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State of the Birds Report

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Other Islands

Guam and the Northern Marianas


            Northern Marianas capital of Saipan photo by P. Miller.
Inset: Guam Micronesian Kingfisher courtesy USFWS.


Guam and the Northern Marianas, an island chain proximate to Asia, is home to members of several bird families that occur naturally nowhere else in the United States or its territories. Typical of isolated small islands, 95 species inhabit Guam and 103 species inhabit the Northern Marianas. The avifauna includes five endemic species and several other endemic subspecies. This limited avifauna has been adversely impacted by habitat loss and degradation from agriculture and development, and use of pesticides. Introduced species have taken a significant toll especially on Guam where the brown tree snake has caused many bird extinctions. Today six species are listed as endangered under the Endangered Species Act in each of Guam and the Northern Marianas.

The Threat of the Brown Tree Snake

Brown tree snake on Guam.
Photo courtesy of USGS

The wave of bird extinctions in Guam since the introduction of the brown tree snake is considered one of the most dramatic cases of the impact of an exotic species ever recorded. The snake arrived just after World War II, probably in cargo from the Admiralty Islands of Papua New Guinea. By 1968, it had spread to the entire island, and by the 1980s, densities of the snakes approached 100/ha and marked declines of many bird species were observed. By the time many species were listed under the Endangered Species Act in 1984, a number were already extinct or fast approaching extinction. In all, nine species of forest birds have been eliminated by the brown tree snake. Of the five species of native forest birds listed as endangered on Guam, the Guam Flycatcher (Myiagra freycineti) and the Guam Bridled White-eye (Zosterops c. conspicillatus) are almost certainly extinct.

The Guam Rail (Gallirallus owstoni) exists in captive breeding populations on the island of Guam and in stateside zoos. A program is currently underway to maintain rails in a snake-free enclosure on Guam, and an experimental population has been established on the island of Rota. Similarly, the Guam Micronesian Kingfisher (Todiramphus c. cinnamominus) has been maintained in captivity on Guam and in mainland zoos after being extirpated from Guam. Like the rail, the plan is to reintroduce kingfishers as snake control measures are developed over time. The Mariana Crow (Corvus kubaryi) is the only native Guam forest bird with populations still existing in the wild, but the population is down to four males.

Dispersal of the brown tree snake is a serious and ongoing concern. The snake has already spread to Saipan, Tinian, and Rota in the Northern Marianas, although conditions on Rota seem to be especially favorable to its establishment. However, it has yet to be implicated directly in bird declines there with the other leading threats enumerated above still be the drivers of bird declines. As on Guam, Black Drongos (Dicrurus macrocercus) have been introduced on Rota and have become among the most abundant birds. Predation by and competition from Black Drongos has been suggested as factors in the decline of the Rota Bridled White-eye (Zosterops rotensis) and the Mariana Crow. The crow’s Rota population has dropped from 1,318 birds in 1985 to less than 50 pairs in 2007.

What Can We Do?

Important conservation strategies for the birds of Guam and the Northern Marianas include captive breeding and reintroduction, snake-proof exclosures on Guam for selected releases of captive reared birds, research on and implementation of snake control techniques, predator control for other species such as rats (Rattus sp.), habitat restoration, and direct protection of critical habitat. Brown tree snakes have already been transported to multiple locations from Guam, including as far away as Texas. It is essential that cargo screening for both air and sea going freight is maintained at the highest possible level of thoroughness.


American Samoa

Bristle-thighed Curlew by Daniel R.
Ruthrauff, courtesy of USGS

American Samoa has a fairly limited island avifauna of 52 regularly occurring species, comprised of 40% each of landbirds and seabirds. The remainder includes a few species of waterbirds and transient and wintering shorebirds. Rose Island, a National Wildlife Refuge administered by the Remote Pacific Islands Refuge Complex, holds a 14 species seabird colony of nearly 140,000 birds. Among the shorebirds are an unknown number of wintering Bristle-thighed Curlews (Numenius tahitiensis), which are considered globally vulnerable to extinction with a total population of less than 7,000 individuals and a species of High Concern under the U.S. Shorebird Conservation Plan. There are no endemic species. No species are listed under the Endangered Species Act, although the Mao (Gymnomyza samoensis) is extirpated and the Pacific Black Duck (Anas superciliosa) has declined. The Shy Ground-Dove (Gallicolumba stairi) and Spotless Crake (Porzana tabuensis) are candidates for listing under the Endangered Species Act. Both have small populations of no more than a few hundred pairs and are threatened by introduced cats and rats, and disturbance from natural events such as cyclones, and from human damage to forests and wetlands. There are few data available about status and population trends of other bird species.

