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I have always found bird migration fascinating. The thought of these creatures, sometimes just small scraps of feather, covering such huge distances year after year is just staggering. So, when I found out about this book by Scott Weidensaul, I was keen to read it to find out more about the awe-inspiring feat that is migration. Weidensaul’s passion for birds has led to him becoming involved in a number of ringing (known as banding in the US) projects. These, and the many other research programmes he explores in A World on the Wing have revealed some incredible stories. More importantly, this research is helping conservationists to protect some of the most vulnerable species on the planet.
Weidensaul begins with a trip to the Yellow Sea in China. This is one of the most important stopover areas for migrating waders in the world, with countless birds passing through in spring and autumn on their way to breeding or wintering grounds. The whole area is under huge pressure, however. Industrial, agricultural, aquacultural and recreational developments are all eating up the mudflats that feed these migrating birds. Although charismatic species like the spoon-billed sandpiper are helping to persuade politicians that it needs preserving, the situation is on a knife-edge. Stopover sites like these are crucial because if waders can’t feed up sufficiently en route they will be in poor condition when they arrive at their breeding grounds, if they get there at all. They may then not have the energy to breed successfully.
Waders are understandably a key bird group when it comes to migration. Most wader species make long-distance migrations, with some making non-stop journeys of more than 5,000 km. Those bar-tailed godwits breeding in Alaska travel the furthest without stopping. (Just this autumn, scientists revealed that a young godwit born this summer had set a new record, travelling 13,560 km non-stop from Alaska to Australia.) These long migrations mean that waders are extremely vulnerable. Conditions at the breeding grounds, wintering grounds, stopover sites and on the journeys between all these mean they face a multitude of risks at each stage. Unsurprisingly, most wader species are in decline globally.
The Mechanics of Migration
The book also explores some of the mind-blowing aspects of exactly how birds are able to perform these feats of migration. The physiological changes alone are almost beyond belief. Some of the wader species we met in the first chapter essentially cannibalise their muscles and organ tissues to provide the necessary energy to travel such huge distances. Godwits not only double their weight by feeding up first, but they then shrink their intestines and gizzards whilst increasing the mass of their pectoral and heart muscles. Lung capacity also grows. In addition, they increase the amount of red blood cells in their systems, which means they can access more oxygen when breathing. These are amazing physical changes, and that they undergo them twice each year is almost beyond comprehension. No one knows how they survive these enormous physical strains.
Scientists are also only just beginning to understand how birds navigate, according to Weidensaul. Although a few species, including geese, swans and cranes, travel together in groups, most young birds find their own way. An inbuilt genetic road map tells them to fly in a certain direction, for a certain length of time and at a particular time of year. Scientists used to think that many birds navigated using the tiny deposits of magnetite found in their beaks. They assumed this acted in tandem with the Earth’s magnetic field as a compass of sorts. However, it now seems there is actually some much more complicated quantum entanglement stuff going on. I have to admit, this was one small section that was beyond my understanding!
Although there is still so much we don’t understand about migration, including the routes some birds take and where they winter, advances in technology mean we are now able to find out so much more about individual species. For decades, researchers had to rely on ringing, and then recapturing, birds to find out where they went and how long they lived. More recently, scientists began using radar to look for migrating birds. Used primarily to look at weather systems, the system can show the speed and direction of migrating birds. Audio recording and citizen science databases such as eBird can then be tied in to identify individual species, something radar can’t do.
Miniaturisation has also had a massive impact on research. As tracking devices have got smaller, the range of species that scientists can put them on has dramatically increased. Small warblers can now have trackers fitted that show migration routes and timing. The bird doesn’t even have to be recaptured the following year with some trackers. These tags are revealing some incredible facts. They have shown that some Hudsonian whimbrels, for example, deliberately fly into Atlantic storms in order to sling shot themselves south. They have also revealed that common swifts spend ten whole months on the wing, without landing at all. Crucially, these advances and the information they reveal mean conservationists can target their efforts and often limited funds much more effectively.
Climate Change and Migration
One of the biggest threats to migrating birds that Weidensaul identifies is climate change. Weidensaul points out that conditions on a birds’ wintering grounds have an enormous impact on how likely they are to breed successfully the following spring. As the planet warms, important wintering locations such as the Caribbean and Africa’s Sahel region are receiving less rain. This means birds have less access to food and often arrive on their breeding grounds late and in poor condition. Some birds, such as the pied flycatcher, are unable to keep up with earlier and earlier springs. They arrive back on their breeding sites too late to take advantage of their usual food sources. Short distance migrants seem to be able to shift their migration to keep up, but not longer distance ones. Rising sea levels could also shrink suitable habitats
Research shows that some species have been able to adapt, however. Sometimes this is down to a quirk of genetics. That genetic road map that tells a bird which direction to fly in sometimes goes wrong. Often, this would spell doom for the bird as it flies to somewhere with no suitable habitat or that is too cold to survive in. Climate change, though, means that some places that were previously too cold for these accidental visitors are now bearable. Despite their faulty genes, they can now survive to return to breed the following year. The new genetic road map is in turn passed on to their offspring and a new migration route opens up. Some European blackcaps are now wintering in the UK as a result of this phenomenon. Increased bird feeding in gardens is also helping them. In the US, a number of hummingbird species that accidentally head east are now able to survive and feed up before reorienting themselves to head south.
Hunting and Hope
Hunting is also a big problem for migrating birds. An estimated 1.3 to 3.2 million birds are killed each year in Cyprus alone. Birds migrating in the Mediterranean basin are also killed in other countries including Malta, Italy, France, Egypt and Lebanon. The methods used are often indiscriminate, such as mist nets. This means that targeted songbirds, such as blackcaps and thrushes, are caught alongside bee eaters, owls and other birds. There is hope though. Weidensaul ends with a look at how the people of Nagaland in India were persuaded in an extremely short space of time to stop hunting the thousands of Amur falcons passing through each autumn.
Weidensaul is a wonderful communicator. His practical experience in the field means that he has seen many of these amazing feats first-hand. However, that he is first and foremost a birder means he is able to explain some complex topics clearly for a general audience. The book is extremely readable. His passion shines through at all times. Despite the fact research has cleared up some of the mysteries of migration, he retains a sense of wonder at this amazing phenomenon. Ultimately, he has complete reverence for the ‘endurance and tenacity’ of migrating birds. After reading this book, it is impossible not to feel the same.
For a more scientific and less personal look at bird migration, try Ian Newton’s New Naturalist title. It is extremely readable for such a dense book.
The BTO’s Flight Lines project linked researchers, artists and photographers to tell migration stories. Beautifully illustrated, this is a different approach to showing the wonders, and perils, of migration.