Alexander Lees & James Gilroy, 2022. Vagrancy in birds. ISBN 9781472964793. Hard cover, 400 pages, 360 colour photographs. Bloomsbury/Christopher Helm, London. Price: € 41,17.

Birds are capable of long-distance displacements, because they can fly - and sometimes swim or even walk… Most displacements are biologically ‘planned’ (or programmed) and take birds from their breeding range to wintering areas where food availability and climate are more suitable to survive. This migration may cover short distances or altitudinal movements but many species fly enormous distances, up to several 1000s of kilometres one way. Not surprisingly, this phenomenon is a driver for birds being (literally or virtually) blown off course, and to turn up at places far away from their regular areas. This is known as vagrancy, and it has drawn interest from both scientists and amateur birders for centuries. In fact, for a large and growing community of birders, vagrancy is the ultimate spicing of their hobby and some (the so-called ‘twitchers’) undertake enormous efforts to see as many rare birds as possible in a certain geographical area, and sometimes within a given time frame. But also birders less fanatic and even the general public will often be captivated by stories of lone birds ending up at forlorn places after long and - without doubt - often hazardous flights, crossing mountain ranges, oceans or deserts to reach their unplanned destination. Often considered 'just' a curiosity, vagrancy is much more than that. This book tells you why… Vagrancy learns us what birds can be capable of in terms of long-distance flights and endurance and it must be considered an essential part of the long-term survival strategy of (many) migrant species. So, vagrancy is not a freak phenomenon attracting equally freaky birders but serious business that can and should be studied from a scientific point of view, making use of the enormous pile of valuable data collected by predominantly amateur birders who find, visit and document rare birds. This book makes the case for vagrancy as a biological phenomenon with important implications for avian ecology and evolution.

Avian vagrancy has fascinated natural historians for centuries, from the old-day collectors spending fortunes on a rare specimen to modern-day fully equipped twitchers. This book explores both patterns and processes in avian vagrancy, based on broad recent research, to answer fundamental questions concerning the occurrence of rare birds. These include the causes of vagrancy, the geographical patterns (including vagrancy hotspots) and the differences between species (groups) when it comes to vagrancy. Everything that is currently known about the subject comes together in this book.

The book starts with a 70-page review of how birds migrate and therefore what might go wrong with that behaviour and lead to a bird arriving far away from where it is expected to be if it had followed the trails of its fellow conspecifics. Almost 20 years ago, the authors published their first paper on the subject of vagrancy: (Vagrancy theories: are autumn vagrants really reverse migrants? Br Birds 96 (2003): 427-438). This paper questioned the than popular and leading theory of 'reversed migration' as the best explanation for vagrancy patterns in migration species (first described by Cottridge, D & Vinicombe, K 1996. Rare birds in Britain and Ireland: a photographic record). Instead, they offered a much more layered and sophisticated set of factors influencing vagrancy patterns. The authors then expanded on that in an essay in Slack, R 2009 (Rare birds, where and when). In the current book, that pioneering work has accumulated into an in-depth treatment of vagrancy from a broad perspective - reversed migration still plays a role but surely not the only role. Every factor or cause is discussed in detail. The authors were the first to propose the term ‘pseudo-vagrants’ for birds that are neither regular migrants nor lone vagrants but something in between: birds following a different migration path from the vast majority of their population but in such numbers and with such strong (or emerging) patterns, that labelling them as vagrants deflects from what is happening here: species trying and finding new migration routes and wintering areas to boost the survival chances of their kind. Examples in the Palearctic region include Yellow-browed Warbler Phylloscopus inornatus, Richard’s Pipit Anthus richardi and Little Bunting Emberiza pusilla, which have become regular (mostly autumnal) pseudo-vagrants to western Europe that appear to be establishing tiny winter populations in southern Europa and/or North Africa. Finally, in the introductory chapters, human-driven vagrancy – both direct and indirect – is also discussed.

Apart from closing words, no less than 40 pages of references and the index, the remainder of the book is devoted to accounts of vagrancy for every bird family - from ostriches (flightlessness is obviously a handicap for vagrancy…) to tanagers. These 250+ pages are also well served by plenty of good photographs. The large number of excellent images which enliven the text add much to the attractiveness of the book. Many photographs come from Britain and North America – not surprisingly, because in these countries twitching is popular, observer density is high and there is a long tradition of collecting and now documenting rare birds. However, one of the appealing aspects of this book is that it is probably the first to cover vagrancy all over the world, showing that it occurs on all continents, in many directions and in a large number of families. And it underscores that any common bird in a given region can be an extremely rare vagrant elsewhere. This is well illustrated by records of, eg, European Robin Erithacus rubecula and Willow Warbler P trochilus in Japan, showing that vagrancy in the Palearctic region is not an east-to-west thing but that it occurs in both directions. Of course, vagrancy is linked to long-distance migration and therefore is most common in species breeding at higher latitudes (on either hemisphere) and it is not known in most tropical (and resident) species. But even in species or families where you would not expected it, vagrancy occurs, and this book shows several examples of most unexpected occurrences. Examples of this include Phainopepla Phainopepla nitens in Ontario, Canada, Roufous Vanga Schetba rufa on Aldabra, Seychelles, or the apparent colonization of Europe by Elegant Tern Sterna elegans. Birds never cease to amaze and this book is a living testament to one of the most intriguing aspects of that.

Enno B Ebels