Book of Birds: Introduction to Ornithology

By John Faaborg and Claire Faaborg

In Book of Birds: Introduction to Ornithology, John Faaborg, renowned expert on avian ecology and conservation, brings a fresh and accessible sensibility to the study of ornithology. In this beautifully illustrated volume, Faaborg’s approachable writing style will engage students and birders alike while introducing them to the study of the evolution, taxonomy, anatomy, physiology, diversity, and behavior of birds. With its unique focus on ecology, the text emphasizes birds’ relationships with the environment and other species while showing the amazing diversity of avian life.

Faaborg pays special attention to the roles that competition, community structure, and reproductive behavior play in the astonishingly varied and interesting lives of birds seen around the world. He discusses variations in anatomy, morphology, and behavior; explains why such vast diversity exists; and explores the ways in which different birds can share the same spaces. Artist Claire Faaborg brings the science behind this diversity to life through her unique, hand-drawn artwork throughout the book.

Combining vibrant visuals and knowledgeable insights, Book of Birds offers readers a firm foundation in the field of ornithology and an invaluable resource for understanding birds from an ecological and evolutionary perspective.

• Language: English
• Publisher: Texas A&M University Press
• Release date: Nov 11, 2020
• ISBN: 9781623497774

Book preview

BOOK OF BIRDS

Gideon Lincecum Nature & Environment Series

Sponsored by Jerry B. Lincecum and Peggy A. Redshaw

BOOK of BIRDS

Introduction to Ornithology

John Faaborg

Illustrations by Claire Faaborg

TEXAS A&M UNIVERSITY PRESS

COLLEGE STATION

Copyright © 2020 by John Faaborg, illustrations by Claire Faaborg

All rights reserved

First edition

This paper meets the requirements of ANSI/ NISO Z39.48–1992 (Permanence of Paper).

Binding materials have been chosen for durability.

Printed in Canada by Friesens.

Library of Congress Cataloging-in-Publication Data

Names: Faaborg, John, 1949– author. | Faaborg, Claire, illustrator.

Title: Book of birds: introduction to ornithology / John Faaborg; illustrations by Claire Faaborg.

Other titles: Gideon Lincecum nature and environment series.

Description: First edition. | College Station : Texas A&M University Press, [2020] | Series: Gideon Lincecum nature and environment series | Includes bibliographical references and index. | Summary: In this beautifully illustrated volume, Faaborg’s approachable writing style will engage students and birders alike while introducing them to the study of the evolution, taxonomy, anatomy, physiology, diversity, and behavior of birds. With its unique focus on ecology, the text emphsizes birds’ relationships with the environment and other species while showing the amazing diversity of avian life—Provided by publisher.

Identifiers: LCCN 2019034266 (print) | LCCN 2019034267 (ebook) | ISBN 9781623497767 (hardcover) | ISBN 9781623497774 (ebook)

Subjects: LCSH: Ornithology. | Birds--Ecology.

Classification: LCC QL673 .F27 2020 (print) | LCC QL673 (ebook) | DDC 598—dc23

LC record available at https://lccn.loc.gov/2019034266

LC ebook record available at https://lccn.loc.gov/2019034267

With gratitude for her long commitment to the mission of Texas A&M University Press, her colleagues, the Press’s Faculty Advisory Committee, and its Advancement Board publish this special book in appreciation and honor of SHANNON DAVIES, the Press’s distinguished director, upon her retirement.

CONTENTS

Preface

Acknowledgments

Chapter 1. An Introduction to Ornithology

Chapter 2. General Traits of an Avian Flying Machine

Chapter 3. Flight

Chapter 4. Speciation and Radiation

Chapter 5. Constraints on Avian Diversity

Chapter 6. Systematics and Taxonomy: Classifying Birds

Chapter 7. Foraging Behavior

Chapter 8. Adaptations for Survival in Extreme Environments

Chapter 9. Migration

Chapter 10. Anatomy and Physiology of Reproduction

Chapter 11. General Patterns of Reproductive Behavior

Chapter 12. Adaptive Variation in Avian Reproduction

Chapter 13. Economic and Cultural Values of Birds

Notes

Bibliography

Suggested Reading

Index

PREFACE

Obviously, this book deals with ornithology, which is quite simply the study of birds. But ornithology is an exceedingly broad topic, particularly if you include all the conservation-related work done on birds in recent decades. A recently produced handbook of bird biology is so long (1302 pages) that each chapter starts with page 1. The most recent best seller in ornithology is a book of nearly 800 pages. As a university teacher, I have found those options to be too much material to cover in a semester, particularly when we also have laboratory exercises, and also pretty expensive, when we also force students to buy field guides and binoculars for the labs. On the other hand, a recent book from England attempted to cover essential ornithology in just 167 pages, which appeared to miss too many interesting topics for me. Based on my having taught ornithology and avian ecology courses for 40 years, I have attempted to provide a book that is reasonably comprehensive, but not too big and not too expensive, and that has just the right mix of topics.

Which brings us to the ecological approach in the title. My editors often asked me, What is an ecological approach to ornithology? My response was that it is really an ecological and evolutionary approach to understanding birds, but that including both topics in the title would be too confusing. My goal is to try to show the diversity of birds in a way that explains why such diversity exists and how these diverse forms can coexist, using all the wonderful variation in anatomy, morphology, and behavior that exists in the world of birds. To avoid writing an 800-page book, I put little emphasis on such topics as internal anatomy, physiology, and neurobiology, and more on the role of competition, community structure, and reproductive behavior in driving nature to produce such an amazing diversity of birds around the world. With this ecological approach, I hope that after reading this book, you better understand how the birds in your life work. Why does that blackbird have such a red shoulder? Why is that robin attacking your window? What are those bird chirps that you hear late at night during spring and fall migration?

