Fly Me to the Moon: The Incredible Migratory Journey of the Arctic Tern

tags: evolutionary biology, behavioral ecology, migration, microtechnology, geolocator, natural history, biological hotspots, longest migration, seabirds, Arctic Tern, Sterna paradisaea, researchblogging.org,peer-reviewed research, peer-reviewed paper

For decades, it was widely suspected that a small seabird, the Arctic Tern, Sterna paradisaea, migrates an estimated 40,000 km each year -- the longest migratory journey of any animal.

"This is a mind-boggling achievement for a bird of just over 100 grams," says Carsten Egevang, a seabird researcher with the Greenland Institute of Natural Resources.

Despite generally being acquainted with Arctic Terns' migratory flyway, no one really knew precisely where these birds went nor how far they actually traveled on each trip.

"From ringing, we knew where the Arctic tern traveled," explains Mr Egevang, who spearheaded a new study of the birds' migratory habits. "There have been all kinds of theories, but now, for the first time, we've been able to show what the birds are doing out there."

Until recently, only larger birds could be followed on their journeys, but thanks to new microtechnology, it is now possible to track much smaller birds, such as the diminutive Arctic Tern. Data collected by these new devices show that these birds travel an average of 70,900 km each year, confirming that Arctic Terns really are the world's champion commuters.

If you do the math, you'll find that during the average tern's lifetime of up to 34 years, they fly a remarkable 2.4 million kilometers. This is equivalent to traveling around the Earth 60 times or completing three round trips to the Moon!

"We have shown that Arctic Terns can annually migrate up to 81,600 kilometres -- twice as far as previously thought," Mr Egevang marvels. "They are the true kings of commuters, not the sooty [shearwater]."

Using similar technology, the much larger Sooty Shearwater, Puffinus griseus, was found to travel 64,000 kilometers (nearly 40,000 miles) whilst flying from New Zealand to the North Pacific Ocean in search of food (DOI: 10.1073/pnas.0603715103).

An international team of scientists from Greenland, Denmark, the US, the UK and Iceland made this startling discovery after attaching tiny "geolocator" tracking devices to the legs of Arctic Terns and recording each individual's journey for one year. Seventy birds were originally tagged with the new devices, which weigh just 1.4 grams or one-twentieth of an ounce (see below).

The geolocators were provided by the British Antarctic Survey (BAS) and are similar to devices used previously to follow the migratory journeys of North American passerines (DOI: 10.1126/science.1166664). With an accuracy of 185 km in flying seabirds, the geolocators were used to document the migration routes, stopover sites, and wintering areas of the Arctic terns.

The devices function by recording light intensity. These archived data provided the researchers an estimate of the local day length, and the times when sunrise and sunset occurred. After analyzing each geolocator's sunrise and sunset data, the researchers estimated the geographical position for each of the birds and plotted it on a map (Figure 1).

"The use of these devices on seabirds is not only revolutionizing our understanding of migration patterns, but the resulting data on distribution also help address the requirement to identify important biological hotspots," comments one of the study's co-authors, Dr Richard Phillips, an evolutionary ecologist and albatross expert at BAS.

After being tagged in their high-Arctic breeding colonies in Greenland and Iceland, eleven of the birds were recaptured when they returned to their breeding colonies the following year.

The analyzed geolocator data revealed some surprises. First, these data showed that the birds do not follow a direct route; instead, they wander south along either the African or Brazilian coasts -- even zigzaging across the Atlantic Ocean -- between their breeding colonies and winter feeding area.

Along the way, the birds stop at a deep water area in the middle of the North Atlantic Ocean. This previously unknown stopover is located roughly 1,000km (620 miles) north of the Azores, according to the researchers. This stopover is a rich "biological hotspot" created by the confluence of cold northern waters and warm southern waters.

"We were able to compare biological productivity in the ocean from satellite imagery and we could see a high productive area that the birds will spend time in," reports Mr Egevang. "Even more importantly, it's the last high productive area before they enter tropical waters where we know productivity is low."

The terns remained in this biological hotspot for 10 to 30 days, resting and refueling by eating fish and zooplankton before resuming their southward journey to Antarctica.

Another surprise was that when the birds returned to their Arctic breeding grounds the following year, they did not retrace their southbound journey nor did they follow the shortest route. Instead, the birds traveled along a different route entirely; an "S"-shaped path that crossed the middle of the Atlantic Ocean. Their northbound journey ends up being roughly 10,000 km shorter than their more convoluted southbound trip.

"They make a detour of several thousand kilometers," explains Mr Egevang when comparing their different seasonal migratory routes. "But once we start comparing the route to the prevailing wind system, it makes perfect sense -- moving in a counter-clockwise direction in the Southern Hemisphere, and clockwise in the Northern Hemisphere. It's just more energy-efficient for them to do that even though they are traveling several thousand more kilometers than if they flew in a straight line."

Interestingly, despite following a different seasonal migratory routes, the birds somehow manage to maintain a high level of breeding synchrony: nearly all birds arrive at their breeding colonies within one week of each other.

"The new thing is that we've now been able to track the bird during a full year of migration, all the way from the breeding grounds to the wintering grounds and back again," states Mr Egevang.

There are several more interesting biological features about Arctic Terns that this study helps emphasize. First, it raises the question of how these birds "know" which season they are experiencing. This is important because many birds rely on increasing daylength to trigger both northbound migration and moult into breeding plumage. However, when Arctic Terns depart the Weddell Sea to return to their breeding colonies, daylengths are actually decreasing.

Further, Arctic Terns apparently never experience daylengths of less than 12 hours (when they pass over the equator) and often experience daylengths as long as 24 hours. Due to their habit of "flying towards springtime" and spending significant periods of time at either pole, this species experiences more daylight than any other on the planet.

Equally intriguing; how do the birds know where in the world they are? And how do they know the date so they can maintain their high level of breeding synchrony? Do they track both the angle of the sun as well as reading the stars to provide a biological compass reading?

This new study suggests that overfishing and global warming could endanger the terns by reducing winter ice and krill in the Weddell Sea, where these terns spend their winters. Anders Møller, an ecologist at the University of Paris-South in France who was not involved with this research, agrees, and adds that climate change already is causing problems for Arctic Terns in their northern habitat.

"My research suggests that they can no longer find suitable conditions in Denmark and are being forced to fly further north before settling down to breed, causing more adults to die." [Abstract, PDF]

Source:

Egevang, C., Stenhouse, I., Phillips, R., Petersen, A., Fox, J., & Silk, J. (2010). Tracking of Arctic terns Sterna paradisaea reveals longest animal migration. Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0909493107