The widths of the Pacific continued unaltered for millions of years. Temperatures scarcely dropped there in the Ice Ages. Generation after generation of Pacific birds were able to evolve in an almost completely stable world. Birds which somehow or other had arrived on remote islands branched into different species. In the Atlantic, there was hardly time to do that between the Ice Ages…in the Atlantic endemics — species confined to particular places — only rarely evolved.
What you see when the puffins arrive in the spring is a product of this history. The Atlantic, for the past 2.74 million years has been a place of coming and going, unsettled at the deepest of levels, a system always ready to flip from relatively beneficent to deeply unaccommodating. Life does not have the time here to develop the mass of differentiated variety it has within the security of the Pacific.
…The result is that now in the North Atlantic there is relatively little local variation. Species have evolved to cope with the variability and have wide ranges across the latitudes. The Pacific is a mosaic of local land-based varieties; the Atlantic the exclusive realm of the ocean travellers, birds which have distance embedded in their way of being.
That is from the new and excellent The Seabird’s Cry: The Lives and Loves of Puffins, Gannets and Other Ocean Voyagers, by Adam Nicholson. Whether it is covering the sex lives of guillemots or how gannets abuse their children, this book is first-rate.
The puffin chapter closes with this:
Next time you sit among the puffins on a summer evening, looking at their elegance and anxiety, that is what to hold in mind: not clowns but beauties, Ice Age survivors, scholar-gypsies of the Atlantic, their minds on an everlasting swing between island and sea, burrow and voyage, parent and child, the oscillating nomad masters of an unpacific ocean.
By the way, that is a UK Amazon link above, so they had to ship my copy across the Atlantic.