One of the most important things I would like you to understand with this book is why a bird looks the way it does. Birds have evolved almost unbelievable patterns of color and shape as part of their reproductive behavior. There is no other animal group with quite as much variation in color as birds, and by using color in most of the graphics in this book, we hope to visually give you some sense of that variation while explaining it in the text. With modern access to the internet and the amazing photographs available there, we do not attempt to compete with photographs; rather, my daughter, Claire, and I hope to give you a visual sense of what is going on as you read the text and begin to understand the variation that is the world of birds.

ACKNOWLEDGMENTS

A larger version of this textbook first appeared in the late 1980s as Ornithology: An Ecological Approach. At that time, the ornithology books that were available tended to be much more laboratory and museum oriented, which reflected the state of the science of ornithology at that time. The goal of my book was to be both comprehensive in scope (covering all of ornithology) and to incorporate all of the new discoveries made about avian ecology and evolution during the 1970s and 1980s that were not in the textbooks available to me when I was in school. Because my university taught ornithology at a sophomore-junior level, I also wanted a book that was fairly comprehensive but not as large as the older books or the new ornithology books that appeared at the same time as mine.

My original publisher was bought out by a larger publisher who was not as enthusiastic about this text as I had hoped, so after a single printing my publisher dropped its support. I was able to round up enough copies to teach ornithology on campus for a few years, but it was difficult. About 15 years ago, our book store developed the ability to make books on campus, and I immediately converted 12 of the original chapters into a short but decent quality text. It also was very cheap, which made the students happy, particularly when they also had to buy field guides, lab manuals, and binoculars for this class. I used various versions of this book for many years, updating chapters every few years.

A few years ago, I was talking with an old friend from bird meetings, Shannon Davies, who had developed an impressive list of bird books for the University of Texas Press before she moved to Texas A&M University Press. We felt that there was a market for my kind of ornithology book, one that was fairly comprehensive but not too large or expensive. We also hoped it would be easy to read and could be marketed not only to undergraduates but to anyone who might be interested in understanding birds.

For a while, we were hung up on the sort of design we might want. As it turned out, while we were discussing options, my daughter, Claire, was growing up and developing spectacular artistic abilities. With formal training in art history and lots of experience being dragged into the field with her father, Claire showed tremendous talent at doing bird and other science illustration. With some great examples, we convinced Shannon that a textbook done almost completely with hand-drawn art would allow us to fulfill the goals of a textbook art program while also being colorful and attractive. Thus, we have an unusual father-daughter textbook.

My colleague Susan Chaplin, who was at the University of Missouri for some time, helped with major parts of the first version of this book. She was originally listed as a coauthor for chapters 2, 9, and 11, although she helped with other parts of the book. She is responsible for much of the anatomy and physiology in this book, although I should be blamed for any mistakes in this version.

I have many people to thank for their help on this revision. Mark Ryan was very helpful with the first version of the text, but most of the changes in the uncounted number of revisions were comments from undergrads who volunteered to do some extra editing. Heather Tearney of the university bookstore at the University of Missouri helped us develop the sample books that were modified over time. Claire did an impressive amount of art; every father should have the chance to work with his daughter on a bird book. Maybe. Working with Claire on this project has been one of my favorite lifetime memories.

Claire would like to thank her public school art teachers, whose years of tutelage, encouragement, and purloined classroom art supplies contributed directly to this project. She’d also like to thank me, though I told her you shouldn’t thank one’s coauthors. She writes, Without John Faaborg as a father, I might never have known about or painted the iridescence in the wing of a Myna, the piercing gaze of a Hawk Eagle, or the spirited dance of a Grebe. It is both because of and for him that I have illustrated this book.

We thank Shannon Davies of Texas A&M University Press for many years of support while the book developed in our minds. Katie Duelm, Mary Ann Jacob, and all the other behind-the-scenes personnel at TAMU Press, as well as copyeditor Laurel Anderton, helped put the pieces together and move the book through production and publication.

My wife, Janice, has dealt with my joys and frustrations regarding this text for most of our married life. Adding our daughter to the mix has been, in general, a wonderful thing, although none of this happened fast. We do think that the mix of text and art explains things and entertains at the same time. Hope you agree.

BOOK OF BIRDS

CHAPTER 1

An Introduction to Ornithology

The term ornithology has its roots in the Greek ornis, birds, and logos, the study of. Thus, ornithology is simply the study of birds. Long, complex definitions of ornithology have been offered, but these are unnecessary. Any study that deals with birds is ornithology, although it might also be considered ecology, paleontology, behavior, or some other branch of science.

Today’s birds comprise the class Aves of the subphylum Vertebrata and phylum Chordata. The other primarily terrestrial vertebrates with which birds currently share the world are the reptiles (class Reptilia), mammals (class Mammalia), and terrestrial amphibians (class Amphibia) such as salamanders. While virtually everyone can recognize a bird, a close examination of these three groups of animals has historically shown only two distinctly avian traits. The most obvious of these is the covering of feathers found on all birds. Although sharing the same origin and composition as lizard scales, feathers are very different from the scales of lizards or the hair of mammals. The other distinct avian trait is the presence of a right aortic arch that carries pure blood from the heart to the body. While the location of this arch is distinctive, the four-chambered heart with double circulation is a trait shared with mammals (which have the aortic arch on the left side). Such a system seems to be a necessary part of maintaining homeothermy (warm-bloodedness), another trait shared by birds and mammals. Birds are distinctive in their highly developed, powered flight and its diverse forms, but among the mammals, bats are also excellent fliers. Additionally, the evolution of wings has led to bipedal walking in birds, a trait that has become so developed in some forms that the powers of flight have been lost. While most mammals and reptiles are quadrupedal, bipedalism is obviously not purely an avian trait. Some authors make a big deal about the beak, a feature of birds shared only by the beaked mammalian Duck-billed Platypus (Ornithorhynchus anatinus).

