Another point is obvious everywhere: the longer a place has been studied, the greater the number of species recorded there. New species are continually added to local lists. Even in an area as well watched as Britain, more than 70 newly recorded species were added in the 25 years after 1980, an average of nearly three per year (Pitches & Cleeves 2005). The rate of addition would be expected to slow over time, as the cumulative total begins to reach a plateau, but the terminal stages of this process clearly take a long time - more than a century. And all the time, bird distributions are changing, bringing additional species within range. In the first 17 years that the bird ringing station had operated at Eilat in Israel 139 354 individuals of 268 species had been trapped. The cumulative species total increased at a progressively slowing rate over the years, but even in the later years some 2-3 new species were added annually (Yosef & Tryjanowski 2002a).
For obvious reasons, vagrancy is much harder to study in pelagic seabirds than in landbirds, because it depends largely on birds being sighted from land or washed up dead on shorelines (to which they may have been transported long distances by currents). Away from their nesting places, pelagic birds sometimes fly over land, either deliberately or accidentally during storms. This habit may explain some occurrences in the 'wrong' ocean. Thus, did the Mottled Petrel Pterodroma inexpec-tata from the Pacific reach New York State by crossing the Isthmus of Panama to get into the Atlantic Ocean, or did the Great Shearwater Puffinus gravis that reached California cross from the Atlantic in the opposite direction, or simply start its northward migration from the wrong sector of the Southern Ocean? Other examples of long-distance vagrancy in seabirds include two Short-tailed Shearwaters Puffinus tenuirostris and two Laysan Albatrosses Diomedia immutabilis seen in the Indian Ocean rather than the Pacific, and Jouanin's Petrel Bulweria fallax seen in the Pacific (near the Hawaiian Islands) rather than the Indian Ocean (Bourne 1967, Warham 1996). As oceanic bird distributions are still poorly known, however, some records may relate to individuals from small undetected nesting colonies well away from the known range, rather than genuine vagrants.
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