Seasonal movement patterns of streaked shearwaters Calonectris leucomelas during the non-breeding period 非繁殖期におけるオオミズナギドリの季節移動パターン
Seasonal movement patterns of streaked shearwaters Calonectris leucomelas during the non-breeding period
Organisms in polar and temperate regions live in a seasonal environment where they experience regular changes in resources and/or weather conditions. Exploring how organisms respond to seasonality in environment, and what kind of environmental characteristics influence their distributions is fundamental for understanding their habitat requirements as well as adaptations in behaviour, physiology and morphology to the environment. Recent advances in tracking technologies enable us to examine the foraging movements of a variety of seabird species during the breeding season, and to identify inter-specific and sexual segregations as well as individual consistency in foraging area. In contrast, seabird distributions outside the breeding season are much less documented due to the technical and practical difficulties. Therefore, understanding of how individual seabirds respond to seasonal changes in the marine environment during the non-breeding period, and if such the response differs between sexes, among colonies, or within individuals has been elusive. The aims of this study were to examine the seasonal movement patterns of a pelagic seabird, the streaked shearwater (Calonectris leucomelas), during the non-breeding period (including post-breeding, migration, wintering, and pre-laying periods) in relation to (1) seasonal changes in the marine environment, (2) sex-related, inter- and intra-colony, and inter-annual differences, and (3) individual consistency over successive years.The study was conducted at three breeding colonies, Sangan Island (39°18’N, 141°58’E, Iwate, Japan) and Mikura Island (33˚52΄N, 139˚14΄E, Izu Islands, Japan) located in the Pacific Ocean, and Awa Island (38°27’N, 139°13’E, Niigata, Japan) located in the Sea of Japan, from August 2006 to October 2010. I recorded the seasonal movement patterns of streaked shearwaters during their non-breeding period using leg-mounted global location sensors. I obtained data of 223 tracks, including 47 birds that tracked over two successive years.Streaked shearwaters migrated to four wintering areas in November-February during the non-breeding period: most to the seas off northern New Guinea (73.1% of migrations), and others to the Arafura Sea (17.0%) and South China Sea (9.0%), and two birds to the seas off northwestern Australia (0.9%). Furthermore, streaked shearwaters from Sangan Island predominantly migrated to the seas off northern New Guinea over four non-breeding seasons. Therefore, the seas off northern New Guinea represent the key wintering area for streaked shearwaters from the study colonies. The surface concentration of chlorophyll a was low in the seas off northern New Guinea, in contrast to previous studies showing a close relationship between primary productivity and the occurrence of marine top predators. The western equatorial Pacific is generally characterized as an oligotrophic region with deep mixed and isothermal layers, but also shows a deep chlorophyll maximum at subsurface depth. Thus, biomass, including zooplankton and mesopelagic fish, is relatively high in the epipelagic layer despite low chlorophyll a concentration at the sea surface. Streaked shearwaters showed diurnal changes in their activity in the tropical oceans, as they flew for longer periods and landed on the water more frequently around dawn and dusk. This pattern of activity is similar to that of subsurface predators, such as tuna, and to that of tropical seabirds that are known to feed with subsurface predators. It possibly indicates the feeding association of streaked shearwaters with sub-surface predators in wintering areas. The seas off northern New Guinea have one of the largest fisheries for several tuna species in the Pacific Ocean. Feeding association with subsurface predators, such as tuna, is an important foraging strategy for tropical seabirds, because subsurface predators drive prey fish nearer to the surface where the seabirds can reach them.Although most shearwaters migrated to the seas off northern New Guinea, there were sex-related, inter- and intra-colony differences in the proportion of individuals that migrated to four different wintering areas. Males were more likely to migrate to the South China Sea (11.8% in males vs. 5.2% in females), while females were more likely to migrate to the Arafura Sea (9.5% in males vs. 29.2% in females). Among birds, including males and females, migrants to the Arafura Sea started the southward migration relatively earlier, and migrants to the South China Sea started the migration later than those that migrated to the seas off northern New Guinea. Between the sexes, females started the southward migration earlier than males. Among the colonies, shearwaters from Awa Island were less likely to migrate to the seas off northern New Guinea, compared to the other breeding colonies. There were two different migration routes in shearwaters from Awa Island: Pacific Ocean route and Sea of Japan route. Fewer birds that traveled along the Sea of Japan migrated to northern New Guinea (33.3% of 18 birds) than did those that traveled in the Pacific Ocean (61.3% of 31 birds). Migratory routes along the Sea of Japan were closer to the South China Sea and Arafura Sea, compared to those in the Pacific Ocean. Therefore, the South China Sea and Arafura Sea were more accessible to migrants that traveled along the Sea of Japan from Awa Island.In this study, I found remarkable individual consistency not only in wintering areas, but also for the timing of southward migration. Individuals that were tracked for two successive years migrated to the same wintering areas at a similar timing to the previous year. This may suggest that streaked shearwaters possess individual-specific migratory schedules that are possibly under the control of endogenous time programmes.After returning to the seas around the breeding colony in March (the pre-laying period), streaked shearwaters changed their foraging areas in relation to seasonal changes in sea surface temperatures of the North-western Pacific. Females moved their foraging areas northwards from April to July, but not being apparent for males. This was probably because females followed changes in the distribution of their prey, such as Japanese anchovy, that are known to conduct a seasonal northward migration in relation to the increase in sea surface temperature from spring to summer. In contrast to females, males mainly foraged in the areas around the breeding colony from April to June. During this period, males returned to the colony more frequently than females, probably to defend their nests from prospectors. Thus, sexual differences in breeding role may limit foraging ranges, leading to the different seasonal movement patterns between the sexes.The results from the present study showed that streaked shearwaters have a unique seasonal movement pattern compared to other shearwater species, as they forage in a temperate region during the breeding season and winter in tropical oceans during the non-breeding season. Pelagic seabirds often occupy similar latitudinal areas year-round either by conducting a trans-equatorial migration or remaining in areas around the colony, because general characteristics of the marine environment differ between temperate/polar and tropical oceans, and may require different foraging tactics. Streaked shearwaters forage mainly at the sea surface and are considered as an excellent glider in terms of their morphological characteristics. These behavioural and morphological characteristics appear to be adaptive to the tropical marine environment, and may enable this species to occupy both temperate and tropical marine environments.