When compared to modern reptiles (excluding the crocodile, which as we will see is an exceptional reptile), mammals and birds are distinctive in the relatively large amount of care that parents provide their offspring. Whereas this care in most mammals is directed toward young that are born alive, the existence of such primitive egg-laying mammals as the platypus and Spiny Echidna (Tachyglossus aculeatus) again reduces the distinctiveness of the avian reproductive system.

Among living forms, it is the reptiles that share the most traits with birds. The scales covering most reptiles are identical in general composition to the scales on bird legs and some bird beaks.¹ The egg of reptiles and birds is much the same, and both have young that use an egg tooth to crack the shell to emerge. The general internal arrangement of organs is similar, and the air sacs of birds are placed in a manner similar to that of the air sacs of turtles and chameleons. A number of skeletal traits are shared by these groups, including the bone at the skull-neck hinge (the occipital condyle), the basic jaw structure, the lateral brain case, the structure of overlapping ribs, the intertarsal ankle joint, and the presence of a single bone in the middle ear. Birds are particularly similar to crocodiles in the structure of the pleural (body) cavity, the shape and structure of the brain, the inner ear structure, and the characteristics of the blood proteins. Parental care is also shared only with the crocodiles among the present-day reptiles.

Origins of Birds: A Look at Avian Paleontology

With so many similar characteristics, it is not surprising that birds share a closer evolutionary history with reptiles than with mammals. Yet, to think of birds as hot-blooded reptiles derived from forms we see today is misleading and simplistic. It is clear that ancestors of birds and present-day reptiles diverged millions of years ago, at about the same time as mammals arose from a common ancestor. From this radiation came the groups we still see today (birds, turtles, snakes, lizards, crocodilians, and mammals), but there was a period when at least two major types of dinosaurs dominated land animal communities. Of course, in all these cases, the original forms that separated from a common ancestor looked quite different from the forms they became over millions of years of evolution. Additionally, the fossil record did not always record these forms, or it left only a record of bones for us to use to try to re-create history. Thus, in recent years, a serious (and often rancorous) argument has developed about the origins of birds, with two major theories involved, one supported mostly by the old-school, classically trained, mostly near-retirement types, and the other by younger faculty who like to think outside of what they see as the traditional box.

To understand the origins of birds, we need to take a brief look at avian paleontology, the study of the fossil record of birds. All paleontology faces problems associated with the chance occurrence of fossilization, with no guarantees that the bones or other evidence discovered represent a cross section of the animals living at a particular time. Most fossils are best preserved in such sediments as lake beds or ocean bottoms, a characteristic that immediately biases the fossil record toward aquatic forms and away from land-dwelling forms, especially those of the dry uplands. Generally, only hard parts, particularly large bones, are preserved, so little can be said about soft internal structures, skin color, and similar features. Since birds are generally small and have light, often hollow, bones, it is not surprising that bird fossils are hard to find. On the other hand, it has been suggested that our knowledge of bird evolution is presently limited more by the scarcity of paleornithologists than by the lack of fossils. At one point in the past, only 15 authors were responsible for three-fourths of the published descriptions of fossil species²,but this has changed somewhat recently. In particular, a great many new fossils have come from China in recent years, in deposits that are fine enough to preserve the detail of bird skeletons and even feathers.

The difficult fossilization process for birds has resulted in relatively little information about fossil birds. The same undoubtedly holds true for any small, fragile reptilian forms from which birds might have evolved. With this paucity of information, it is not surprising that paleornithologists disagree about the origins of birds and have been doing so for at least 150 years. Nearly 30 years ago, a reviewer of a volume titled The Beginnings of Birds³ suggested that this volume has more arguments per page than I have seen in a long time.⁴

To an ecologist reading the paleornithological literature, it appears as if there are only two topics about which paleornithologists agree concerning the origins of birds. First, they agree that birds separated from the line of present-day lizards, snakes, and turtles very early in the evolutionary sequence. Therefore, to understand the evolution of birds, we must go back to the beginning of the radiation of terrestrial animals on this planet. This radiation also included the traditional dinosaurs, which dominated the earth for millions of years but are now completely gone. Second, they agree that Archaeopteryx is the earliest-known fossil that can be classified as a bird. This does not imply that it serves as the primitive ancestor for all modern birds (most agree it does not), but simply that it is the oldest-known bird in the fossil record. There have been some claims of a fossil (Protoavis) that is as much as 75 million years older than Archaeopteryx, but the consensus still favors Archaeopteryx as the oldest bird. Let us examine these two general areas of agreement before we look at the two major arguments for the evolution of birds.

The reptilian ancestry of birds

To understand the separation between modern reptiles and birds, we need to go back about 300 million years to the end of the period known as the Carboniferous. At this time, the first reptiles (order Cotylosauria; fig. 1.1) had recently been derived from amphibian forms. These early reptiles were amphibious in general behavior, but they had evolved an egg that could be laid and would develop on land. The development of this egg meant that these early reptiles were no longer forced to be near water, which allowed them to penetrate large landmasses. Because the terrestrial environment was essentially unfilled at this time, a variety of evolutionary developments occurred. The cotylosaurs are considered the stem reptiles because all reptilian forms arose from them, plus the mammals and birds.

