How Birds Evolve

What Science Reveals about Their Origin, Lives, and Diversity

Douglas J. Futuyma

• Publisher: Princeton University Press
• Year: 2021
• ISBN:
  9780691227269
• Topic: Biological Sciences
• Subtopic: Zoology
• Index: Biological Sciences

A marvelous journey into the world of bird evolution How Birds Evolve explores how evolution has shaped the distinctive characteristics and behaviors we observe in birds today. Douglas Futuyma describes how evolutionary science illuminates the wonders of birds, ranging over topics such as the meaning and origin of species, the evolutionary history of bird diversity, and the evolution of avian reproductive behaviors, plumage ornaments, and social behaviors.In this multifaceted book, Futuyma examines how birds evolved from nonavian dinosaurs and reveals what we can learn from the "family tree" of birds. He looks at the ways natural selection enables different forms of the same species to persist, and discusses how adaptation by natural selection accounts for the diverse life histories of birds and the rich variety of avian parenting styles, mating displays, and cooperative behaviors. He explains why some parts of the planet have so many more species than others, and asks what an evolutionary perspective brings to urgent questions about bird extinction and habitat destruction. Along the way, Futuyma provides an insider's perspective on how biologists practice evolutionary science, from studying the fossil record to comparing DNA sequences among and within species.A must-read for bird enthusiasts and curious naturalists, How Birds Evolve shows how evolutionary biology helps us better understand birds and their natural history, and how the study of birds has informed all aspects of evolutionary science since the time of Darwin.

1

In the Light of Evolution

BIRDS AND EVOLUTIONARY SCIENCE

A few years ago, I joined a birding tour of Ghana. After several days of enjoying such exotic species as drongos, hornbills, and pratincoles, we encountered a beautiful red and black finch, the Black-bellied Seedcracker (Pyrenestes ostrinus).¹ I was delighted to see this species because I had long known, and had described in my textbook of evolutionary biology, a study of this species by Thomas Smith,² a professor at University of California–Los Angeles. Smith had followed the life of members of a population in Cameroon by fitting each individual with a unique combination of colored leg bands. Bill size is highly variable in seedcracker populations; most birds have either small or large bills, although a minority are intermediate (plate 1). Smith found that large-billed birds feed more efficiently on the large, hard seeds of one species of sedge and small-billed birds handle the small seeds of another sedge more efficiently. Large-billed and small-billed birds both had higher rates of survival than intermediate birds: a striking example of natural selection in action. By occupying somewhat different “ecological niches,” birds with different genotypes (specific combinations of genes) persist. Years later, when the study of genomes had advanced, Smith and his collaborators determined that the inherited difference in bill size is caused by different forms of a single gene (called IGF1, or insulin-like growth factor 1).³ As we admired the seedcracker, I told my companions this story. One of them exclaimed, “So that’s why it doesn’t look like the picture in the field guide! I wondered if the book was wrong.” He was intrigued by the idea that different members of a species have different diets and ways of life.

Some birders are focused on seeing and listing species; others are curious about the lives and features of the birds they see. Once in a while a fellow birder, knowing that I’m a biologist, will ask me a question. Sometimes it is along the lines of “how can birds fly so fast through dense vegetation without hitting it?” or “how can a tiny Blackpoll Warbler (Setophaga striata) fly nonstop from New England to Venezuela?” I awkwardly answer that I don’t know much about how birds achieve these amazing feats because those are topics studied by biologists who specialize in bird physiology or brain function, and I haven’t followed those fields since I was a student. Some other questions, though, tempt me to say more than they may want to hear. (And I can resist anything but temptation.⁴) Why do some bird species have different color morphs? Why are males more brightly colored than females in some species but not others? Why do albatrosses and many other sea birds lay only one egg? How come I can see more bird species in a two-week birding tour in Peru than in an entire year in eastern North America? Why do they keep changing bird classifications, and how do they know falcons are closer to parrots than to hawks?

