The streaked shearwater flies thousands of miles each year.
Seasonal produce: how shearwaters exploit the ocean's resources
7 October 2011
Shearwaters migrate thousands of miles each year in search of food, but short of taking to the air themselves, scientists have had no way of following their journey. Phil Trathan and colleagues explain how, thanks to some handy new kit, tracking shearwaters is no longer a flight of fancy.
The north-western Pacific is one of the most productive of our oceans, rich in life. It's a highly seasonal environment, so the amount of nutrients it contains, and where they are concentrated, varies over the year. This means the amount of food available to marine organisms and the animals that depend on them changes too, and food has a powerful influence on the distribution of many marine creatures, including seabirds.
The streaked shearwater, Calonectris leucomelas, flies many thousands of miles each year in response to this seasonality. Shearwaters are pelagic birds, in other words they spend the majority of their time out at sea, coming to land to breed around the coasts of east and south-eastern Asia.
After breeding, shearwaters migrate south towards the equator, to spend the winter in warmer tropical regions. But though they have been seen at various different wintering grounds, until now there has been no way of knowing where individual birds have come from, nor what influences their choice of migratory route.
Working with Akinori Takahashi and Takashi Yamamoto from the National Institute of Polar Research, we spent four years investigating the links between the region's seasonality and the birds' behaviour - how they choose their wintering grounds and migratory routes, and find food along the way. Our breakthrough came from the British Antarctic Survey's (BAS) development of some micro-electronic equipment which means that, for the first time, we have been able to track the birds' epic journeys.
The migratory routes of the shearwater where the black line indicates the route to northern New Guinea, the white line to the Arafura Sea and the red line to the South China Sea. The star indicates the breeding colony. Orange and yellow arrowed lines indicate the direction of the ocean currents.
Our information came from global location sensors which we attached to the legs of breeding birds at two shearwater colonies off the east coast of Japan: Sangan Island and Mikura Island. These clever devices weigh just 4.5g and measure 25 x 18 x 7mm but BAS has packed a lot in that small space. The sensors have a real-time clock and measure light levels, so that we can estimate longitude and latitude to work out the bird's location. They also have a saltwater activity sensor which indicates whether conditions are wet or dry - in other words whether the birds are flying (or on land) or are on the sea surface. The light sensors enable us to produce two positions for each bird each day while the activity sensor generates a wet/dry record every ten minutes; together the results effectively allowed us to produce a map of the route each bird had taken, showing when and where it had landed on the sea to feed or rest.
Our first discovery was that streaked shearwaters migrated from the seas around Japan to three main wintering areas in the tropics. Most of our birds wintered off northern New Guinea, but some went to the South China Sea, and others as far as the Arafura Sea, between New Guinea and Australia: respectively 4000km, 3500km and 5400km from the breeding colony. These different wintering sites may be traditional areas, but it remains to be seen whether individuals always use the same site in subsequent years, and if so what causes birds to select particular sites.
The area to the north of New Guinea is an area of low primary productivity (in the oceans primary producers are generally phytoplankton which form the base of the food chain), but the birds still seemed to like it there. The trackers revealed that, around dawn and dusk, the shearwaters flew for longer periods and landed on the water more frequently. What could this mean? This pattern is similar to that of other tropical seabirds that we know feed in association with subsurface predators such as tuna. We think the birds may benefit from easy access to prey driven to the ocean surface by the predator, or from scraps of fish it leaves behind. So it looks like our streaked shearwaters also have the knack of using these big fish to find their own food during the wintering period. Shearwaters are certainly efficient gliders, so they could cover large areas without using too much energy: a real issue in ecosystems where productivity is low.
We also know that the moon affects the behaviour of some marine animals, particularly zooplankton and their predators. But we don't know very much about how seabirds and marine mammals respond to the lunar cycle, mainly because it has been so difficult to record their behaviour during long periods at sea. Our GLS loggers proved sufficiently sensitive for us to record the birds' behaviour at sea throughout the winter, including how this varied with the lunar phase. Our results showed that streaked shearwaters do change their behaviour in response to the phase of the moon: birds flew for longer and landed on water more frequently on nights with a full moon than when there was a new moon. So it looks like the at-sea behaviour of these pelagic seabirds is also closely associated with the lunar cycle.
Once we knew more about the shearwaters' winter life, we turned our attention to how they lived for the rest of the year. It turned out that the birds stayed in Japanese coastal waters and their main foraging areas moved north as the breeding season progressed. The sea off the east coast of Japan is dominated by the Kuroshio and Oyashio currents, which cause sea-surface temperatures to increase rapidly from spring to summer. Shearwaters from Sangan Island focused their foraging activities along the coastal areas where the two ocean currents meet, while birds from Mikura Island moved northwards along the Kuroshio current, probably following the migration of warm-water pelagic fish - including their main prey the Japanese anchovy Engraulis japonicas.
We also saw differences in the behaviour of male and female birds. For example, during the pre-laying period, males foraged much closer to the colony than females; they spent less time at sea and returned to the colony more often. But once the eggs were incubating there were no obvious differences in foraging areas between the sexes.
So the shearwaters travel great distances as the seasons change, but once the breeding season kicks off it looks like the location of their colony, and the different roles played by the sexes, become the primary influences on their foraging.
Thanks to the detailed information the tiny data loggers recorded, we've been able to join up many of the pieces of the streaked shearwater puzzle. We now know a huge amount more about their lives on land and out at sea, and why they behave the way they do.
Phil Trathan is a senior research scientist with the British Antarctic Survey in Cambridge. Akinori Takahashi is an associate professor, and Takashi Yamamoto is a PhD student, both at the National Institute of Polar Research in Japan. E-mail: email@example.com
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