In part because of their rapid variation and adaptation to these new terrestrial conditions, the stem reptiles rather quickly disappeared as they evolved into a diversity of better-adapted forms. One of the earliest separations was a set of primitive reptiles that led to what are present-day mammals. Depending on when one decides the mammal-like reptiles became true mammals, the class Mammalia may be older than the class Aves. Mammals sort of just hung around for the next couple of hundred million years until they became more prevalent after the great extinction of 65 million years ago (mya). Several primarily aquatic forms evolved from the stem reptiles. Two of these were large, carnivorous types that are now extinct. Another included the various types of turtles, which have not changed greatly in the last 180 million years.

Fig. 1.1. Radiation of land animals from the early reptiles (order Cotylosauria) to present-day groupings. The first split involved separation of mammals and thecodonts, which radiated into the highly diverse ruling reptiles. Note that birds may have evolved directly from thecodonts, through crocodilians, or through the sauropod dinosaur group.

For our purposes, the most important separation of the stem reptiles led to two major divisions of land-dwelling reptilian forms. One line retained a skull structure much like that of the amphibians from which they had only recently been derived. This line also possessed a simple, sprawling form of quadrupedal locomotion powered by legs that were only about 50 million years removed from being fleshy fins on lobe-finned fishes. Although this form of locomotion can be considered primitive, and it puts some limits on the size and flexibility of movement of animals using it, the relatively small lizards and snakes that were derived from these forms are still with us in great numbers. Despite possessing what we consider primitive structural design, these forms have been successful in their ecological roles for 250 million years with little change.

The other major radiation of land-dwelling reptiles was first characterized by a newer, more effective form of locomotion. The most apparent change here was a tendency to increase the size of the hind legs and shift them so they were situated below the body. Initially, this allowed short bursts of bipedal motion from a quadrupedal lizard, a behavior that we see in some lizards today. With greater development of the hind legs, though, forms evolved that were totally bipedal.

In addition to providing greater agility and speed of locomotion, the evolution of bipedalism effectively freed the front limbs for other activities. The line of mobile reptiles evolving in this fashion was called the thecodonts (fig. 1.1). Early thecodonts had smaller front legs (suggesting only some bipedalism), conical teeth set into deep sockets, and a variety of skeletal shifts to support this new body plan. The thecodonts are considered the ancestors to the full line of archosaurs that dominated during the Age of Reptiles, a period of nearly 150 million years. This group included the crocodiles, the flying and gliding pterosaurs, and the dinosaurs. The dinosaur line split rather quickly into two groups, the Ornithischia (dinosaurs that had birdlike pelvises but were not further associated with bird evolution) and the Saurischia (dinosaurs with reptilian pelvises). The latter group included carnivores such as Tyrannosaurus rex, the large, amphibious sauropods, and some ostrichlike dinosaurs.

While everyone agrees that birds developed through the thecodont line of reptiles, there seem to be several possibilities for when and where the avian line separated from the reptilian (fig. 1.1), or if such even occurred. The flying reptiles (pterosaurs) might seem a logical choice for avian ancestors, but the differences between the pterosaur wing and the bird wing are so extreme that no way can be seen to get from one to the other. Certainly, the membranous pterosaur wing was an alternate means of gliding and flying; pterosaurs were successful during most of the Age of Reptiles and ranged in size from sparrowlike to those with 27-foot wingspans. Many varieties were carnivores, often specializing on fish, while the largest may have scavenged dead dinosaurs. All the pterosaurs disappeared at the end of the Cretaceous with the rest of the dinosaurs. Another alternative for the evolution of birds is that they split from a crocodilian ancestor sometime during the early radiation of the archosaurs. Birds and crocodiles share almost identical inner ear structure, several sets of bones that are very similar to one another but differ from those of other existing vertebrates, and aspects of parental care that are rare in other current reptiles. Several scientists have made detailed studies of the similarities of these forms, but support for this idea has declined as evidence in favor of one of the other models has advanced. In particular, the behavioral similarities are not that special, as over 100 species of snakes and lizards show some form of brooding behavior for eggs or nests. Thus, most look elsewhere in the thecodont line for the avian ancestor. Before we examine the two most popular theories available, let us jump ahead and look at the next generally recognized fact, that Archaeopteryx is the oldest-known bird.

Fig. 1.2. How a fossil Archaeopteryx may have looked upon its discovery.

Archaeopteryx

It has been only about 150 years since workers in a limestone quarry in Germany uncovered a peculiar fossil (fig. 1.2). The rock formation in which they were working was believed to date from the Jurassic, about 130 million years ago. Although the bone structure of this fossil (including teeth in sockets) was similar to that of many of the smallest dinosaurs found in the same deposits, this animal had a distinct covering of feathers. The specimen was given the genus name Archaeopteryx, meaning ancient wing, and the species designation lithographia, since the limestone was being mined for use in lithographic printing. It is our good fortune that a few of these primitive birds were preserved in these fine limestone sediments, for without them the picture of avian evolution would be even more confusing. As it is, fewer than a dozen specimens that everyone seems to agree are Archaeopteryx have ever been found.

Fig. 1.3. Our interpretation of how Archaeopteryx may have looked when alive.

Although the covering of feathers suggests that Archaeopteryx was a bird (fig. 1.3), it is in fact almost a perfect intermediate between modern birds and archosaurs. The bone structure differs from the reptilian only in the presence of a furcula, also known as the wishbone. In fact, several of the Archaeopteryx fossils were initially misidentified as dinosaurs because scientists did not look closely enough to see the sometimes vague imprint of feathers. In addition to teeth, Archaeopteryx had a long, bony tail, a small sternum, and wing bones unlike those of modern birds. This structure suggests to some that it was a glider but not really developed for powered flight, although others disagree. Even though there was a long way to go from Archaeopteryx to modern birds, we know that at least by 130 million years ago, birds did exist.

