[nemo]

Seeds can float on the sea for years. Shore birds that are used to flying over the open sea do spread across much larger gaps, but the passerines and other birds adapted to life in trees or on the ground generally avoid long flights over open seas and only wind up there if blown by storms - the ocean is a very deadly place for a typical bird with no desirable resources.*

* In the Americas and some parts of Eurasia there's migratory birds which are passerines that fly over the open seas, but they evolved this behavior for adaptive reasons and only fly over the ocean for migration and then when they've fattened themselves up for weeks and prepared for a dangerous over water journey that not all will survive. In their cases body morphology evolved to allow that migratory flight.