Crossbills and Conifers: One Million Years of Adaptation and Coevolution, by Craig W. Benkman
Pelagic Publishing, 2025
224 pages, paperback
Charles Darwin once wrote that in the Galapagos Islands, “We seem to be brought somewhat near to that great fact—that mystery of mysteries—the first appearance of new beings on this earth.” Darwin’s famous “finches” (actually tanagers in the family Thraupidae) stand as testament to this legacy, acting as some of the most powerful examples of evolution known. Now, a genus of genuine finches is poised to take the spotlight.
Before 2017, Red Crossbills were one of the easiest birds in the ABA Area to identify. Large, brick-red finches with bizarre bills were unique, it seemed. But groundbreaking research by Dr. Craig Benkman of the University of Wyoming showed that a little-known population of crossbills in southern Idaho formed a distinct species, and birders across the country became fascinated with crossbill flight calls—the only diagnostic field mark for the newly-minted Cassia Crossbill. Now, in his new book, Crossbills and Conifers: One Million Years of Adaptation and Coevolution, Dr. Benkman presents a comprehensive overview of the ecology, evolutionary history, and not-too-certain future of crossbills across the world, illuminating one of the most complex genera of birds in a book accessible to all readers.
Benkman begins with basic crossbill biology, outlining in detail how seeds are removed from conifer cones, husked, and consumed by hungry crossbills. This seemingly simple interaction is central to a crossbill’s life, and Benkman frames the rest of the book through a pleasingly simple lens: how can crossbills maximize seed intake rates? And what happens when they do? Answering these questions is the real focus of the book. Having such a solidly mathematical basis for the more abstract theorizing that follows is just one way that the book makes its science approachable. Of course, that has more to do with the crossbills themselves than the writing, but nevertheless, the book keeps the science logical and clear at all times.
Two other assets quickly become clear. First is the genuine enthusiasm Benkman feels for these birds. All science is in some way personal, and the book hits a sweet spot in terms of personal touch, making it an even more enjoyable read. Even if you aren’t a total crossbill nut at the start, Benkman’s infectious wonderment at these birds is quick to rub off. Secondly, it’s about conifers just as much as crossbills. Conifer phenology, ecology, and biology are all major topics, so if you’re more intrigued by ponderosa pines than ponderosa pine (Type 2) Red Crossbills, this book is still for you! Benkman dives deep into the complex and utterly absorbing coevolution between conifers and their dependent crossbills, all while remaining grounded in the concrete experiments and observations that inform his dramatic ecological conclusions. And indeed, both experimentation and drama are on full display. Every chapter seems to showcase a different study that could take up an entire book, demonstrating just how dynamic crossbills can be. For example, Cassia Crossbills have been found to be evolving a more distinctive flight call, in real time, with measurable differences detected over just a few years, and “new” and “old” flight calls produce consistently different responses when presented to crossbills of different types. Such an example of evolution on a human time scale is remarkable (though not unheard of) and is just one of many striking ways in which crossbills seem to flout much of what was known or assumed about avian evolution. This book would be worth the sticker price just as a vehicle to appreciate Benkman’s knack for experimental design!
Special attention is rightly paid to the Cassia Crossbill. While some readers may remain skeptical about bestowing species status on this bird, Benkman’s in-depth research and engaging account of decades of study are seem solid. One point to be clear on, however, is that this book is not a traditional field guide. It won’t help you visually separate Cassia and Red crossbills, but there is information on how to differentiate the call types, which should aid in identification.
In addition to Benkman’s comprehensive study of crossbill origins and diversification, attention is duly paid to the all-too-uncertain future of these lovely birds. Benkman details how climate change affects the phenology of conifer seed shedding and is directly responsible for both well-established range limits of Parrot Crossbill and for year-to-year population declines in Cassia Crossbill.
The book is also filled with high-quality charts and photos, displaying foraging crossbills and graphs of seed intake rates, call type plasticity, reproductive isolation, and dozens of other variables. Because crossbill ecology is so tied to the basic physics of foraging, these clear and well-designed diagrams are immensely useful in understanding the evolutionary pressures and responses described in the text.
Overall, this book is a landmark in science writing, and its contents are (in my opinion) some of the most dramatic findings in evolutionary biology. It is quite simply one of those rare, near-perfect bird books, uniting a user-friendly approach with some of the most sophisticated science in evolutionary biology right now. It’s not quite the next On the Origin of Species, but it would blow Darwin’s mind.
Owen Robertson is an 18-year-old birder from Louisville, Colorado. An incoming first-year at Yale University, he is a multi-year participant in the ABA’s Young Birder of the Year program and has received first-place honors for his writing and conservation and community leadership work. He has been a birder for over ten years and is incredibly grateful to his loving family for their support. The bird he’d most like to see next is White-faced Storm-Petrel.