Theories of Bird Evolution

The evolution of birds in general, and the evolution of birds from dinosaurs in particular, attracts a great deal of attention from the public. Unlike most of the topics in this book, the evolution of birds and dinosaurs is a regular subject of papers in such upper-level journals as Science and Nature, with articles released to local newspapers for general readership. Everybody loves birds, dinosaurs, or both, and the possibility that birds are dinosaurs is almost magical to most people.

Scientists are also deeply involved in this controversy about the possible evolution of birds from dinosaurs, perhaps sometimes too deeply. Although new fossil discoveries sometimes appear that help provide new evidence to fuel the controversy, much of the discussion revolves around a small number of fossils that have been analyzed and reanalyzed over decades by different scientists, with one person convinced that a new discovery is important while another strongly disagrees. Some of these comparisons are real, as several of the original Archaeopteryx specimens were classified as dinosaurs until someone looked closely enough to see the feather impressions; others are simply individual interpretations. As the two theories of avian evolution have developed over the past two decades, the discussion has sometimes been less than scholarly. Rather than a comparison between a theory that has been around for 100 years versus one that is relatively new (but was actually suggested first), some of the discussion has degenerated into attacks on how different people do science, with mostly older paleontologists who do things a certain way on one side arguing with younger, nonpaleontologists with new methodologies on the other side. Name calling is sometimes involved, with one person talking about the other’s rhetorical sham or how he or she supports an argument with only the sociology of science rather than data. To get a feel for these arguments, check out the series of commentaries and rebuttals by Richard Prum (2002, 2003), Alan Feduccia (2002, 2013), and Alan Feduccia, Larry Martin, and Sam Tarsitano (2007).

Much of this discussion occurs because the fossil record is incomplete, with large anatomical steps between existing fossils and millions of years with no information. Bird bones are light and hard to preserve as fossils, which means that feathers are even harder to find. We have already noted that several of the Archaeopteryx specimens were misclassified until someone looked closely enough to see feather impressions. Avian scientists are searching for their own missing link, just as anthropologists who study the evolution of humans are. New fossils that might provide evidence toward these arguments are of great value. Many of these have recently been coming from China, where the proper deposits to preserve bird skeletons exist. Most fossils are collected by professional fossil hunters, who then sell them to scientists and museums. These collectors are well aware of what the paleontologists want to find and how much money such a fossil might be worth. A few years ago, one ingenious fossil gatherer put together what appeared to be the perfect missing link, one that would have ended the arguments about the origin of birds. A renowned national society paid a great deal of money to acquire this fossil. The society had its own paleontologists examine the fossil but would not let others look at it before the highly publicized unveiling of the fossil and a story about it in the society’s magazine. Unfortunately, the members of this group let their enthusiasm get the best of them. Once the fossil was open to viewing, several scientists were able to rather quickly show that it was a fraud concocted by a very clever Chinese fossil collector.

The dominant theory until fairly recently was the pseudosuchian thecodont hypothesis. This was developed early in the 1900s⁵,although until it was attacked by the alternative proposal, it was not particularly detailed. This theory posited that since Archaeopteryx was so distinctly different from coexisting animals when it occurred, it had to be the result of a separation from sister forms much earlier in its history. It was suggested that an early thecodont such as Euparkeria, which was quadrupedal but tended toward bipedal, provided an excellent form to turn into a bird, and perhaps into other archosaur groups, including dinosaurs and maybe crocodilians. Unfortunately, there is no evidence linking this and Archaeopteryx for about 100 million years.

The theropod hypothesis has been developed mostly in the past 40 years, but in some ways it is by far the oldest hypothesis for the evolution of birds. In the 1860s, after looking at a fossil Archaeopteryx and fossils of some of the terrestrial coelurosaurian dinosaurs, Thomas Huxley (1867), friend and colleague of Charles Darwin, declared that birds were simply glorified reptiles. In its most basic form, this theory suggests that paleontologists’ confusion of Archaeopteryx with some of the small, terrestrial dinosaurs had to mean something regarding evolution. Such similar skeletons as shown by primitive birds and coelurosaurian dinosaurs had to come from similar ancestors in relatively recent times. Proponents of this theory suggested that these two groups split from a common ancestor not that long before they could be found coexisting on the earth. Examinations of many of these theropods and early birds seemed to reveal the existence of feathers in a wide variety of forms, both avian and nonavian. By compiling much information, Paul Sereno (1999) developed a phylogeny suggesting that the theropod dinosaurs split from the major sauropod dinosaur group and then split into three further groups, including the coelurosaurs (fig. 1.4). Coelurosaurs included a variety of bipedal dinosaurs and birds, and this group may have been where feathers first appeared in animals. Further down the line, a split divided the tyrannosauroids (including everyone’s favorite, Tyrannosaurus rex) and a diverse group of forms, one of which appeared to have well-developed, modern feathers and was considered the birds. Such a phylogeny shows birds easily nestled within such dinosaur sister species as T. rex and Velociraptor, the star of the movie Jurassic Park. Feathers of some form, which we suggested earlier were key in distinguishing birds, were widely present among these birds and their dinosaur relatives, with the avian groups distinctive because they had well-developed feather structure.

Fig. 1.4. One of the hypothesized evolutionary trees of birds through the coelurosaur dinosaur group, showing the split between tyrannosaurs and maniraptors, which led to the branch in which all bird groups seem to occur. It has been suggested that all these groups had some sort of feather-like covering, while well-developed feathers appear only in the bird groups, and perhaps one or two of the nonavian maniraptors. Modified from Sereno (1999).

Which Theory Is Correct?

