The oceanic birds were probably among the earliest birds to reach the
islands of Micronesia. The presence of phosphate deposits on islands
(Fais, Angaur), denoting deposition of guano by oceanic birds (possibly
boobies, noddies, sooty terns), indicates long time residence by these
birds. A person is prone to think that these deposits must have been
made by larger concentrations of oceanic birds than are found in these
islands today. Whether there were actually more individuals present
during the period of deposition of phosphate in the lagoons of these
islands is not known, although the elevation of the lagoons (forming the
raised islands of Fais and Angaur) with the resulting freshening of the
water probably was a great attraction to oceanic birds, especially to
those which prefer to drink fresh water. According to Leonard P. Schultz
(_in litt._), the abundance of fish in the areas about these Pacific
islands has been approximately the same since Pleistocene times, so that
there was apparently no greater concentration of fish near these islands
to attract large populations of fish-eating sea birds. Probably the time
element is of sufficient magnitude to account for such deposition by
birds with a population similar to that found there today.
The oceanic avifauna of Micronesia contains birds which are apparently
from ancestral homes in the Palearctic Region, in the North and Central
Pacific, in Polynesia, in Melanesia and Malaysia, and from homes the
positions of which are uncertain because of the widespread
circumtropical occurrence of the birds. There are no sea birds that are
endemic in Micronesia.
Oceanic birds whose range is in the Northern Hemisphere (especially
Palearctica) reach the northern and western edges of Micronesia as
winter visitors. These include _Larus argentatus_, _Chlidonias
leucopterus_, and _Sterna hirundo_. Another northern gull, _Larus
ridibundus_, has been reported in the Marianas.
One bird of the North and Northcentral Pacific, _Diomedea nigripes_,
reaches the northern Marianas where it has been taken at Agrihan. It is
not unlikely that other birds of the North Pacific reach northern
Micronesia as occasional visitors.
Species of oceanic birds which are restricted in their distribution to
Polynesia and some adjacent islands and which range to Micronesia,
either as visitors or residents, include _Puffinus tenuirostris_, _P.
nativitatis_, _Pterodroma rostrata_, _P. hypoleuca_, _Sterna lunata_,
and _Procelsterna cerulea_. The islands of the vast Pacific basin offer
havens for many kinds of oceanic birds. Apparently there has been
considerable speciation among sea birds in Polynesia, especially in its
marginal areas. Micronesia has received only a small part of this
avifauna.
Two terns, _Sterna sumatrana_ and _Thalasseus bergii_, have reached
Micronesia, either directly or indirectly, each from a dispersion point
somewhere in the Melanesian or the Malayan area. These two birds are
restricted in their ranges to the western Pacific and the Indian oceans.
Many of the species of oceanic birds found in Micronesia have
circumtropical ranges. These include _Puffinus pacificus_, _P.
lherminieri_, _Phaëthon_, _Sula_, _Fregata_, _Sterna anaethetus_, _S.
fuscata_, _Anoüs stolidus_, _A. tenuirostris_, and _Gygis alba_. Some of
these kinds range along continental shores as well as in island
archipelagoes. Others, like _Gygis alba_, are rarely found along the
shores of continents or even at coastal islands.