Most questions about birds fall into two categories—how and why—that correspond to two major kinds of biological research. Much of biology poses “how” questions: it aims to understand how organisms function—how the molecular, cellular, and organ components of an organism work, here and now, without reference to how they came to be. “Why” questions are the province of evolutionary biology. We ask why a Eurasian Golden Oriole (Oriolus oriolus) or an American Baltimore Oriole (Icterus galbula) is brightly colored because we understand that it could have been otherwise: something in its history—in its evolution—caused it to be bright rather than drab. For every characteristic of every species, we can ask “how” questions about its functional role (if any) in an organism’s lifetime, complemented by “why” questions about its origin. All species of birds have evolved from a single ancestral species (“common ancestor”), which was one of a great many species of vertebrates that all evolved from a single, more ancient, common ancestor; this, in turn, was a descendant of the ur-ancestor of all animals, from sponges to primates. And so every feature of every bird, from its DNA sequences to its behaviors, has come into existence—has evolved—during this history of descent.

Evolutionary biologists attempt to develop broad principles that can explain all these features of all species. Evolutionary biology illuminates every area of biological research and every group of organisms. The geneticist Theodosius Dobzhansky, who helped to shape modern evolutionary biology, rightly wrote that “nothing in biology makes sense except in the light of evolution.”⁵ There are biologists who study biochemical processes within cells and biologists who study how these processes evolved—and likewise for the structure and function of genomes, brains, and hormones. Among ornithologists, some take a mostly functional approach, and others a more evolutionary approach, to bird physiology, morphology,⁶ behavior, and life histories. Others are devoted to understanding the history of bird evolution—how and when birds’ form, behavior, habitat use, and geographical distribution diversified during their descent from their common ancestor. The amount of research that bears on bird evolution is immense: when I entered “evolution and bird*” in a search engine (Web of Science), it yielded 73,200 articles in scientific journals.⁷ Variant search terms would add many more.

So for almost any question we might ask about how birds evolved, there is plenty of research on which to draw. Nevertheless, the known is far less than the unknown. Questions such as “how do new species form?” and “why do female birds prefer flashy males?” are debated and are the subjects of active research. And while we may be able to provide a general answer to a question (e.g., why do birds’ bills differ in shape?), there may not be a definitive answer for a particular species. (I don’t know of any research about why the bill of the Groove-billed Ani [Crotophaga sulcirostris] is grooved.) Evolutionary biologists strive, instead, to develop theories that should apply to a wide range of species but which require detailed information to explain particular cases. For example, there are several models⁸ to account for genetic polymorphism—the persistence of two or more genetically different types within a population, such as the color “phases” of the Tawny Owl (Strix aluco) and the Eastern Screech Owl (Megascops asio). Information about the survival and reproduction of each form, under several environmental conditions, may be needed to match a particular instance to one of the models.

I can imagine someone thinking, at this point, “I watch birds because I’m entranced by their beauty and their behavior or because I enjoy the challenge of finding and identifying as many species as I can. It’s an aesthetic, emotionally rewarding experience. Doesn’t looking at a bird with the cold analytical eye of science ruin the experience?” Of course, I can’t speak for everyone, but for me, birding certainly has those rewards, and the more I know, the more my appreciation is enhanced. As many as I have seen, I still am overwhelmed by a peacock’s beauty, but it also spurs me to ask why and how it came to be, and having an answer enlarges and makes whole my experience. We integrate intellectual and aesthetic appreciation when we want to know the names of the birds we encounter and to which family or group a species belongs.

With knowledge of their biology, the most common, everyday birds take on new interest. Take the ubiquitous House Sparrow (Passer domesticus).⁹ When I stop to look at a House Sparrow, I sometimes think of its broader evolutionary context: other species in the genus Passer. For example, the Italian Sparrow (Passer italiae) originated as a hybrid between House and Spanish Sparrows (Passer hispaniolensis) (see chapter 10), and the Eurasian Tree Sparrow (Passer montanus) replaces the House Sparrow as a human associate in southeastern Asia. The House Sparrow itself shows interesting geographical variation in Europe: northern birds are bigger than birds in the south. This is one of many species of birds and mammals that have this pattern due to adaptive evolution: larger bodies lose heat more slowly than smaller ones and are advantageous in colder regions. What is more, since House Sparrows were introduced from Europe into North America in 1851, they have spread widely, and northern populations have evolved larger size. This was one of the first examples of how rapid evolution can be; Darwin never imagined that evolutionary changes could happen within a few human lifetimes.

