Migratory shearwaters integrate oceanic resources across the Pacific Ocean in an endless summer

Abstract

Electronic tracking tags have revolutionized our understanding of broad-scale movements and habitat use of highly mobile marine animals, but a large gap in our knowledge still remains for a wide range of small species. Here, we report the extraordinary transequatorial postbreeding migrations of a small seabird, the sooty shearwater, obtained with miniature archival tags that log data for estimating position, dive depth, and ambient temperature. Tracks (262+/-23 days) reveal that shearwaters fly across the entire Pacific Ocean in a figure-eight pattern while traveling 64,037+/-9,779 km roundtrip, the longest animal migration ever recorded electronically. Each shearwater made a prolonged stopover in one of three discrete regions off Japan, Alaska, or California before returning to New Zealand through a relatively narrow corridor in the central Pacific Ocean. Transit rates as high as 910+/-186 km.day-1 were recorded, and shearwaters accessed prey resources in both the Northern and Southern Hemisphere's most productive waters from the surface to 68.2 m depth. Our results indicate that sooty shearwaters integrate oceanic resources throughout the Pacific Basin on a yearly scale. Sooty shearwater populations today are declining, and because they operate on a global scale, they may serve as an important indicator of climate change and ocean health.

Fig. 1.

Shearwater migrations originating from breeding colonies in New Zealand. (a) Interpolated geolocation tracks of 19 sooty shearwaters during breeding (light blue) and subsequent migration pathways (yellow, start of migration and northward transit; orange, wintering grounds and southward transit). The 30° parallels, equator, and international dateline are indicted by dashed lines. (b–d) Representative figure-eight movement patterns of individual shearwaters traveling to one of three “winter” destinations in the North Pacific. These tracks also represent those of three breeding pairs to reveal the dispersion and extent of each pair. The image was created by using the Blue Marble data set (15).

Fig. 2.

Sooty shearwater diving depths and frequency (a), sea surface temperatures experienced (b), and primary productivity (c) at each dive location, in relation to latitude. Sea surface temperatures (SST) were recorded by the archival tags on each bird just before a dive. Primary productivity (PP) was measured remotely by satellite and overlaid onto the locations of each dive (see Materials and Methods). Note the paucity of dives, warm sea surface temperatures, and low primary productivity at low latitudes of the South and North Pacific when shearwaters cross the equatorial region.

Fig. 3.

Sooty shearwater latitudinal movements (white open circles, n = 4,375 filtered locations) and primary productivity in the South (yellow) and North (red) Pacific throughout the year. Primary productivity is represented by the mean (±SD) of 8-day productivity from 1997 to 2005 encompassing the geographic regions where sooty shearwaters most frequently occurred (see Materials and Methods). The shaded regions represent the time periods for breeding (BR), migration, and prenuptial (PN) phases. Note that chick-hatching (A) occurs during the austral summer when productivity becomes higher in the South Pacific than in the North Pacific. Upon completion of breeding, the onset of migration coincides with the period when primary productivity becomes higher in the North Pacific (B). Primary productivity in the North Pacific peaks several months before shearwaters return to the South Pacific. Shearwaters conduct the reverse migration in October when productivity is still higher in the North Pacific. However, the timely return is required so adults can court during the prenuptial phase and lay an egg that will hatch (A) when productivity is the highest available at that time (i.e., between A and B).