Before we get into the details of the strengths and weaknesses of these two theories, we must remind ourselves that both suffer from giant gaps in the fossil record. For the thecodont model, we have a gap of 90–100 million years between Archaeopteryx and some fossils that might serve as prototypes for the developing bird. In contrast, the theropod model is based on a variety of shared characteristics found in a coexisting group of animals from the time of Archaeopteryx; this information can be put together through taxonomic methods to present a proposed relationship among developing forms as shown in figure 1.4, but this relationship is based on the results of evolution and shared traits. There are no fossils to support the splits shown in the figure; rather, clustering techniques suggest the various groupings, and the splits in early periods are also just suggestions. The theropod model involves a lot of development in a much shorter period, as the evolution of birds occurred well past the split between the two major dinosaur types.

Because most ornithologists are not paleontologists, the origins of birds have been discussed mostly by paleontologists and ignored by ornithologists. By the year 2000, virtually all paleontologists believed in the theropod origin of birds. When Richard Prum wrote a commentary for The Auk, the journal of the American Ornithological Society, the world’s largest ornithological society, and titled it Why Ornithologists Should Care about the Theropod Origin of Birds, the intellectual battle was on. In one corner was Prum, a broadly trained ornithologist who became interested in fossil birds later in his career. In the other corner was Alan Feduccia, a lifelong avian paleontologist. Here are some of the factors they discussed in their original papers and rebuttals:

Fig. 1.5. Evolutionary change in which digits are kept in different groups. All vertebrates start with five digits but often reduce the number to three. Theropods keep the first three digits, while birds keep digits II, III, and IV.

(a) DIGITS: Most vertebrates have hands or feet with five fingers or toes (digits) at some point in their development (fig. 1.5). Both birds and theropods have reduced the number of functioning digits on their forearms to three, with two vestigial digits. It appears that theropods have kept the first three digits (the thumb plus the next two fingers, labeled I, II, and III), while in birds, it appears that the thumb and little finger have become vestigial, such that digits II, III, and IV remain functioning. This is a basic enough difference that we would not expect it to occur later in evolution. Thus, the digits suggest that birds do not share a basic ancestor with the theropods, which then favors a thecodont model. Scientists favoring the theropod model have suggested that some sort of frameshift might have occurred to explain this difference between the two groups. Some interesting work on the development of the fingers in embryonic ostriches has supported the separation of birds and dinosaurs, but this argument is difficult to test because dinosaurs are extinct.

(b) TEETH: Bird teeth are very similar in all the old bird families in being peg-like and set in sockets. The teeth of most theropod dinosaurs are blade-like, which is appropriate for vicious carnivores but suggests separate lines of evolution for these groups.

(c) FEATHERS: We have already noted that feathers are diagnostic for birds among existing animals. Obviously, if feathers were shared between birds and theropods, it would suggest a close lineage. All the groups shown in figure 1.4 may have had some kind of feather-like covering, but only a few of the nonavian maniraptoran groups had the fully developed feathers characteristic of all birds. Some have suggested that Tyrannosaurus rex was feathered, at least when young, but there has been great controversy about whether feathers really exist on anything other than birds. Some have suggested that some sort of dino-fuzz did occur on all the coelurosaurs, but that this was not necessarily a feathered covering. As we noted earlier, the chemical composition of feathers and scales is almost identical, which makes it difficult to be sure which side is correct regarding widespread feathering on dinosaurs and its role in showing relationships with birds. On the other hand, a recent paper seems to show pronounced evidence of feathers on Velociraptor.⁶

(d) THE TEMPORAL PARADOX: As we noted above, birds and birdlike theropods were known to coexist at similar times in history. Unfortunately, though, it appears that primitive birds such as Archaeopteryx appeared earlier in the fossil record than the theropods, in many cases by tens of millions of years. Obviously, you cannot be your own grandmother, so it would be impossible for theropods to evolve into birds if birds already existed.

(e) BEHAVIOR: As we have learned more about dinosaurs in recent years, we have seen that they may have had much more sophisticated forms of social and reproductive behavior than most of the existing snakes and lizards on the planet. Such dinosaur behavior has been described as very avian in nature, which would make sense if the two groups were related. While this relationship between the behavior of the two groups makes sense, one worries that as science considers a bird-dinosaur connection, it assigns avian traits to dinosaurs when the evidence for such behavior is sparse. For example, it is not very diagnostic to recognize that over 100 species of existing snakes and lizards show parental care at nests, and some are even communal (nest in groups).

(f) SKULLS: Although the skulls of theropods and primitive birds seem quite similar, the controversy over the relatedness of these two groups has caused some to take a very detailed look at the skulls of these groups. It has been suggested that the diapsid arches in the skull show distinct patterns of evolution within the birds, eventually leading to what is called an avidiapsid arch, which is distinctly different from the structure of theropod skulls.

(g) EVOLUTION OF FLIGHT: Because flight is so distinctive to birds, the arguments about avian evolution have to consider the possibilities of flight evolving in these two alternative models. The thecodont theory suggests that early archosaurs that climbed trees may have developed the ability to extend their glide by modifying their front legs into wings; such modifications extended eventually to powered flight. Archaeopteryx serves as a good example of an early stage of such evolution, as there is skepticism about the extent to which this species was able to perform powered flight rather than just extended gliding. This arboreal theory for the evolution of flight makes a certain amount of sense, and we can find snakes, lizards, and mammals that glide today. If birds evolved from running dinosaurs, though, the evolution of flight had to occur from the ground up, the cursorial theory. This theory involves modified front legs where extended scales or feathers allowed a powerful runner to extend a jump over great distances. With a little more modification, these feathers could provide a power stroke and, eventually, powered flight. Arguments in favor of the thecodont model suggest that the aerodynamics of a cursorial route to flight were a hurdle too significant to overcome, particularly with such strongly bipedal organisms with heavy back legs. Several recent fossils show primitive birds that had feathered hind legs, and there is even some evidence that Archaeopteryx had these legs.⁷

Another problem with the cursorial theory for the evolution of flight is the development of large wings after the front legs had become much reduced in size as these creatures became bipedal. How could big front legs become small front legs, and then how could these tiny front legs become modified into large wings that eventually led to flight? This seems to require a step where a short wing that was not actually used for flight had some value, after which the wing could evolve. But how does a half wing or quarter wing develop that serves some purpose that could eventually lead to flight?

