Bird Migration

Author Ian Newton, Harper Collins, 2010, 596 pages, ISBN 978-0-00730731-9 (HB) 978-0-00730732-6 (PB)

There is no doubt that the story of bird migration is huge – not just the distances covered, but the reasons for it, and the consequences of it. Just trying to comprehend the immense number of facts involved in this phenomenon is exhausting, but here, in one volume, Ian Newton has condensed everything that is known about the subject, and in his characteristic style he has explained complex theories with a clarity and conciseness that makes the subject available to everyone.

Importantly, this book (like all in the New Naturalist series) only tries to assess the subject from a British Isles perspective and as a result it is both readable and highly relevant. Starting with a summary of migration studies, Newton then sets the scene by outlining the different types of migration through each season, and he then looks at the ways that birds both large and small tackle the challenge of long-distance flight. The positive and negative effects of weather and altitude are explored, as is the importance of food both before and during migration. Many examples are given of enormous journeys undertaken by birds in circumstances that are hard for humans to comprehend. Incredibly, some species can cross continents without the aid of parents showing them the way – and having been born just a few weeks previously!

But for me the most fascinating aspect of bird migration is the way that birds navigate their way across the globe – often at night. This whole area is assessed, summarising the main findings from experiments to check the way birds use the Sun, stars, magnetic forces and an incredible memory for land features that would leave even the best pilot in awe.

Another section describes the way birds disperse after breeding, and the way individuals return (or not) to wintering sites. Vagrancy is also discussed, with explanations for overshooting and drift, mirror-image and reverse-direction migration. There is an extensive section on the evolution of migration, showing how birds choose to migrate in response to factors beyond their control. This is particularly interesting given the climatic changes that are predicted for the next fifty years. Many recent changes are discussed, and clearly more species may change their travel patterns as a result, with some needing to travel further north in order to find their optimal habitat. A chapter on the effect of the last glacial age (20,000 years ago) is fascinating, and shows how some species probably altered their migration patterns due to the climatic conditions. While the research on this is nowhere near complete, it seems that some species still take unnecessarily long migrations – probably because it has not yet been to their disadvantage.

Newton continues to explore more aspects of migration such as the fact that many species undertake moults in their winter quarters, and explores the reasons why they do not take the opportunity to breed twice – once in the north, and then again in the south.

While we might think of the sudden arrival of Waxwings as an irruption in search of food rather than a planned migration, this subject is also considered along with movements within a winter period by wildfowl, waders and thrushes. The differences in migratory patterns between males and females are also explained.

The final sections discuss the circumstances for birds wintering in Africa, and the current population trends for our migrant species. It means that the book has to end on a less positive note, but it does so at a time when the BTO’s “Out of Africa” research is highlighting the challenges that our birds are facing from massive overgrazing in key parts of West Africa.

Compared to our awareness of birds’ breeding behaviour, I would suggest that most of us are comparatively less informed about their migratory habits. Indeed many of us have probably never considered whether the birds that we see every day ever undertake migrations. This book opens the door for birdwatchers on a subject that has traditionally been the domain of scientists. It does so in a way that translates detailed research into everyday facts without losing the rigour of scientific expertise.