Wednesday, December 12, 2018

The Ascent of Birds

In honor of the centennial of the Migratory Bird Treaty Act, National Geographic has deemed 2018 the Year of the Bird. It is therefore appropriate that this year saw the publication of a general audience book about the evolutionary history of birds, and that book is The Ascent of Birds by haematologist and birder John Reilly. This book is not focused on the origin of birds from non-avian dinosaurs, which has already been the subject of several existing titles. Instead, it covers recent advances in our understanding of evolution within neornithine (crown-group) birds, a topic that certainly receives less attention in popular writings.

When I first saw the table of contents for The Ascent of Birds, I was instantly reminded of one of my favorite books on evolutionary biology, Richard Dawkins's The Ancestor's Tale. (Yes, I'm aware that Dawkins is not exactly a popular figure these days, but his writings have had a major influence on the development of my own understanding of evolutionary biology, and I know I'm not the only one to make that claim.) The Ancestor's Tale traces the origin of humans (and eventually of all extant life) backwards through time, starting with the last common ancestor of all modern humans and ending with the last universal common ancestor (LUCA) of life on Earth, with some discussion of the origin of life itself. Along the way, other modern life forms are introduced in decreasing order of relatedness to us (i.e.: chimpanzees join first, bacteria last), and some of them are assigned "tales" that explain different aspects of biological evolution. For example, "The Howler Monkey's Tale" discusses the evolution of color vision in primates and "The Fruit Fly's Tale" talks about Hox genes.

Lo and behold, in the prologue to The Ascent of Birds, Reilly cites The Ancestor's Tale as the main inspiration for how his own book is formatted. In The Ascent of Birds, specific types of birds are similarly given "stories" about more generally applicable evolutionary concepts that the birds exemplify in some way. Unlike The Ancestor's Tale, this book starts out with the oldest divergence among crown-birds (i.e.: the split between paleognaths and neognaths) and then goes on to cover more recent evolutionary events, instead of being a journey backwards in time. Additionally, even though the chapters are arranged in a roughly phylogenetic order, they do not strictly adhere to a specific topology except in some cases where a general consensus has been established. I consider that a good decision, given that the phylogenetic relationships between major neornithine groups remain controversial. (Reilly does cite Jetz et al., 2012 as his main source for how his chapters are ordered, but some of the chapter arrangements nonetheless deviate from this study. It should be noted that the phylogeny presented by that paper has since been superseded by more recent studies such as Jarvis et al., 2014 and Prum et al., 2015.)

With this arrangement, the book starts out with "The Tinamou's Story", which discusses how the traditional thinking of paleognaths as textbook examples of remnants from Gondwanan breakup was dismantled by later scientific research (a story that, incidentally, is also covered in the second edition of The Ancestor's Tale). This is followed by "The Vegavis's Story", the only chapter headlined by a fossil bird, which tells of the Mesozoic origin of neornithines and their survival across the Cretaceous-Paleogene (K-Pg) mass extinction. "The Waterfowl's Story" covers several subjects, including the role of isolation in anatid evolution, adaptations to high altitude flight (as seen in bar-headed geese), and how the penis (which is infamously elaborate in many waterfowl) was lost in most other birds, the last of which provides a segue into neoavian birds.

Unlike other extant paleognaths, tinamous can fly. However, it is now known that they are more closely related to some flightless paleognaths than to others, suggesting that the different flightless paleognath groups lost flight independently. Photographed by "Stavenn", under CC BY-SA 3.0.

Neoavians start off with "The Hoatzin's Story", which talks about how fossil evidence suggests that the ancestors of the modern hoatzin underwent a trans-Atlantic dispersal from Africa to South America, likely involving "natural rafts" of vegetation. (This same journey has been inferred for some other South American groups like New World monkeys and caviomorph rodents.) "The Penguin's Story" talks of how penguins gained their many unusual characteristics that make them superbly adapted to cold marine environments. "The Storm Petrel's Story" explains allopatric and sympatric speciation, using the Monteiro's storm petrel as an example of how speciation can occur without geographic separation. "The Albatross's Story" covers the ever-contentious question of what constitutes a "species", and "The Godwit's Story" looks at the evolution of migration.

"The Buzzard's Story" discusses speciation as primarily a chance event, as suggested by recent studies. "The Owl's Story" looks at adaptations to nocturnal life, and is followed by a chapter on another nocturnal bird, "The Oilbird's Story", which talks about the concept of evolutionary distinctiveness (famously used by the EDGE of Existence program as a way of prioritizing species to focus on for targeted conservation efforts). "The Hummingbird's Story" deals with the origins of those miraculous dinosaurs trying to be butterflies, a topic closely related to my own research interests. "The Parrot's Story" covers vicariance and dispersal, and how both may have contributed to the present-day distribution of parrots.

Hummingbirds are remarkable in every way, from their lifestyle to their morphology to their evolution from swift-like ancestors. Photographed by "Nature's Pics Online", under CC BY-SA 3.0.