John Ostrum, one of the first to push the modern ideas of birds as dinosaurs, suggested that some of the early theropods developed extended scales on their front legs that they used as swatters for capturing the foot-long dragonflies and other insects that occurred in those days. After developing these long, modified scales on their front legs, these animals started using them for gliding, and eventually powered flight. But no fossils appear to actually show the developing stages of these fly swatters.⁸

An intriguing and perhaps more reasonable model of how an undeveloped wing could be adaptive has been developed by the work of Ken Dial.⁹ Dial studies locomotion of baby game birds. These birds hatch as very mobile chicks that immediately start moving around their environment. They quickly develop short flight feathers on their wings, and Dial has done experiments showing how even partial wings can help these small birds move, including allowing them to climb steep rocks by gathering friction with their legs because of the flapping of the small wings (which he calls wing-assisted incline running). We could rather easily envision a small, bipedal dinosaur that developed small wings for this advanced mobility for either catching prey or avoiding being caught as prey, and the small wings then developing into something adapted for flight.

As we noted earlier, recent behaviorists have suggested that dinosaurs had much more sophisticated social behavior than we generally assign to modern lizards. Such advanced mating behavior could easily have resulted in strong sexual selection, that form of natural selection that favors males (usually) that can attract the most mates through a variety of secondary sexual traits. It has been suggested that early feathers in large dinosaurs served this purpose, which could have resulted in long feathers on the wings and tail that were then used to develop flight.¹⁰

What Is the Truth about Avian Origins?

In the decade since the Prum/Feduccia exchanges, discussion has settled down somewhat. Paleontologists seem to have universally accepted the birds-as-theropod-dinosaur-relatives model, while the journal Science suggested in 2010 that the bird-dinosaur link [had] firmed up. Numerous exciting fossils have been found in China over this decade, some of them suggesting rather bizarre birds with full wings on both front and back legs. Paleontologists have made good cases that many of these early birds were colorful (fig. 1.6), perhaps even with iridescent (shiny) plumage. These discoveries are almost always presented within the framework of a theropod model. Among several recent books on avian evolution, one titled "Glorified Dinosaurs: The Origin and Early Evolution of Birds" examines the evidence for theropod origins of birds with virtually no mention of thecodonts.¹¹ Another book is titled Living Dinosaurs: The Evolutionary History of Modern Birds.¹² One can even purchase The Field Guide to Dinosaurs,¹³ although I am not sure how much use this will get in the field.

Fig. 1.6. Our version of a colorful, four-winged bird that may have occurred early in the evolution of birds.

On the other side, a monograph by Frances James and John Pourtless (2009) attempts to reconcile the methodological arguments that were part of the Prum/Feduccia interaction by using cladistical techniques in what they suggest is a less biased fashion to explore the validity of six different models of avian evolution. Their results support some of the alternative models as well as they do the theropod models, and they end with the suggestion that at present, the origin of birds is an open question.

Building in part on the James and Pourtless work, Alan Feduccia wrote Riddle of the Feathered Dragons: Hidden Birds of China (2012), and another perspective (2013) in ornithology for The Auk. In both, Feduccia makes exceedingly reasonable arguments about what we really do and do not know about the origins of birds, with insights on how science works and how science can sometimes become biased. It is interesting reading, no matter which side you favor. Feduccia and Stephen Czerkas (2015) also present intriguing evidence that some of the large, walking maniraptoran dinosaurs that were suggested to be avian ancestors possessed a propatagium. A propatagium occurs only in flying birds or birds that once flew. Their argument is that formerly flying birds moved to the ground, became large and flightless, and acted like terrestrial dinosaurs even though they were birds. In a review of Feduccia’s book, Walter Bock (2015) makes a convincing case that we should keep an open mind about the evolution of birds until we have better evidence from the fossil record.

On the prodinosaur side, two major works appeared in late 2014 and early 2015.¹⁴ A group of 11 of the most influential avian paleontologists in the country present material that is highly critical of Feduccia and his approach. They present a great deal of supporting material for their decision that this debate has been settled in the minds of all but a handful for decades and the majority of the scientific community has moved away from arguments over the origin of birds and on to other more compelling specific questions. Another multiauthored review paper relies on a very broad, integrative approach to understanding the evolution of a variety of avian traits, including feathers, flight, reproductive behavior, and pulmonary systems. This suggests that the transition from ground-living to flight-capable theropod dinosaurs now probably represents one of the best-documented major evolutionary transitions in life history. This review provides extensive new evidence regarding evolution of digits, feathers, flight, and other related traits.

Is the debate over? Perhaps. It is hard to go against the material presented by N. Adam Smith and his ten colleagues or Xing Xu and his six coauthors regarding the evolution of birds, but I still personally find it difficult to give up on the old explanation. That may be my bias, as I was raised on the thecodont model and I tend to be the same age as most of those who support it. Perhaps I have just bought into what was described as the rhetorical sham of support for this model, or my age affects the impact of the sociology of science on me. Most of my graduate students seem to favor the theropod model, perhaps for the same reasons I am less comfortable with it. If that model is correct, then modern ornithology is simply the study of the surviving dinosaurs. This may be way cool, or perhaps it means that ornithology should be just a section of herpetology, dividing time with the snakes, turtles, and lizards.

