Birds in Micronesia comprise the most outstanding animal life of the
islands, as far as vertebrates are concerned. No fewer than 206 kinds,
belonging to 37 families and 91 genera have been found there. Although
this number upon first consideration may seem large, actually any large
land mass in the same latitude has many more kinds of birds than does
Micronesia. In this connection it is pertinent to recall that the
islands of Micronesia are oceanic and have apparently been formed
independently of any continental land mass. Thus, animal life found on
these islands has reached them by overseas migration, either by some
passive means or by individual effort. Zoogeographers have had some
difficulty in explaining the presence of snails and other nonflying
animals on isolated oceanic islands. Crampton, in his studies of the
land snails of the genus _Partula_ at Guam and Saipan (1925:10), writes,
"Despite the geological difficulties, the biological findings strongly
support the view that the dominant process in this part of Oceania has
been one of subsidence and of insular dissection." Although there exists
today some question as to how certain forms of life have reached these
remote dots of land, the ornithologist has not been much in doubt as to
the actual _means_ of arrival of birds. With the exception of six kinds
of birds which are definitely known to have been introduced by man, the
birds have apparently reached these islands by flying there from
somewhere else. The ornithologist is, therefore, concerned with learning
_from where_, _by what route_, _when_, and _why_ the various species of
birds came and _how_ they have become established on these islands of
Micronesia. These birds exist in small populations; often less than 100
individuals of one kind may be found on a small island. How have such
small numbers had the ability to survive and what environmental
adaptations have occurred, are two additional questions which confront
the student of Micronesian birds.