From ancient days the migration of birds has excited the wonder of thoughtful observers. (J. A. Thomson 1913.)

The phenomenon of bird migration has long fascinated its human observers, who have been continually impressed by the sheer scale and regularity of the movements. It has repeatedly prompted familiar questions about birds, such as where do they go or come from, how do they know when and where to travel, and how do they find their way? For more than a century now, bird movements have been subjected to scientific study, and by increasingly sophisticated methodology. In the past 25 years, hardly a year has gone by without the publication of a new book or symposium volume dealing with some aspect of bird migration, and each year dozens of papers have appeared in the scientific journals. In this book, I hope to provide an up-to-date synthesis of much of this information, taking account of both older and newer findings. However, the emphasis throughout is on ecological aspects: on the different types of bird movements, how they relate to food supplies and other external conditions, and how they might have evolved. It is mainly in the weight of attention devoted to ecological aspects -which have received scant attention in previous reviews - that this book differs from earlier ones. It is also in these aspects that, with my own background, I feel most at home with the subject matter.

After a brief introduction and survey of methodology, the book is divided into five main sections. The first deals with the journeys themselves: with the constraints and limitations of bird flight, the influence of weather, fuelling needs, migration strategies, travel speeds, the problems of navigation, and vagrancy. The second section is concerned with the annual cycles of birds, with how migration relates to breeding and moult, and with the physiological control of these various processes. The third section describes geographical patterns in bird movements across the globe, and the various types of bird movements, such as dispersal, irruption and nomadism, emphasising the ecological factors that underpin them. The fourth section is concerned with the evolution of migration and other movement patterns of birds, with the role of glacial history in influencing current migration patterns, and with recent changes in migration related to climate change and other human influence. The fifth section discusses how the population ecology of migratory birds differs from that of sedentary ones, and the influence of migration events on the population levels of birds. In particular, it considers the extent to which migratory bird numbers are limited by conditions in breeding, migration or wintering areas. This section is followed by a glossary, references and index.

Although the book is intended mainly for research students, I have tried to write simply, in the hope that the text will appeal to anyone with an interest in this fascinating subject, including the many bird-watchers and ringers who have contributed so much over the years to its development. To keep the book within bounds, I could not mention all recent work on bird migration, and have sought to cite examples rather than every study. Nevertheless, the reference list (up to and including 2006) relates to more than 2500 scientific papers and more than 50 books. It is inevitable in a book of this type that the same topics recur in different chapters, as they are relevant to more than one aspect of the subject, but I have tried to reduce this repetition to a minimum, and cross-refer between chapters. Nevertheless, each chapter is intended as a stand-alone read. So much of the book is concerned with geography that, while I have tried to provide some helpful maps in the text, some parts would be better read with an atlas, or preferably a globe, close at hand.

For permission to reproduce diagrams and other material from scientific journals, I thank the various publishers, ornithological societies and individuals involved, and for providing electronic copies of particular diagrams, I thank John Croxall, Thord Fransson, Mark Fuller, Sidney Gauthreaux, Yossi Leshem and Richard Phillips.

I owe a great deal to the many colleagues in the field who have discussed various aspects of the subject with me over the years, and to several friends for helpful comments on particular chapters, namely Bill Bourne (Chapter 4), Bill Clarke (Chapter 7), Alistair Dawson (Chapters 11 and 12), Barbara Helm (Chapters 11, 12, 20 and the Glossary), Lukas Jenni (Chapters 5 and 6), Peter Jones (Chapters 22, 24 and 25), Mick Marquiss (Chapters 15 and 18) and Tim Sparks (Chapter 21). Other colleagues, in their capacity as referees, commented helpfully on certain papers which preceded the book. I owe a particular debt to David Jenkins, who read the whole book in draft (some parts more than once), and offered many constructive suggestions for improvement. Finally, my wife, Halina, supported me through the writing process, and commented helpfully on the penultimate draft.

Ian Newton

Common Cranes Grus grus on migration

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