I love a decisive experiment to help understand how nature works, and we humans are conducting a decisive, if unplanned, experiment on the consequences of heating our planet rapidly using vast quantities of greenhouse gases. We have already learned much from this experiment, but unfortunately it is so complex and messy (poorly controlled scientifically) as to render the results difficult to comprehend, let alone forecast. Moreover, we're committed to this experiment for at least another 50 years, given lags in global greenhouse-gas dynamics—no matter how catastrophic the results. Both these new books address how this experiment has already affected birds, and Møller et al. set the stage to accelerate figuring out what's in store.

Both books are timely and valuable contributions to the proliferating literature on climate change (for reviews of multiple books on climate issues in general, see Kitcher 2010, Trenberth 2010). Møller et al. have published an earlier book on birds and climate change (Møller et al. 2004), but their new book 6 years later builds on new literature and new ideas. They state that “No other field of scientific inquiry into the biological sciences is currently of greater significance than an understanding of the consequences of climate change for all living beings, including humans” (p. 4). Both books justify the focus on birds in terms of existing knowledge, diverse data sets, and tractability of a charismatic taxon. Both books are clearly written and organized, well indexed, and referenced by chapter; both conclude, among other things, that long-distance migratory birds, in particular, are vulnerable to present and future effects of global warming; and both acknowledge explicitly the importance of interactions between global warming and other global change phenomena in threatening mass extinctions involving birds. This is where the resemblances end.

As a single-authored book, Cox's is the more cohesive and consistent in structure and content, and it is by far the more accessible by targeting a general audience such as bird watchers (land managers and conservationists too, in my opinion). Cox's consistently succinct chapter summaries are helpful, as are the many well-designed tables and an appendix of common and scientific names of all species mentioned in the text. I understand Cox's choice to include just a list of references for each chapter without specific literature citations, given his audience, but I found it frustrating to link some assertions with their sources.

I like Cox's fundamentally optimistic take—hopefully not misguided—“that many migratory birds exhibit a high degree of ecological and evolutionary adaptability and that many are now showing rapid adjustment to climate change” (p. ix), but he acknowledges that those species that cannot adjust are likely doomed (e.g., some long-distance migrants). His book treats migratory birds comprehensively, with many chapters organized geographically (all continents and both hemispheres) and taxonomically, including seabirds (three chapters), raptors, shorebirds, waterfowl, and tropical (e.g., elevational migrants) as well as high-latitude terrestrial migrants. He attempts where possible to link changes in birds' behavior and ecology (phenology, migration routes and ranges, population sizes) to what's known about climate fluctuations, including El Niño-Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO), the Pacific Decadal Oscillation (PDO), and long-term changes (“regime shifts”) in some of these climate patterns.

Cox's book includes treatment of the taxonomy of migration in birds, an overview of climate change patterns globally, as well as other global change threats to birds—also treated, albeit less thoroughly, by Møller et al. Cox is careful to include discussion of diverse effects of greenhouse gas emissions and global warming, including changes in rainfall (e.g., in the subtropics), extreme weather events, and even effects of CO₂ acidifying the oceans. The final four, more synthetic, chapters focus on evolutionary adaptability in land birds and water birds (chapters 17 and 18, respectively), the capacity for migratory birds to adjust to climate change (chapter 19), and conservation priorities (chapter 20). This final chapter (based on earlier chapters) highlights the current plight of seabirds (44% of the 311 endangered migrant species on the 2008 IUCN Red List) due to changing ocean conditions and prey base. About 8% of the endangered migrants are land and water birds, mostly in the Asia-Pacific region. Cox also calls attention to the plight of shorebirds (especially tundra breeders), nomadic species (of Asia, Africa, Australia), birds breeding on oceanic islands (due to sea level rise), and cloudforest migrants; but he also acknowledges the plight of the far more speciose resident, especially tropical birds. His conservation recommendations include expanding on existing monitoring programs (well described), enhancing corridors and stopover areas, anticipating with new preserves the geographic ranges to which migrant birds are shifting, developing an assisted range adjustment program for species that cannot keep apace with the geography of climate changes, and increasing protection of colonial birds' nesting and roosting areas (against human harvesting).

If Cox's book is the “lite” version, Møller et al.'s is anything but lite. Møller et al. bring together many of the most talented, active ecologists and evolutionary biologists in western Europe (77% of authors) and North America (the other 23%) to address the effects of climate change on birds in general, and this abundant talent helps account for this book's more complete and more recent references (many from 2008–2010) compared with Cox's book. Møller et al. capture the intellectual excitement of an explosively expanding field. The intended audience, appropriately targeted by most chapters, is scientists and graduate students, making this an ideal starting point for new avian-climate research programs. Considerable credit for the strengths of the book go to Møller—lead editor, author, or co-author of five chapters, conscientious reviewer of many chapters, and the contributor of many groundbreaking discoveries (and collaborations), many of these contributions based on his long-term studies of Danish Barn Swallows (Hirundo rustica). In addition, many chapters are state-of-the-art reviews of important methods and contributions the other authors are making to the field, and many chapters emphasize how important multiple, integrated, and novel modeling methods will be to this field (some chapters are technically challenging). Many authors recognize that understanding and predicting the future of such complex, global physical-biological systems that affect and are affected by bird populations and communities will require diverse new tools.

Møller et al.'s book is also a sobering account of just how much less we know about climate-change effects on birds than we thought, and how challenging it will be to improve our toolbox, our models, and the science. Lacking space to describe all the chapters—none are duds, and many emphasize the need to develop more biologically based, mechanistic models—I will highlight just a few. Hurrell and Trenberth (prominent climate researchers) provide a fascinating review of some complexities of Earth's climate changes and fluctuations. Sheldon emphasizes how challenging it is to distinguish microevolutionary change from phenotypic plasticity, asserting that

Such precaution is also evident in Newton's (2008) chapter on climate change in migratory birds, but not in Cox's book. Sheldon's and Both's chapters, among others, emphasize the different responses to climate even within species over short distances. Different authors help us appreciate how—rigorously—to identify avian population trends in relation to climate (Lindström and Forchhammer), recognize real changes in geographic range (Brommer and Møller) and migration timing (Lehikoinen and Sparks), and determine population consequences of climate forcing (Saether and Engen) such as how synchronously populations fluctuate spatially, and whether species belong to the “tub” or “tap” category. A chapter on host-parasite interactions (Merino and Møller) is one of several that acknowledge the need to include in our models the complexities of species interactions, in contrast to most habitat-suitability models and other single-species approaches (well reviewed by Thuiller and Münkemüller).

Møller et al.'s book suffers from a lot of typographical errors and considerable repetition among chapters (e.g., multiple references to the interaction of invasive malaria and warming threats to endemic Hawaiian birds). Taxonomic and regional biases of the authors are also fairly obvious: besides Barn Swallows, we hear much about Great Tits (Parus major), Blue Tits (P. caeruleus), and Pied Flycatchers (Ficedula hypoleuca).

Both books, but Møller et al.'s in particular, identify in every chapter numerous foci for future effort, and the latter's penultimate (conservation) chapter (Miller-Rushing, Primack, and Sekercioglu) is excellent. Sadly and ironically, despite the desperate need for further research toward understanding the effects of probably the most decisive experiment in human history, including the incredibly complex ways this experiment will affect birds and their ecosystems, funding for this kind of research is drying up because of budget crises and growing opposition to inconvenient science by a powerful political-industrial complex (especially in the United States; see Oreskes 2010, Sherry 2011). I hope that both these books will rouse diverse audiences to increase conservation of birds on the basis of improved scientific understanding of their responses to global change.