I encourage everyone to keep an open mind regarding the question of avian origins. Other points of view may end up being important in figuring out what really happened. For example, a recent study done by morphologists who are totally separate from the controversy about bird evolution shows that birds are knee runners, animals that run with the thigh bone almost fixed.¹⁵ All other land animals move their thighs as they run. For birds, knee running is needed to keep their air sacs functioning properly, which is necessary for flight. For this reason, most physiologists are almost certain that birds could not have evolved such an unusual mechanism for breathing unless they separated from their dinosaur relatives long in the past.

The Radiation, Extinction, and Radiation of Modern Birds

Rapid evolution of birds occurred during the Age of Reptiles, when dinosaurs ruled the earth and mammals simply tried to hang on. While it is believed that both the Archaeopteryx and the four-winged bird lines went extinct rather quickly, two major groups of birds evolved into a variety of types during the period from 150 mya to 65 mya (fig. 1.7). One of these groups was the enantiornithines (known as the opposite birds because the articulation between the scapula and the coracoid was completely the reverse of that found in modern birds), which developed into birds ranging from sparrow to vulture sized. Another group is considered the primitive ornithurines, which includes such groups as the Hesperornithiformes (heavy-bodied divers), Ichthyornithiformes (tern-like aquatic birds), and the poorly known Apsaraviformes (Ambiortiformes).¹⁶ Little evidence exists that any of these early forms ended up leading to modern birds, although the world 66 million years ago appeared to have a broad diversity of birds.

A recently discovered new fossil of a species named Archaeornithura meemannae is estimated to be 130 million years old.¹⁷ This fossil is very similar to modern birds and is considered the oldest example of something that may have led to modern birds. But few such avian fossils exist that are that old, and most of the first records for modern forms of birds occur at only around 70 mya or more recently, although the new species suggests that there was a lineage of modern birds throughout the period from 130 mya to the giant change of 65 mya.

Fig. 1.7. A summary of avian radiation, then extinction, then radiation over the past 150 million years. Note that Archaeopteryx and its relatives went extinct early in time, while the enantiornithines and ornithurines became diverse before disappearing during the K-T collapse. Some of the ancestors of modern birds survived the K-T disaster and have diversified greatly in the past 65 million years. The recently discovered bird Archaeornithura sits by itself because it is an extremely old fossil but has no clear evolutionary pathway from the fossil record.

Most know that this Age of Reptiles ended about 65 mya when an asteroid hit Central America, changed the world’s climate, and caused the extinction of nearly all the dinosaurs. This piece of rock was about 10 km in diameter and hit the earth at 40 times the speed of sound, producing an impact equivalent to 100 trillion tons of TNT. This impact resulted in debris being sent high into the atmosphere, with some of it going halfway to the moon before coming back because of the earth’s gravitational pull. The dust from this event, plus an outbreak of volcanic activity that may or may not have been related to the asteroid, resulted in widespread fires, massive cloud cover, and a dramatic increase in the earth’s atmospheric temperatures. It is suggested that 75% of the species in existence on earth went extinct, but some areas served as refuges for some species.

This period is known as the Cretaceous-Tertiary (K-T) boundary, as it ended one age (Cretaceous) and opened another (Tertiary). Although the K-T boundary is famous for its effects on dinosaurs, recent evidence suggests that it had tremendous effects on birds too.¹⁸ The enantiornithines and primitive ornithurines all disappeared. Some birds survived, perhaps some ostrich relatives and some birds called transitory shorebirds, and it is from these that all modern birds have evolved. Data suggest that an amazing amount of change occurred during the first 10 million years after the K-T boundary, such that most modern avian forms except the Passeriformes appeared within that time. The Passeriformes started to radiate about 30 million years ago. This model suggests that most of the evolution of modern birds can be traced back only about 65 million years to the relatively few forms that were able to survive through the darkness and cold of the postcollision period. Recent work suggests that all the placental mammals that occur today arose after the K-T boundary, suggesting that most of our birds and mammals are of relatively recent origin.

There is considerable controversy about which forms of birds had evolved before the K-T extinctions, and the whole idea of massive avian extinctions at that time is fairly new. As you can see by the references, some of the same people are involved in this discussion as were involved with the controversy about birds and dinosaurs in the earliest evolution of birds. With so much evolution followed by a great deal of extinction, we are a long way from clearly following the exact path from the first bird to the modern birds we see today.

CHAPTER 2

General Traits of an Avian Flying Machine

Birds are distinctive for their powers of flight. Although other animals fly, in no major group is flight such a dominant part of the general lifestyle. Because the constraints of flight are severe, birds as a group are relatively uniform in shape and overall structure. Even the small number of flightless birds have evolved from flying forms, so they show relatively few deviations from the standard, aerodynamic design. While we will focus here on adaptations for flight, remember that all birds alight and move around on ground, water, or both, so compromises must also be struck with terrestrial or aquatic locomotion. Much of the variation in the avian form is related to the proportion of time spent walking or swimming versus flying. Yet even with this variation, we can rather easily visualize a standard version of an avian flying machine. Before we look at all the components in detail, let’s see how they must fit together.

Building a bird is not unlike building an airplane, a fact not lost on early engineers. The ultimate requirements for flight are high power, low weight, and a balanced yet streamlined design.¹ A bird is unlike an airplane in that it also must carry some structures (such as a reproductive system) that are not related to flight. High power comes directly from highly developed breast muscles, which may account for half of the bird’s body weight in a strong flier. Keeping these muscles working requires a hot, hard-working engine (bird body temperatures run from 107° to 113°F).