Bird Skeletons

Before studying this page and the accompanying specimens in lab, you should read the Skeleton lab introduction page.

Objectives:

• Identify & name bird bones corresponding to the mammal bones you learned earlier.
• Describe how bird skeletons differ from those of mammals and explain the functional significance of these differences.
• Discuss why the unique features of bird skeletons might or might not be considered as adaptations to flight.

Specimens

You should apply the information on this page to the following specimens in lab:

• Bird skeletons, including chicken & duck
• Bat & bird wing skeletons (compare the wing of this flying mammal to the wing of a bird)
• Bird skulls

Warning: these skeletons are unbelievably delicate. Please be gentle with them.

Introduction

Birds and mammals are the two groups of large-brained, endothermic (“warm-blooded”) animals on Earth. In this lab you’ll compare bird and mammal skeletons, and you’ll see some clear homologies between these two groups. This shouldn’t be too surprising, since both birds and mammals are descended from the same early vertebrate ancestors. On the other hand, you will also see some distinct differences between bird and mammal skeletons. There are two main kinds of explanations for these differences: ancestry and adaptation. While birds and mammals both came from the same original terrestrial vertebrate ancestors, they came from different branches of the vertebrate tree. If bird skeletons have unique features, it could be because birds inherited those features from their recent ancestors. Therefore, we can't interpret birds' skeletal features without knowing something about their phylogeny. On the other hand, birds do one thing that is dramatically different from mammals: they fly. If bird skeletons have unique features, it could be because birds are adapted to flying. Therefore, we can't interpret birds' skeletal features without considering how the skeleton functions in flying and in other activities.

Both of these factors, ancestry and adaptation, are likely to be important; in this lab you'll consider both of them. As it turns out, many of the unique features of birds have traditionally been interpreted as adaptations to flight. However, some major fossil discoveries have been made in recent years, overturning long-held beliefs about bird evolution. With that in mind, this lab isn't only about the bones of birds; it's about the ways that biologists understand the evolution of body structures.

Before you start looking at skeletons, here is some basic background on the ancestry and unique features of birds.

Birds are dinosaurs

Consider this highly selective phylogenetic tree showing some major groups of land vertebrates. I put boxes around two clades: amniotes and dinosaurs. "Amniotes" includes all the vertebrates with amniotic eggs (yes, mammals have eggs!). Amniotes, unlike frogs or fish, can reproduce on dry land. I will discuss the importance of amniotic eggs in lecture.

Now look at the dinosaurs. Allosaurus, Tyrannosaurus, and Velociraptor are are clearly dinosaurs; after all, dinosaurs like these were featured in the Jurassic Park movies. Notice that there is no clade that includes all these dinosaurs but does not include birds. If Dinosauria is a valid taxonomic group (and it is), then it has to include birds. As it turns out, not all the dinosaurs became extinct; birds are the last surviving dinosaurs. Most people tend to think that the age of dinosaurs ended long ago; however, given that there are over 10,000 species of birds on Earth (compared to 5500 species of mammals), one could say that we are still living in the age of dinosaurs.

What kind of evidence supports this surprising conclusion? Comparisons of the genomes of living vertebrates have shown their evolutionary relationships quite clearly. For the phylogenetic tree shown above, the relationships among crocodiles, turtles, lizards, birds, and mammals could be determined without even looking at the fossils. As it turns out, the conclusions based on DNA evidence agree with those based on fossil evidence. Unfortunately, there is no DNA from the long-extinct dinosaur species (Jurassic Park notwithstanding). Therefore, to understand the relationships of animals known only from fossils, paleontologists must study the fossils themselves.

Dinosaur fossils are old, and have been studied intensively since the late 1800s. You might think that all the important discoveries would have been made long ago, but some remarkable bird-related fossils have been found in recent years. These fossil finds have shown how similar birds are to some of their extinct dinosaur ancestors. Before you read on, take a moment to think about what characteristics make birds different from a Velociraptor or Tyrannosaurus. What makes a bird a bird? Here are a few of those traits; you could probably think of others.