The Superb Fairywren (Malurus cyaneus) in Australia (plate 2) is another example of a common bird that poses interesting questions. A group usually has two or more bright blue and black males and several brown birds that include both males and females. Biologist Andrew Cockburn and his associates studied the extraordinary breeding behavior of fairywrens for more than twenty-five years.¹⁰ The bright-plumaged and brown males all cooperate to rear nestlings. Cooperative breeding is known in many birds, and why it has evolved poses a very interesting question (chapter 7). But there is more: female fairywrens, to a greater extent than any other bird yet known, engage in “extra-pair copulation,” or adultery: they will travel across intervening territories to mate with a “hotshot” male. The female’s male associates dutifully help raise babies that usually aren’t their own offspring. Why are females so unfaithful, and why do males stay and rear the offspring?

These are fascinating questions that evolutionary biology can help to answer—as it can shed light on countless other aspects of birds, ranging from their coloration and structure to their geographic distribution and diversity. My aim in this book is to pose such questions and show how insights from evolutionary biology can answer them. Also, research into these topics has revealed features of many species that I think will amaze and delight anyone who likes birds and help them appreciate birds all the more. And if some readers learn more about evolution and how it is studied, the book will have served another purpose—sharing some of the richness of evolutionary science that I have found so rewarding.

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By “evolution,” biologists usually mean change in the features of a single species over time (that is, across generations) as well as the division of a single species into two or more descendant species, both of which undergo change. The alterations of a feature must be inherited to count as evolutionary change. Some features can be affected by an individual’s environment, but these changes are generally not inherited. A generation of people might be lighter skinned than their grandparents because they work in offices instead of fields and so are less suntanned, but this doesn’t count as evolution. As inheritance is a defining feature of evolution, evolutionary change of organisms’ features (their phenotype) is accompanied by evolution at the level of the genes. There is also evolution at the genetic (DNA) level that may not affect any features of the organism.

In The Origin of Species, Darwin developed two main themes: that all living things have descended, with modification, from common ancestors; and that the chief cause of modification is natural selection of inherited variations. The wealth of insights, hypotheses, and information in Darwin’s writings is staggering. Every time I read a few pages of The Origin of Species, I’m simply floored by the questions he thought to ask, the possible answers he advanced, and the evidence he found in an extraordinary range of facts, some of them seemingly trivial. During his voyage on the Beagle, he notices, in South America, that a flycatcher, the Great Kiskadee (Pitangus sulphuratus) (figure 1.1), sometimes acts like kestrels and kingfishers when foraging. Later he cites this, in The Origin of Species, to illustrate that species might change and perhaps become adapted to new ways of life. Not everyone can see a world in a grain of sand, but Darwin realized that a coherent explanation or theory must be able to accommodate, and build on, every fact, however trivial it might seem.

Evolutionary biology today is devoted to Darwin’s two great themes: what has happened in the evolution of the world’s organisms, and what have been the causes of these evolutionary events?

In studying the history of evolutio...

Table of contents

• Cover
• Title Page
• Copyright
• Contents
• Preface
• 1. In the Light of Evolution: Birds and Evolutionary Science
• 2. Parrots, Falcons, and Songbirds: The Bird Tree of Life
• Color Plates
• 3. After Archaeopteryx: Highlights of Bird History
• 4. Finches and Blackcaps: How Bird Populations Change and Adapt
• 5. The Ruff and the Cuckoo: Variation within Species
• 6. Hoatzin and Hummingbirds: How Adaptations Evolve
• 7. Owls and Albatrosses: Life Cycle Events and Variations
• 8. Auklets’ Crests and Peacocks’ Trains: Sexual Selection in Birds
• 9. Anis, Swallows, and Bee-eaters: The Social Life of Birds
• 10. Bird Species: What Are They and How Do They Form?
• 11. A World of Birds
• 12. Evolution and Extinction: The Future of Birds
• Notes
• Bibliography
• Index