And yet, bird migration has fascinated people since ancient times. The Greek philosopher Aristotle was one of the first to write about the subject, describing how cranes traveled from the steppes of Scythia to the marshes at the headwaters of the Nile. However, he was also the originator of two common superstitious explanations for birds disappearing in winter: that they either hibernated, or turned into other species. A third common misconception, originating from a pamphlet published in 1703, was that birds actually spent the winter on the moon. The truth about bird migration involves neither hibernation nor visits to the moon, but still goes beyond any imagination. Birds traveling from their breeding areas in temperate latitudes to wintering grounds in the tropics, or even farther, often cover thousands of kilometers each way. Absolute record breaker is the Arctic tern (Sterna paradisaea), which flies a phenomenal round trip of around 32,000 km per year, from the Arctic to the Antarctic and back. Bar-tailed godwits (Limosa lapponica) breeding in Western Alaska are hardly less impressive. To spend their non-breeding season in New Zealand, they make an 11,000 km journey, flying for six days without ever stopping to rest or refuel. Upon arrival in New Zealand they are near starvation, having lost nearly half their bodyweight. "The main question, of course, is why on earth these birds go through so much trouble twice a year", says prof. dr. Theunis Piersma, an evolutionary biologist at the Royal Netherlands Institute for Sea Research and at the University of Groningen. Piersma has spent his entire career studying bird migration, and continues to be amazed. "Why do birds fly all the way to the Arctic to breed? One theory says that it has to do with food being more abundant in the Arctic summer, due to the continuous sunlight. Another stipulates that predation pressure is lower up north. Both might play a role, but personally I think there is more to it than that."

Bottleneck
Some waders, explains Piersma, show an astonishingly low genetic diversity. This indicates that their populations might have experienced a bottleneck in recent history, meaning that the current population originates from one that was much smaller at one point in time. Piersma names the red knot (Calidris canutus) as an example. "Red knots probably suffered towards the end of the last ice age, ten to twenty thousand years ago", he says. "The relatively sudden rise in temperature reduced their tundra habitat. Also, due to the fast sea level rise, there were hardly any tidal mudflats in temperate zones. And it is these habitats that the birds depend on during most of the year." The small, surviving population expanded to its current size, but genetic diversity remained low. Piersma: "This means that the birds today are much more sensitive to diseases and parasites. You can easily house other waders in a filthy cage, but if you do that to a red knot it will die." The Arctic, contrary to temperate zones, is poor in parasites and other pathogens. This might explain why chicks in the tundra grow so much faster than their temperate counterparts, despite the Arctic weather conditions. "Birds in these areas need to spend far less energy on their immune system. Instead they can use all their energy to keep themselves warm and to grow fast." The same is true for marine coastal areas. These, too, are relatively 'clean'. "This is why there is a link between breeding in the Arctic and wintering on tidal mudflats", notes Piersma. "There is a stunning correlation." Red knots, however, have not developed their migration only during the past ten thousand years. "Migration has always existed", he says, "but the way in which it exists and develops depends on the circumstances. There are some species in which certain populations migrate and others do not. And in red knots there are six subspecies and six entirely different migration systems. It is impossible for all these systems to have developed only so recently." Piersma suggests that all traveling birds may have blueprints of their migration in their genes, remaining inactive when they are not needed, and being activated when the circumstances demand it. "Ice ages have occurred many times in history", Piersma explains, "so some of these systems have been switched on and off repeatedly."

Conservation challenge
Fact remains that migration is extremely demanding. Birds have developed intricate physiological mechanisms to help them fly days on end at speeds exceeding seventy kilometers per hour. When preparing for migration, bar-tailed godwits gain two ounces of fat in just a few weeks, nearly doubling their minimal body weight. "When the fat store is depleted during their non-stop flight, the birds switch to burning proteins instead. They actually consume some of their own muscle tissue", says Piersma. This extreme struggle for survival makes migratory birds highly vulnerable to disturbances. In order to store sufficient amounts of fat and protein while preparing for migration, the birds need not only good-quality food, but also a safe and quiet habitat. Hunting and habitat destruction are common stress factors throughout the flyway. Scientists have also started to note the effects of climate change. In fact, birds serve as early indicators. Pied flycatchers (Ficedula hypoleuca), for instance, depend on caterpillars to feed their chicks. They time the hatching of their eggs to coincide with the peak in caterpillar abundance. "For the past few years biologists have noticed that flycatcher populations were decreasing", says Piersma. "Spring is setting in earlier and earlier, and the caterpillar peak has followed. Flycatchers, however, time their migration according to day length rather than temperature, so in areas with higher spring temperatures they tend to arrive too late." The conservation of migratory species is an international affair. For one thing, tackling issues like climate change demands international cooperation. Secondly, the birds themselves fly across country borders. Protecting migratory birds against hunting and habitat loss in one place is only effective if similar measures are applied throughout the flyway. One mechanism for international conservation is the African-Eurasian Migratory Waterbird Agreement (AEWA). This Agreement, which celebrates its tenth anniversary this year, is the largest of its kind, and was concluded under the Convention on Migratory Species. Currently 51 countries, ranging from Sweden, Lithuania and Uzbekistan to Senegal, South Africa and Mauritius, have signed AEWA. It is not only a framework for legislation, but also for research and education. Piersma finds this last aspect by far the most important. "If you want to achieve anything in conservation, you need the support of the public at large", he argues. "A strong public opinion will influence policy makers, more so than international law. In that regard I think that conservation organizations, including AEWA, have a very important role to play, especially in terms of practical activities such as awareness raising campaigns."

Exciting future
Piersma, himself an avid champion of conservation of the Dutch tidal mudflats, believes that his own role lies in unveiling the persistent secrets surrounding migratory birds. How exactly do they find their way? Which factors determine survival? Why do certain populations decline while others do not? And how do birds pull off these amazing physiological tricks? Piersma: "For some species we don't even know exactly which route they follow, and whether they stop at all on the way, or even where they spend the winter. I see an exciting future for satellite and advanced radio tracking. I can't wait until the technology is further refined. And of course I'd like to continue working on my immunology theory. Although getting into that is like opening a can of worms." Each answer, after all, will evoke even more questions. Which is maybe just as well, with bird migration remaining to provide intriguing mysteries.

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