Remote Pacific Islands

Baker Island, Howland Island, Jarvis Island, Johnston Atoll, Palmyra Atoll (National Wildlife Refuges administered through the Remote Pacific Islands Refuge Complex), and Wake Atoll are home to some of the largest and most diverse seabird colonies in the world. Baker and Jarvis each hold over a million seabirds and several of the islands host mixed colonies of 12-15 species. Palmyra supports the second largest colony of nesting Red-footed Boobies (Sula sula) in the world with total population estimates around 6,250 pairs. Two hundred of the world’s six thousand Bristle-thighed Curlews winter on Palmyra.  The curlew is a species of High Concern under the U.S. Shorebird Conservation Plan and considered globally vulnerable to extinction. In addition, all of the islands receive small numbers of transient and wintering Ruddy Turnstone (Arenaria interpres), Wandering Tattler (Heteroscelus incanus), and Pacific Golden-Plover (Pluvialis fulva). Northern Pintail (Anas acuta) is among the species of waterfowl that occasionally use the islands in small numbers.


Puerto Rico and the US Virgin Islands

Puerto Rican Parrot courtesy of USFWS

Puerto Rico and the U.S. Virgin Islands hold significant tracts of moist and dry tropical and sub-tropical forests that are habitat for birds not found anywhere else in the United States. Puerto Rico has 177 regularly occurring species, including 16 endemic species. Among them is the Puerto Rican Tody (Todus mexicanus), a member of the family Todidae which is not represented anywhere else in the United States or its territories. There are seven species listed under the Endangered Species Act, including the Puerto Rican Parrot (Amazona vittata), which numbers only about 35 individuals and has been the subject of intensive recovery actions since 1968. Its habitat in the Luquillo Mountains lies entirely within the El Yunque National Forest, the sole tropical rain forest in the U.S. National Forest System. Other important endemic species include the endangered Puerto Rican Nightjar, whose stronghold is the dry forests of southwest Puerto Rico, especially Guánica State Forest, and Yellow-shouldered Blackbird, also restricted to the southwest. Each of these species has a population of less than 1500 birds. Puerto Rico has a large number of introduced species with 40 of 117 landbird species (34%) being nonnative. The U.S. Virgin Islands have 212 regularly occurring species, with no endemic or listed species.

Importantly, both Puerto Rico and the U.S. Virgin islands are significant wintering and migration stopover sites for several species of Nearctic-Neotropical migrants that breed in the eastern United States, especially Northern Parula, Cape May, Prairie, Black-throated Blue, Black-and-white, and Worm-eating warblers; American Redstart; Northern Waterthrush; and Ovenbird. Puerto Rico is important for migratory and wintering shorebirds. Cabo Rojo National Wildlife Refuge and the Jobos Bay Estuarine Reserve are the best known Puerto Rican sites, with Cabo Rojo having the largest numbers. Monthly average numbers of shorebirds there range from 2500-3500 during August-December. The site is nesting habitat for Killdeer, Black-necked Stilt, Wilson's Plover and is the only nesting site for Snowy Plover.


Shiny Cowbird by Daniel Talevi

Threats to birds in Puerto Rico and the U.S. Virgin Islands include forest clearing for agriculture and urban development. Thousands of hectares of wetlands have been drained for agriculture or degraded by irrigation systems. For example, Laguna Cartagena formerly supported tens of thousands of migrating and wintering waterfowl and shorebirds, but is now largely invaded by cattails. A project is underway to restore it. Conversion of shade coffee to sun coffee monocultures threatens agricultural land with significant value for birds. A wind energy development threatens a significant proportion of the range of the Puerto Rican Nightjar at Guayanilla and as many as half of the 46 known breeding territories in the area. Enforcement of environmental laws is inadequate and incentives for private landowners to manage land in ways compatible with birds are lacking. Introduced predators such as rats, cats, dogs, and mongoose take a toll on many species of birds especially ground nesters. Puerto Rico has many introduced bird species, especially parrots and finches which may compete with native species for food and nest cavities. The Shiny Cowbird has invaded naturally, but is a serious nest parasite, especially of Yellow-shoulder Blackbirds. Most successful nesting of the blackbird now occurs only on offshore cays.


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