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Extinctions and Loss of Species from
Beginning in the mid 1960's, the brown Treesnake decimated
Guam's native avifauna. The birds of Guam evolved in the absence
of snake predators. They had no experience with such a predator and lacked
protective behaviors against the brown Treesnake. Consequently,
they were easy prey for these efficient, nocturnal predators. As
the snakes spread across the island, the number of snakes began to grow
exponentially and bird populations declined. Nine of the 11 species
of native forest-dwelling birds have been extirpated from Guam.
Five of these were endemic at the species (*) or subspecies (**) level
and are now extinct on Guam. Two of these species, the Guam rail
and the Micronesian kingfisher, are being captively bred in zoos in the
hope that they can eventually be released back into the wild. Several
other native species exist in precariously small numbers, and their future
on Guam is perilous.
The history of this decline starts shortly after the
introduction of the brown Treesnake, although appreciable losses were
not evident until the 1960s. By 1963, several formerly abundant
species of native birds had disappeared from the central part of the island
where snakes were most populous. By the late 1960s, birds had begun
to decline in the central and southern parts of the island and remained
abundant only in isolated patches of forest on the northern end of the
island. Snakes began affecting the birds in the north-central and
extreme northern parts of the island in the 1970s and 1980s, respectively.
Most native forest species were virtually extinct when they were listed
as threatened or endangered by the U.S. Fish and Wildlife Service in 1984.
Nesting and roosting in caves, the island swiftlet has
persisted in at least one refuge on Guam. The one active colony
in south-central Guam is in a cave where they roost and nest, apparently
out of reach for brown Treesnakes, so the young are able to reach maturity.
At one time, there were perhaps a dozen swiftlet caves on Guam harboring
thousands of these insectivorous birds. These populations have nevertheless
declined along with the other forest birds.
Sali (CNMI and Guam)
Although its numbers have been greatly reduced, the Micronesian
starling is the only native forest bird that may survive into the future
on Guam. The starling is a cavity nester and has been able to colonize
a few urban areas using artificial nest sites and thus avoiding predation
by the snake. A small population of starlings also resides on Cocos
Island, an islet 3 km south of Guam.
Aga (CNMI and Guam)
The only native forest bird to persist in snake-infested
habitat, the Mariana crow has probably managed to persist on Guam due
to its large size, long life span, and human intervention. However,
its survival is now extremely tenuous. The population on Guam is
believed to number only 12 individuals, including 4 birds recently translocated
from Rota. Little or no successful recruitment into the adult population
has occurred since 1986. Intensive efforts have been made by the
Guam Division of Aquatic and Wildlife Resources to protect nests in trees
from snakes and monitor lizards with electrical barriers on Andersen Air
Force Base, but the numbers of nests available for such efforts have declined
considerably over the last decade.
Chuchuko atilong (CNMI and Guam)
A native to the wetland and coastal areas of Guam, the
Pacific reef-heron is uncommon on Guam but still nests on the nearby island
of Cocos. It is equipped with a long bill and defensive manner that
have probably helped it combat the brown Treesnake in confrontations.
The reef-heron typically nests on rocky, offshore islets where it is protected
Pulattat (CNMI and Guam)
The common moorhen is now rarely seen on Guam, although
effects of the brown Treesnake on this population are poorly documented.
Fairly large in size and aggressive, moorhens would be difficult prey
for most snakes and may be able to defend their nests against snakes.
Also, the moorhen usually nests in wetland areas, which are not prime
habitat for the snakes. Losses to snake predation most likely are
concentrated on eggs or young birds.
Halcyon cinnamomina **
Sihek (CNMI and Guam)
Once found throughout Guam's forests, the Micronesian
kingfisher has been extirpated from the wild by the brown Treesnake and
now exists only in captive populations at several mainland U.S. zoos.
As experimental deterents progress and barrier technology improves, it
is hoped that these populations will be introduced back into Guam's environment
as the rail has been. Since this subspecies of kingfisher has a
relatively soft beak and selects only soft, rotting trees in which to
burrow nesting sites, it is possible that these trees could be protected
in ways similar to those used for the Mariana crow. The existence
of appropriate nest sites in areas where snakes have been controlled will
be critical to the success of future introductions.
Kakkak (CNMI and Guam)
The Yellow bittern is now one of the most frequently
sighted native birds on Guam. It moves between a mixture of habitats,
preferring primarily wetland and grassy areas. The bittern is equipped
with a long bill, and like the Pacific reef-heron, its tendency to defend
itself probably has helped it survive on Guam. The bittern is also
found on Cocos.
Myiagra freycineti *
Once widespread and relatively conspicuous, the Guam
flycatcher was small in size and driven to extinction early by the brown
Rallus owstoni *
A flightless bird, endemic to Guam, the Guam rail disappeared
from southern Guam in the early 1970s and was extirpated from the entire
island by the late 1980s. This species is now being bred in captivity
by the Division of Aquatic and Wildlife Resources on Guam and at some
mainland U.S. zoos. Since 1995, more than 100 rails have been introduced
on the island of Rota in the Commonwealth of the Northern Mariana Islands
in an attempt to establish a wild breeding colony. Although at least
one chick resulted from these efforts, predation (largely by feral cats)
and accidental deaths have been extremely high. A small number of
birds potentially persist. A recent effort to introduce rails on
Guam in a 22-ha forested area concentrated on protecting the rails by
limiting snakes using a combination of trapping and a perimetereter barrier
to reduce re-invasion by snakes. This endeavor allowed the tentative
survival of several pairs of rails released into the area. Reproduction
by the rails was reported in this control area on the basis of sounds
attributed to chicks. The preliminary success constitutes one of
the few bright spots in the conservation of Guam's native fauna in recent
years and speaks to future opportunities to recover wildlife.
