Wynne-Edwards (1935:241) defines the offshore zone as extending to the
continental edge; however, in Micronesia where small islands rise
abruptly out of the ocean's depths, there is no useful way to separate
the offshore zone from the pelagic zone. Since certain species go
farther from the land than others, the two zones may be combined as a
single zone extending beyond the sight of land. Birds which frequent
this area beyond the inshore zone but may not range extensively at sea
include _Fregata_, _Sula_, _Sterna fuscata_, _S. hirundo_, _S.
anaetheta_, and others. The Herring Gull (_Larus argentatus_), which has
been taken in the northern Marianas, may be classed with this group
although it probably ranges widely in the open sea. Birds which spend
considerable time at sea and may seldom approach land include _Diomedea
nigripes_, the petrels (_Puffinus_ and _Pterodroma_), and possibly the
tropic birds (_Phaëthon_).
In numbers of individuals the birds inhabiting the inshore zones are
relatively more numerous than those preferring the offshore and pelagic
zones, although 12 of the 18 resident kinds of oceanic birds apparently
prefer the offshore zone, while only 6 kinds appear to be restricted
primarily to the inshore areas.