• Ability to fly
• Small size (compared to larger extinct dinosaur groups)
• Wings
• Feathers (but many non-flying dinosaurs also had feathers)
• High metabolic rate (compared to other extant reptiles)
• Short tail (the bony part, not the feathers)
• Beak with no teeth

These characteristics might seem to be unique to birds, but the surprising thing is that all of them have been found in nonflying fossil dinosaurs. While it's been clear for many years that birds descended from dinosaurs, researchers have only recently learned how much those prehistoric dinosaurs resembled birds. Some of these characteristics have traditionally been explained as adaptations to flight, but if the characteristics first appeared in dinosaurs long before the evolution of flight, then they weren't adaptations to flying.

This lab is about living birds, not extinct dinosaurs, so I will describe just two of the unusual features that birds inherited from their dinosaur ancestors: feathers and pneumatic bones. If you'd like to learn more about why birds are classified as dinosaurs, see the references under bird evolution at the bottom of this page.

Feathers

Feathers are essentially a highly modified form of the scales found on other reptiles. Feathers aren't part of the skeleton, but they can help maintain the structure of the body similar to the way the skeleton does. For example, compare the wings of a bird to the wings of a bat. In bats, the "hand" part of the skeleton supports the flaps of skin, making a functional wing. In birds, the "hand" part is proportionally much shorter. The tip of the wing is composed only of the large primary flight feathers. As a result, the number of bones in the bird forelimb has been reduced compared to their ancestors. In fact, bird skeletons generally have many bones eliminated, reduced in size, or fused together compared to mammals, to the extent that a bird's feathers commonly weigh more than its bones.

Since feathers play an essential role in bird flight, it's tempting to think that feathers originally evolved as an adaptation to flight. However, this idea turns out to be wrong. We know this because feathers evolved long before birds began to fly. Numerous fossils found in the last decade or so have confirmed that a great many dinosaur species had feathers, long before the evolution of flight. If feathers existed before flying, then feathers did not arise as an adaptation to flying.

This doesn't mean that feathers aren't relevant to flight; it simply means that feathers first evolved for other reasons and later became adapted to flight. If this is the case, then how did feathers function in nonflying dinosaurs? In modern birds, feathers are also important for insulation, mating displays, and as aerodynamic aids when running. All these may have been factors in the evolution of feathers in the dinosaur ancestors of birds; it is a matter of some debate among paleontologists as to which factors were most important.

Feathers didn't evolve all at once. Numerous feathered dinosaur fossils have been found, with feathers ranging from simple hairlike structures to complex asymmetrical flight feathers resembling those of modern birds. Functionally, these feathers didn't all do the same thing, so it's likely that a variety of forces acted on the early evolution of feathers. Birds may be the only living group of animals with feathers, but that wasn't always true; birds are simply the last surviving members of a large group of feathered dinosaurs. For more information, see references under the evolution of feathers, below.

Pneumatic bones

Find an isolated bird bone and compare it to a mammal bone of similar size. 

Birds have pneumatic (air-filled) bones; the air spaces may make up the majority of the bone's volume. In mammals, bones are generally solid, or else filled with marrow. Why do birds have pneumatic bones? The traditional explanation is that pneumatic bones evolved because it makes the bones lighter, enabling birds to fly better. There are two problems with this explanation. First, many large, nonflying dinosaurs also had pneumatic bones. If pneumatic bones evolved before flight, they didn't evolve because of flight. Second, birds don't necessarily have lighter skeletons than mammals of equivalent size. Although birds' bones contain air spaces, the bone tissue is denser, so it their pneumatic bones aren't necessarily lighter than the solid bones of a mammal with the same mass. This doesn't say that pneumatic bones aren't relevant to flight, though. The hollow, dense construction of bird bones does give them greater rigidity than comparable mammal bones. However, just because pneumatic bones function well for flying, that doesn't mean that they originally evolved as a result of natural selection for improved flight.

Why did non-flying dinosaurs have pneumatic bones? The air spaces in bones don't only affect the mass and stiffness of the bones. These air spaces are connected to the system of air sacs that runs throughout the bird's body. As you learned in lecture (I hope), birds have one-way air flow through their lungs, aided by air sacs that help to pump the air in and out. These air sacs extend into the bones, and air can move in and out of the bones as it moves through other parts of the system. That's how it works in birds, and it seems likely that at least some dinosaurs had a similar system of air sacs and 1-way ventilation through the lungs. In this respect, birds seem to have simply retained the pneumatic bone structure inherited from their dinosaur ancestors. Still, one important question remains: Air sacs may help pump air through the lungs, but why have them in the bones? Does this help with gas exchange or some other process? Frankly, I don't know the answer to this one.

