Migration Routes of Sooty Shearwaters in the Pacific Ocean

During 17 cruises, 1983 to 1991, we recorded flight directions and densities of Sooty Shearwaters (Puffinus griseus) migrating across the equatorial Pacific, between the Americas and 170°W. Sooty Shearwaters breed in New Zealand and Chile in winter (seasons given as "boreal"), migrate to the North Pacific during spring, and return south in autumn. A two-fold increase in numbers seen flying northwest from the Peru Current in spring compared to the number flying southeast on return in autumn, and a six-fold increase in numbers flying southwest towards New Zealand during autumn compared to the number migrating northeast during spring, indicates that many completed a figure-eight route (ca. 40,500 km) each year. This route would involve easterly flight from New Zealand to the Peru Current in winter, northwesterly flight to the western North Pacific in spring, eastward movement to the eastern North Pacific during summer, and southwest flight to New Zealand during autumn. We suggest that most shearwaters using this route are nonbreeders, possibly from both the New Zealand and Chilean populations. Many birds, probably breeders from both populations, likely use shorter routes to and from the North Pacific (ca. 28,000 to 29,000 km). Annual variation in the number of transequatorial migrants was positively correlated with a progressive, annual increase in sea-surface temperature, due to large-scale ocean warming in the eastern Pacific. A progressive increase in number of shearwaters migrating to the North Pacific mostly reflected increased migration from the Peru Current, consistent with a concurrent sharp decline of these birds in the California Current. These results indicate a distributional shift in feeding location during the nonbreeding period, from the eastern boundary currents to the central North Pacific, which has been exhibiting a cooling trend.