After that, Reilly turns to the passerines, which account for 60% of all living birds. Their origins are explained in "The New Zealand Wren's Story". Of all the chapters in this book, this one seems poised to be the first to become outdated by new scientific research. Although acknowledging that dissent exists, the chapter ultimately favors the idea that New Zealand wrens diverged from other passerines in the Cretaceous, with the separation of Zealandia from the rest of Gondwana. As Reilly notes in the prologue to the book, several recent papers have estimated much younger dates for the origin of passerines, casting further doubt on this concept. These papers include the aforementioned broad-scale studies by Jarvis et al. and Prum et al., as well as more focused research on passerines specifically.

Delving deeper into passerines, "The Manakin's Story" covers the diversification of suboscine passerines. "The Sapayoa's Story" looks at the enigmatic sapayoa, the lone New World suboscine that is more closely related to Old World suboscines. The oscine passerines, or songbirds, are introduced with "The Scrubbird's Story", which discusses their origins in Australia. "The Bowerbird's Story" is used to explain extended phenotypes, another Dawkins-inspired concept. "The Crow's Story" appropriately focuses on the evolution of cognitive skills, and "The Bird-of-Paradise's Story" is dedicated to (what else?) sexual selection.

"The Starling's Story" discusses the evolution of structural colors (which I coincidentally wrote about recently). "The Thrush's Story" goes over the concept of unlikely (sweepstakes) dispersals. Hybrid speciation is covered by "The Sparrow's Story" in describing the origins of the Italian sparrow. "The Zebra Finch's Story" looks at the evolution of birdsong. "The White-eye's Story" talks about characteristics that make a "great speciator". "The Crossbill's Story" is one of coevolution between the birds and the coniferous trees that produce the seeds they feed on. Finally, "The Tanager's Story" examines one of the most recent and diverse passerine radiations, one that gave rise to everything from seedeaters to flowerpiercers to Darwin's finches.

Darwin's finches are not true finches at all, but are tanagers that colonized the Galápagos Islands. Photographed by "putneymark", under CC BY-SA 2.0.

Those are a lot of complex concepts to explain to a general audience. Does The Ascent of Birds do so effectively? For most part, I would say yes. One of the greatest strengths of this book is that Reilly's writing is eminently readable and does an excellent job of clearly explaining otherwise daunting subjects. This does, however, make the (very few) lapses in this regard particularly jarring. At one point, for example, the word "teleost" is used with no further explanation, even though this is a term unlikely to be already present in the lexicon of many prospective readers.

Overall, when it comes to using bird evolution as a vehicle to explain broader evolutionary concepts, I think that this book is quite successful. But does it really tell the evolutionary story of all crown-birds? I do generally approve of Reilly's choices of specific birds as exemplars for the different stories, and those birds do collectively span the diversity of all living birds. However, I can't help but feel that the story could have been more inclusive still.

Naturally, an overview of the precise evolutionary history of every individual bird species would have been impractical for numerous reasons. Reilly understandably had to be selective in his choice of birds to focus on. However, many of the bird groups that aren't given stories are relegated to passing mentions at most. These "omitted" groups include diverse or distinctive clades such as flamingos, cuckoos, and herons. (Come to think of it, parasitic cuckoos could have been given a very interesting story about the evolution of brood parasitism and arms races between parasites and their hosts.) In contrast, The Ancestor's Tale gave a dedicated chapter to every lineage "encountered" during the journey to LUCA, even those that weren't assigned "tales". Those chapters provided interesting details about the biology of these organisms and helped drive home the central theme of common ancestry. It would have been nice to have seen similar treatment given to the non-"storytelling" birds in The Ascent of Birds.

That being said, I wouldn't hold such omissions against The Ascent of Birds too much. It's already an impressive book for what it is. I do, however, have a few nitpicks about the book as it stands. Squid are incorrectly referred to as copepods (presumably "cephalopods" was intended), and I suspect many paleontologists who work on the Hell Creek Formation would balk at the claim that Vega and James Ross Island provide the "best picture" of life immediately prior to the K-Pg boundary. The ability to nest entirely free of sediment is suggested as a possible factor that contributed to neornithine survival across the K-Pg, but the source cited for this statement appears to be a self-published article by a non-biologist, even though this proposal has been made in peer-reviewed literature. Lastly, I did find one unambiguous inaccuracy: the recently extinct piopio are said to be bowerbirds (which they resembled in their behavior), but molecular evidence has shown they were actually unusual orioles.

All of those are minor parts of the book that barely detract from the overall enjoyment of reading it. The evolutionary history of neornithines in deep time is an exciting field of active, interdisciplinary scientific research, and The Ascent of Birds puts up a strong showing in bringing that excitement to a broader audience. If one is looking for a popular overview of neornithine evolution, this title is not one to overlook.

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