Rhipidura rufifrons *
Once widespread and relatively conspicuous, the Rufous
fantail represents another small bird driven to extinction by the brown
Zosterops conspicillatus **
Once the most abundant bird on Guam, the bridled white-eye
was probably the first species to be extirpated by the brown Treesnake,
which constitutes extinction for the Guam subspecies. The smallest
of the forest birds, high stress was placed on the white-eyes during the
snake's irruption. The smaller birds such as white-eyes, fantails,
and broad bills, as well as their eggs and young, were extremely vulnerable.
Non-endemic birds extirpated from Guam:
Gaga karisu (CNMI)
Found on Guam in low numbers prior to the spread of the
brown Treesnake, the warbler has disappeared on Guam; however, its early
disappearance could have been due to snake predation in combination with
Paluman apaka (CNMI)
Paluman Apaka (male, Guam)
Paluman Fachi' (female, Guam)
A somewhat larger species, the ground-dove was able to
persist while the smaller birds suffered heavy predation. However,
in the last decade, it has been extirpated from Guam.
Paluman totut (CNMI)
Like the ground-dove, the Mariana fruit-dove was able
to survive the initial snake irruptions on Guam but ultimately disappeared.
It has not been seen on Guam since 1984.
A colorful and frequent urban resident on Guam, the Cardinal
honeyeater was lost as the brown Treesnake population boomed.
Species extirpated from Guam prior to the introduction of the brown
Nganga' palao (CNMI)
The limited availability of open water habitats placed
the Mariana mallard in jeopardy from over-hunting. It disappeared
from Guam prior to World War II.
Sasangat (CNMI and Guam)
The Micronesian megapode disappeared from Guam prior
to the arrival of the brown Treesnake. The unusual bird's dependence
on placing its nests in mounds of loose soil placed severe limitation
on its numbers on Guam where such soils are scarce.
Of the approximately eight introduced
species on Guam, five are uncommon or rare. Although some of these
birds came from native ranges where snakes are abundant and are better
adapted to resist predation by the brown Treesnake, extreme snake densities
have caused even these species to encounter difficulty. Only three
species are relatively common—the black francolin, the blue-breasted
quail, and the Eurasian tree-sparrow—due primarily to their habitat
selection. They persist in urban areas and developed sites, where
snake density is limited by human activity and non-conducive habitat.
Many of Guam's residents who formerly raised quail in
cages have abandoned the hobby due to problems and losses caused by snakes.
A native of India, the black francolin may have anti-predator
defenses that provide some protection from brown Treesnake predation.
The francolin is found throughout much of Guam but in highly variable
numbers. Because of the francolin's large size, as adults they are
not easy prey for any but the largest snakes. The francolin is a
resident of open fields and savannas, areas that are not favorite habitats
of the brown Treesnake.
Ga'ga' pale (CNMI)
The Eurasian tree-sparrow may have persisted only because
it is so successful in urban areas surrounded by automobile traffic, predatory
domesticated animals, and habitat discontinuities, all of which potentially
reduce the abundance of the brown Treesnake. Yet the sparrow's
abundance in many areas is quite low due to snake predation.
Rare or Uncommon Species on Guam:
Paluman mansu (Guam)
Once abundant in many urban and commercial sites, pigeons
are rarely seen on Guam today.
Salin Taiwan (CNMI and Guam)
This native of Taiwan and eastern Asia harasses other
birds and even humans that approach its nests. It prefers to perch
in high open sites and is probably most successful when it nests on power
poles, which are somewhat protected from intruding snakes. Even
nests on power poles have declined since 1985.
Feral chickens are commonly found on Pacific Islands
but declined drastically and then disappeared from Guam. Many Guam
residents still raise fighting chickens and colorful diminutive chicken
breeds, but a large number of eggs and chicks are lost to snakes.
This diminutive flocking grassland bird has persisted
in low numbers, but undoubtedly many fall prey to snakes.
This habitat generalist was found to suffer nest losses
of 74-93% attributed to snakes, but its high reproductive output and ability
to use a wide spectrum of nest sites and feeding ranges have allowed it
to persist on Guam. However, the poor nesting success demonstrated
by studies done in 1984 and 1986 raise doubts as to this species' continued
survival on Guam. Indeed, population levels have declined 80-90%
or more throughout Guam.
While forest avian species have been conspicuously affected
by the brown Treesnake, seabirds that nested on Guam prior to the snake's
introduction have also declined drastically. Three species of seabirds
nested on Guam prior to the entrance of the brown Treesnake in the environment.
These birds are now absent or extremely rare on Guam, although the brown
noddy and white tern can still be found on offshore islets where they
are protected from snake predation. The cause of the disappearance
of these nesting colonies is presumed to be snake predation on eggs and
Fakpe, Utag (Guam)
This white bird is conspicuous on many Pacific Islands
(e.g., Saipan, Rota, Tinian) that have prominent cliff lines like Guam.
Nests in holes and crevices on limestone cliffs were vulnerable to predation
by the brown Treesnake, ultimately leading to abandonment of Guam as
a nesting site.
Brown noddys nest in numbers on nearby Cocos but have
not successfully nested on Guam since snake populations peaked in the
1970s and 1980s.
Chunge' (CNMI and Guam)
This widespread, common, and conspicuous seabird has
abandoned Guam as a nesting site and is rarely seen except on Cocos where
it nests in large numbers.
Species extirpated from Guam prior to the introduction of the brown
The dependence of this species on loose soils appropriate
for it to dig burrows and deposit eggs restricted its nesting to limited
sites. Adults and young in nest burrows also tended to allow predators
such as snakes and rats to come close instead of fleeing.
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