota, Agiguan, and Tinian have revealed that, as elsewhere on oceanic islands, numerous species of land birds and resident breeding populations of sea birds were extirpated in prehistoric times as a result of overharvesting by humans and predation by introduced predators (Atkinson 1984; Steadman 1995a)
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From page 16... ...
The period of Japanese administration of the northern islands, 1914–1944, witnessed extensive selective logging, phosphate mining, and agricultural development and an associated increase in immigration of Japanese, Korean, and Okinawan workers. On Saipan, Tinian, Agiguan, and Rota, native forest was cleared on nearly all flat, tillable areas for sugar-cane production (Baker 1946; Hezel and Berg 1981; Mayr 1945)
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From page 17... ...
was extirpated from Guam at the turn of the century, the native avifauna of both islands that survived until the century remained largely intact through the 20th century until the 1960s. The resident avifauna of Guam included 28 species (17 land and water birds, 4 sea birds, and 7 introduced land birds)
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From page 18... ...
Rx -- b Eurasian tree sparrow (Passer montanus) I I Guam flycatcher (Myiagra freycineti)
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From page 19... ...
. The loss of the remaining species during the late 1970s and 1980s has been related primarily to predation by the brown tree snake (Engbring and Fritts 1988; Reichel and others 1992; Savidge 1987, 1988)
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From page 20... ...
include predation by brown tree snakes and monitor lizards (Aguon and others in press; Engbring and Fritts 1988; Jenkins 1983; Savidge 1987) , nest abandonment due to unknown causes, egg infertility or nonviability (Aguon 1993; DAWR 1991, 1992; DFW 1990; Grout 1993; Wiles and others 1994)
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From page 21... ...
Estimates of nesting success and adult survivorship are developed to help infer the causes of the population decline and for use in population projections. Nesting success of the aga is different on Rota than on Guam.
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From page 22... ...
. Before the use of electric barriers to protect nests, most nest failure was probably due to predation of eggs and nestlings by the brown tree snake, or to nest abandonment prior to or during egg-laying, perhaps as a result of harassment of nesting females by the snake.
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From page 23... ...
Early embryo death might be caused by inadequate incubation by a female who was being harassed by snakes. The viability of developing eggs declines with exposure to temperatures above 26–27°C (but below optimal incubation temperatures (34–36°C)
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From page 24... ...
Here we estimate survivorship from territory turnover rates in two ways: • If female aga, which are thought to be the only sex that incubates eggs, are susceptible to predation by brown tree snakes when they are incubating, a single bird lost from a territory is perhaps likely to be a female. The possibility of a male-biased sex ratio seems likely because several other species of forest birds on Guam had male-biased sex ratios just prior to their extinction, and because groups TABLE 2-5 Number of Aga Occupying Territories on Guam Number of Birdsb Location Pair Designationa 1993–94 1994–95 1995–96 Conventional-Weapons Mag 8 OL 1 0 0 Storage Area New Mag OL 0 0 2 4th St.
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From page 25... ...
were accounted for two years later. The estimate of annual survivorship is the square root of the proportion of birds remaining after two years (0.56)
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From page 26... ...
; that most avian populations on Rota, Saipan, and Tinian have remained relatively stable despite extensive habitat alterations (Engbring and others 1986; Savidge 1987) ; and that some pairs of crows on Rota have continued to occupy their territories despite relatively extensive habitat modification (Grout 1996)
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From page 27... ...
. Although native birds in the Mariana Islands have evolved with the periodic occurrence of major typhoons, the effects of typhoons on small, remnant populations could be especially severe.
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From page 28... ...
Having accurate, comprehensive pathology data will help answer questions regarding causes of mortality in various age classes and the significance of any subclinical infections or nutritional problems. Predation Various predators of birds have been introduced to the Mariana Islands, including dogs, cats, rats, monitor lizards, and the brown tree snake.
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From page 29... ...
Clearly, the brown tree snake has dramatically altered the fauna of Guam. Some food resources previously used by the aga, such as the eggs and nestlings of other forest birds, have been eliminated by the snake.
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From page 30... ...
Similarly, cases of "nonlaying" might simply represent snake predation of eggs soon after laying. Summary In summary, the principal cause of the decline of the aga population on Guam appears to be predation by the brown tree snake, although the reproductive effects of advanced age, frequent remating due to natural deaths, nest abandonment, or some as-yet-unidentified factor cannot be ruled out as a more recent problem.
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From page 31... ...
For the aga, this would be a serious shortcoming, since changes in the dynamics of the brown tree snake could have a large effect on the population dynamics of the aga. Thus, the committee did not construct a PVA model for the aga because it was inappropriate.
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From page 32... ...
All those participants share one or more of the following objectives: to diagnose the causes of the population declines in the aga on Guam and Rota, to ensure that it survives in the wild, and to eradicate or at least control the brown tree snake on Guam and ensure that it does not spread to other islands. In 1981, the aga received legal protection by the government of Guam for the first time; and in 1984, it was federally designated as endangered (FWS 1984)
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From page 33... ...
The finding that females were laying larger clutches (3 versus 1–2 eggs) in nests in protected trees during the 1992–1993 breeding season strongly supported the suggestion that in previous years many eggs were lost to brown tree snakes during the laying period.
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From page 34... ...
In summary, conservation and management efforts for the aga on Guam have emphasized surveys, behavioral observations of unbanded birds, and placement of tree barriers to protect nests from predation by brown tree snakes and monitor lizards. Most recently, they have included intensive management efforts involving the removal of eggs from wild nests for artificial incubation and hand-rearing of the young.
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From page 35... ...
A captive population of aga was founded with wild birds captured on Rota in 1993–1995 as part of the MARS program. The MARS program includes 7 zoologic institutions (Houston Zoological Gardens, National Zoological Park, Louisville Zoological Garden, Memphis Zoological Garden and Aquarium, Honolulu Zoo, North Carolina Zoological Park, and Philadelphia Zoological Garden)
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From page 36... ...
Habitat Protection As mentioned above, about 9,000 ha of forested habitat on Guam was recently incorporated into the Guam National Wildlife Refuge for the preservation, protection, and management of endemic endangered birds (FWS 1994)
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From page 37... ...
, the life history, social structure, and ecology of the aga are not fully understood, so a full diagnosis of all the factors that might be contributing to its decline is not yet possible. There is an urgent need for new intensive programs of research and management for both the aga and the brown tree snake.
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