Photo Credit: Viola Toniolo

Two Adélie penguins, with the one in the foreground sporting a satellite tag used to track its migration from its southern breeding grounds at Ross Island to the north in the winter and back again.

On the move

Researchers follow Adélie penguin winter migration for the first time

By Peter Rejcek, Antarctic Sun Editor

Posted August 13, 2010

Adélie penguins living at the far southern extreme of their geographic range migrate an average of about 13,000 kilometers during the year as they follow the sun from their breeding colonies to winter foraging grounds and back again.
“They’re definitely making a longer distance migration than we thought,” said Grant Ballard

, lead author of a study recently published in the journal Ecology that examined the birds’ past and present migration patterns, and how changes in climate and sea ice extent might affect migration patterns in the future.
The researchers tracked penguins from two colonies on Ross Island using geolocation sensor tags between 2003 and 2005. The birds came from Cape Royds, a small colony of about 2,500 breeding pairs and the farthest south congregation of penguins in the world, and Cape Crozier, a much larger colony of 150,000 breeding pairs.

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Ballard and his colleagues, including long-time polar research and co-author David Ainley with H.T. Harvey and Associates

, already knew the Adélies ventured north during the winter because they need light and some amount of open water to forage in the ocean.
But the tags revealed many surprises about where the birds went and how quickly they moved. Data collected by the tags also showed that while Adélies need light for navigation and fishing for food, they can operate under extremely low light conditions.
“This is the first time we know where the Adélies go and the environmental conditions they encounter during the winter,” said Ballard, a staff scientist at PRBO Conservation Science

, a California-based wildlife conservation and research non-profit organization.
And while the birds venture far north in the winter, following the setting sun and the twilight conditions they need to survive, they stop about 500 kilometers from the edge of the ice and the open ocean.
“We assumed that they went all the way out to the ice edge. They actually stay fairly well inside,” Ballard said.
The scientists believe the flightless birds save energy on their northbound trip by moving with the ice floes, which follow ocean currents in a clockwise gyre around the Ross Sea. The return trip to their breeding colonies, where they need ice-free land to make nests and breed, is not as leisurely. Ballard said the penguins double-time it back to Ross Island in the spring.

Photo Credit: Emily Stone/Antarctic Photo Library

Scientist David Ainley at Cape Royds.

Photo Credit: Nate Biletnikoff/Antarctic Photo Library

Adélie penguins float on an iceberg near Cape Crozier.

Photo Credit: Emily Stone/Antarctic Photo Library

A penguin preens its feathers at Cape Royds.

“They’re coming back as early as they can,” he said. “They probably need the sun for navigation. We think that’s the main way they navigate. The study strongly suggests that is what is going on.”
The distances covered by the Ross Island penguins, with the longest trek of 17,600 kilometers, represent the longest migration of the species, according to the Ecology paper. In comparison, the most well-traveled seabird in the world, the Arctic Tern, makes a pole-to-pole return journey of 44,000 kilometers.
The ability to migrate over long distances may be an ongoing adaptation in the evolution of the species, the authors suggest.
During the Last Glacial Maximum (LGM) about 20,000 years ago, the West Antarctic Ice Sheet covered almost the entire Ross Sea, meaning most of today’s colonies didn’t exist. However, the authors speculate that one colony might have persisted through the LGM at northerly Cape Adare, which historically has been ice-free during previous glaciations.
Over the last 12,000 years, the ice sheet has retreated, and the Adélies have penetrated farther south to their current location, developing a strategy to move to and fro between summer breeding colonies and winter foraging areas.
However, the authors warn that as climate change finally hits the Ross Sea region in the coming decades, as predicted by some climate models, the Adélies of Ross Island will face difficult challenges.
In such a scenario, winter sea ice — a habitat for the Adélies, as well as key prey like krill and silverfish — will retreat south, as it has in the already-warming Antarctic Peninsula. At some point, the edge of the ice could peel back far enough south that the penguins won’t be within reach of twilight in the winter.
Ballard said it’s possible the Adélies could go extinct locally. The species is notoriously “hard-wired” by its biology, and individuals rarely abandon their birth colony.
However, in a different paper published in June 2010 in the journal Proceedings of the National Academy of Sciences by some of the same scientists, researchers reported that Adélie penguins can indeed move when conditions for raising young deteriorate.
That study was based on a natural experiment that occurred when giant icebergs that calved off the Ross Ice Shelf in 2000 locked in hundreds of additional square kilometers of sea ice for several years. That meant a long, long trek for the Cape Royds penguins to reach open water to forage food for their chicks each summer.
Some birds eventually moved to new nesting sites, choosing colonies that had access to better feeding sites, regardless of the size of the colony.
The same instinct for survival could come into play in the future. After all, the Adélies have survived the ebb and flow of ice ages for hundreds of thousands of years. The question is whether they can adapt in time.
“We still don’t understand all of the dynamics of the Adélies and the ice, but we’re getting closer,” Ballard said.
NSF-funded research in this story: Grant Ballard, PRBO Conservation Science, Award No. 0439759 ; and David Ainely, H.T. Harvey and Associates, Award No. 0440643 . Other co-authors on the Ecology paper include Viola Toniolo, Stanford University; Claire L. Parkinson, NASA/Goddard Space Flight Center; Kevin R. Arrigo, Stanford University; and Phil N. Trathan, British Antarctic Survey.

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