Tens of thousands of pelagic birds return to the site every spring to breed, after wintering at sea or in more southern climates, principal species being puffins, razorbills, kittiwakes, fulmars and guillemots.
This passage describes the annual migration pattern of seabirds that breed in a particular site after spending their winters at sea or in more southern climates.  The main species mentioned are puffins, razorbills, kittiwakes, fulmars, and guillemots.