The vast expanse of the Pacific Ocean is dotted with numerous islands,
most of which are concentrated in the central and western part and are
known collectively as Oceania. Within Oceania three divisions are
popularly recognized: Melanesia, Polynesia, and Micronesia. According to
Krieger (1943:6), the Micronesia islands include the Mariana, Palau,
Caroline, Marshall, and Gilbert islands; they may take in also the
Volcano, Bonin, and Ellice islands (from the standpoint of
anthropology). Zoogeographically, according to Wallace (1876),
Micronesia is to be included in the Polynesian Subregion of the
Australian Region. Mayr (1941a:193), on the basis of the distribution of
birds, ranks Micronesia as one of the four subdivisions of the
Polynesian Subregion, and includes within Micronesia the Palau,
Caroline, Mariana, Marshall, and Gilbert islands. Except in the
discussion of distribution, this report does not treat of the avifauna
of the Gilbert Islands, which straddle the equator south of the Marshall
Islands. This report is concerned only with the birds in the Mariana,
Palau, Caroline, and Marshall islands formerly mandated to Japan, and
with the birds of the island of Guam, which is a possession of the
United States.
[Illustration: FIG. 1. The Islands of Micronesia]
The word Micronesia is, of course, derived from the Greek words _mikros_
meaning small and _nesos_ meaning island, and, as shown in figure 1,
this term is appropriate, for the islands of this area are small. For
the most part they are too small even for inclusion on standard-sized
maps of the world. There are thousands of these islands in an area some
2,400 miles long from east to west and some 1,200 miles broad from north
to south. All of the islands of Micronesia are oceanic islands; that is
to say, they have never been connected to the Asiatic continent or to
other land masses by means of land bridges.
Geologists and oceanographers have shown (see descriptions by Hobbs,
1945), that islands of Micronesia are of two general types: arcuate and
strewn. The Pacific Ocean is surrounded by rising mountain ranges which
are arranged in elongated, near-circular arcs, which form an extended
series of scallops. In the western Pacific these sweeping arcs extend
into the ocean, where the mountain ranges project upward from the bottom
of the sea with only the crests showing above the waves to point out, in
dotted outline, the position of the mountains. The easternmost of these
arcs is marked by the islands of the Aleutians, Kuriles, Japan, Izo,
Bonins, Volcanoes, Marianas, Yap, Palaus, and others continuing
southward into Melanesia. These are characterized by igneous rocks of
andesitic nature.
To the eastward of the arcuate islands in Micronesia, are numerous and
irregularly distributed islands, making up all of the central and
eastern Carolines and the Marshalls, which are known as strewn islands.
Strewn islands mark the places of former volcanoes or volcanic peaks. If
these volcanic peaks have been completely drowned and are now marked by
a series of low islands edged by a protecting reef formed by coral
growth enclosing a lagoon and with all exposures consisting of coral
rock, the island is known as a coral atoll (example, Ulithi Atoll). Some
of the coral exposures lack lagoons; they are known merely as coral
islands (example, East Fayu). Some atolls become elevated by geologic
activity and the lagoons may dry out or drain. The accumulation of guano
of oceanic birds and the residue of fish and other organisms in the area
of the lagoon remains as a rich phosphate deposit; these raised atolls
have been called phosphate islands (example, Fais). Other strewn islands
consist of igneous rocks which are exposed above the surface of the
ocean. These are known as "high" or volcanic islands and may occur as a
single mountain rising out of the ocean (example, Kusaie), or be partly
drowned and surrounded by a coral reef (example, Truk). The igneous
rocks found on these strewn islands are basaltic in nature.
The Mariana Islands consist of a chain of volcanic islands approximately
450 miles long. As shown in figure 2, there are 14 single islands and
one group of three islands (Maug), from Uracas in the north to Guam in
the south. The Palau Islands which are situated in the easternmost part
of Micronesia have often been considered from a political standpoint as
part of the Caroline Islands. As shown in figure 3, the Palau Islands
are a chain of islands approximately 120 miles long from north to south.
Sonsorol, Tobi, Merir, Pulo Anna, and Helen Island occur to the
southward of the Palaus and may be considered as part of the Carolines
or as part of the Palaus. The Palaus together with the Carolines, to the
eastward, extend in an east-west direction for approximately 1,700
miles. The Palaus and Carolines include (as shown in figures 3 and 4) 37
atolls, 34 banks, 11 coral islands without lagoons, 2 uplifted phosphate
islands, 4 volcanic islands, and the Palau chain. The Marshall Islands
to the extreme eastward extend approximately 700 miles from north to
south and, as shown in figure 5, contain 29 atolls and five coral
islands without lagoons. No volcanic exposures occur in the Marshall
Islands.
There is a total land surface of approximately 846 square miles in the
islands of Micronesia. The Palaus and Carolines have 525 square miles,
the Marianas 247 square miles, and the Marshalls 74 square miles of land
surface. Guam has the largest land surface of any of the islands of
Micronesia with 225 square miles, Ponapé has 145 square miles, and
Babelthuap has 143 square miles. Asuncion, in the northern Marianas, has
the highest elevation, rising as an almost perfect cone to a height of
2,923 feet; Ponapé reaches a height of 2,579 feet above the sea level.
The volcanic islands are known as "high" islands, and the coral atolls
are known as "low" islands. The coral islands usually rise but a few
feet above sea level.