Territory of Guam

Guam Kingfisher Todirhamphus cinnamominus ©D W Ross (Creative Commons) Website

Guam is an island in the western Pacific Ocean and is an unincorporated insular area of the United States. It is one of five US territories with an established civilian government. The population is less than 175,000 people the majority of whom live in the island’s capital, Hagåtña. Guam is the largest and southernmost of the Mariana Islands.

Guam has an area of 541 km2 (209 square miles), making it the 32nd largest island of the United States, but is the largest island in Micronesia. This island chain was created by the colliding Pacific and Philippine tectonic plates. The Mariana Trench, a deep subduction zone, lies beside the island chain to the east. Challenger Deep, the deepest surveyed point in the Oceans, is southwest of Guam at 10,911 metres deep. The highest point in Guam is Mount Lamlam, which is just 406 metres high. The island is 48 km long and 6 km to 19 km wide. Being at the meeting point of tectonic plates, the island experiences occasional earthquakes. In recent years, earthquakes with epicenters near Guam have had magnitudes ranging from 5.0 to 8.7. Unlike the Anatahan volcano in the Northern Mariana Islands, Guam is not volcanically active. However, due to its proximity to Anatahan, volcanic ash does occasionally affect Guam. The northern part of the island is a forested coralline limestone plateau while the south contains volcanic peaks covered in forest and grassland. A coral reef surrounds most of the island, except in areas where bays exist that provide access to small rivers and streams that run down from the hills into the Pacific Ocean and Philippine Sea.

Inajaran Falls – ©David Burdick/NOAA Public domain via Wikimedia Commons

The climate is characterised as tropical marine. The weather is generally hot and very humid with little seasonal temperature variation. The mean highs of 30 °C and a mean low of 24 °C with an average annual rainfall of 96 inches. The dry season runs from December through June. The remaining months constitute the rainy season. The months of January and February are considered the coolest months of the year with night time temperatures in the mid to low 70’s and generally lower humidity levels. The highest risk of typhoons is during October and November. They can occur, however, year-round.

Birding Guam

Guam has several ecological issues. From the seventeenth through nineteenth centuries, the Spanish introduced pigs, dogs, chickens, the Philippine Deer Cervus mariannus, Black Francolins, and Water Buffalo (known as carabao locally and having cultural significance). Herds of these animals obstruct military base operations and harm native ecosystems. After birth control and adoption efforts were ineffective, the U.S. military began culling the herds (2002).

Mount Lamlam – ©LittleT889 CC BY-SA 4.0 via Wikimedia Commons

Other introduced species include imported Cane Toads, the giant African Snail (an agricultural pest introduced during WWII by Japanese occupation troops) and more recently frog species which could threaten crops in addition to providing additional food for the Brown Tree Snake Boiga irregularis population, which was the most disastrous accidental introduction of all probably by US troops. Reports of loud chirping frogs, known as coquí, that may have arrived from Hawaii have led to fears that the noise could threaten Guam’s tourism. An infestation of the Coconut Rhinoceros Beetle (CRB), Oryctes rhinoceros, has also been detected on Guam (2007). The invasive snake has been responsible for extirpating the majority of the native bird species. Consequently, there are captive breeding programmes with the remaining native birds and attempts to eradicate the snakes. Guam Kingfishers still only exist in captivity, but the Guam Rail is being released from the captive breeding population into two snake-free islands Rota and Cocos. In some areas the snakes have been reduced in numbers by trapping and poisoning with paracetamol laced mice, but up to two million may still remain.

Number of Species
  • Number of bird species: 146

    (As at June 2024)
  • Number of endemics: 2

    Guam Rail Gallirallus owstoni is only extant in captivity and the re-introduction on Cocos and Rota Islands.
    There is also a captive population of Guam Kingfisher Todiramphus cinnamomius
  • Avibase

    PDF Checklist
    This checklist includes all bird species found in Guam , based on the best information available at this time. It is based on a wide variety of sources that I collated over many years. I am pleased to offer these checklists as a service to birdwatchers. If you find any error, please do not hesitate to report them.
  • Bird ID

    Annotated List
    Alternatively if the bird you are looking for is about the same (size, beak, tail etc) as a bird you know but different colours, you could use the Similar To Search, select the bird you know and then adjust the colours to match the bird you are identifying and and view a few pages of 12 photos to find your bird.
  • Wikipedia

    Annotated List
    This is a list of the bird species recorded in Guam. The avifauna of Guam includes a total of 146 species as of August 2021, according to Bird Checklists of the World. [1] Of them, eight have been introduced by humans and 32 are rare or accidental. 3 species are endemic, of which one is extinct and two are extinct in the wild though their reintroductions are either in progress or planned. Five species have been extirpated.
  • eBird

    PDF Checklist
    125 species (+17 other taxa) - Year-round, All years
Trip Reports
  • 2010 [01 January] - Jon Hornbuckle

    …Golden and Bridled White-eyes, White-throated Ground-Dove, Mariana Fruit-Dove and Rufous Fantail. Taped out Nightingale Reed-Warbler a little lower down at the entrance to the Hadderantanki Trail…
  • 2012 [03 March] - Gail Mackiernan - Western Pacific Cruise (New Zealand to Japan)

    …The Statendam docked at Guam at about 8.00 a.m giving us the whole day to explore the island. The island is virtually devoid of land birds except Drongos and Pacific Doves, thanks to the brown tree snake. Despite this we had rented a car and decided to tour around and see what birds we could find. We had a surprisingly enjoyable day seeing a good variety of migrant shorebirds and other wetland species. Yellow Bitterns were very common in the coastal marshes. We managed to find ten species of waders including beautiful spring plumage Sharp-tailed Sandpipers and Long-toed Stints. Most of these birds were on a small tidal creek that is part of the Sasa Bay Reserve and only a short drive along the coastal road south from the dock…
  • 2023 [04 April] - Doug Whitman

    PDF Report
    Northern Mariana Islands: Rota, Saipan, Tinian Guam Federated States of Micronesia: Chuuk, Pohnpei, Kosrae
Other Links
  • Guam Broadbill Myiagra freycineti

    Species profile
  • Guam Rail

    The Guam rail (Gallirallus owstoni) is a species of flightless bird, endemic to the United States territory of Guam.
  • Native Forest Birds of Guam

    The limestone and ravine forests of Guam have historically supported fourteen species of terrestrial birds. Two of these fourteen birds are endemic to Guam at the species level. Five bird species are endemic at the sub-species level. One of the endemic species is now extinct. Seven have been extirpated from the island.

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