Overall, we can say that pneumatic bones are an important feature of bird skeletons, inherited from older dinosaurs, and adapted to function in a small flying animal. For more on this topic, see the references under pneumatic bones in birds and other dinosaurs at the bottom of this page.

Bird skeleton

Here is a simple diagram of a bird skeleton (from Wikimedia Commons). Many of the bones are clearly homologous to those in mammals, but there are a few important differences.

The numbered bones are:

The words in bold type are things that you might be asked to identify on the lab exam. Rather than focusing only on the names of the bones, it's useful to think about the major differences between bird skeletons and mammal skeletons, as described below.

Here's another bird skeleton diagram.

Neck

Long, flexible neck: Mammals generally have seven cervical (neck) vertebrae, regardless of size (even giraffes have seven!). Birds have typically have 12-25 cervical vertebrae. Not only is the neck proportionally longer for birds than mammals, it is also more flexible. Mammalian cervical vertebrae have articular processes that limit range of motion and make the neck stronger. In birds, the equivalent processes are much smaller. Also, the shape of the vertebrae themselves allows for a greater range of motion. This allows birds to move their heads freely for feeding and flying and to reach any part of the body with the beak for grooming. Since birds don't have hands, this is important.

Trunk region:

The trunk (or torso) is the main part of the body, not including limbs, head, and neck. In mammals, the trunk is fairly flexible; the lumbar and thoracic regions of the vertebral column allow us to bend forward and back. In contrast, birds' bodies are much less flexible in the trunk region. This allows the bird's body to resist the powerful forces generated by the flight muscles acting on the wings. Overall there is very little flexibility between the hip and shoulder, thanks to the structures described below.

• Synsacrum & hip bones: Compared to mammals, the hip and sacral region of birds is more fused and more rigid. In humans, the sacrum is formed by five fused vertebrae which connect to the hip bones at the sacroiliac joint. In birds, the number of vertebrae involved in the synsacrum includes all the lumbar vertebrae, some thoracic vertebrae, and one or more caudal vertebrae. The synsacrum is tightly fused with the hip bones (sometimes called the innominate bones, which includes regions corresponding to the ilium and ischium of mammals).
• Fused thoracic vertebrae: In contrast to the bird's neck, the thoracic region of the vertebral column (between the synsacrum and the cervical region) is fairly rigid.
• Keeled sternum: The pectoral muscles of birds are among the the main flight muscles and are typically the largest muscles in the body. These enormous muscles, which make up the breast meat in a chicken, are attached to a large, projecting structure (keel) on the sternum. In fact, the chest muscles include the muscles that raise the wings, as well as those used for downward flapping. The keeled sternum is clearly an adaptation to flight. It plays an essential role in flying, and it did not exist in birds' ancestors until they began to fly.
• Ribs with uncinate processes: Look closely at a bird's ribs; each has an uncinate process sticking out on the posterior edge. These processes provide leverage for muscles that attach to the ribs. The uncinate processes and their attached muscles have traditionally been described as a mechanism for preventing the bird's chest from collapsing when the large pectoral muscles are contracted. More recent research has shown that the uncinate processes and their muscles also help to expand and contract the chest during breathing.

Pelvic girdle, legs & feet

The leg bones of birds are clearly homologous to those of mammals, but there are some important differences. People sometimes mistake the birds' ankles for their knees, which causes some confusion. Take a look at a bird skeleton in lab and identify the knee and ankle (many people get this wrong at first glance). For a comparison of leg bones in humans and ostriches see fig. 2 from Birds on the run: what makes ostriches so fast? Oddly, for many birds the femur is more or less horizontal when the bird is standing. The femur is short compared to the other bones of the leg, unlike in humans. On the other hand, the relative proportions of the bones in a horse's leg are more similar to those of an ostrich than to a human. Birds have digitigrade posture: they stand on their toes, with their heels above the ground. Some of the bones found in most tetrapods (four-limbed vertebrates) are missing in birds, and some bones have been fused.

• Femur: The upper leg bone is easy to recognize and is homologous to the femur of mammals.
• Tibiotarsus: In birds, part of the tarsus is fused with the tibia. The tarsus in mammals consists of seven bones in the foot.
• Fibula: very small in birds.
• Tarsometatarsus: Part of the tarsus is fused with the metatarsus into a single bone. Thus, the "ankle" of birds is actually within the tarsus, which corresponds to the small bones of the human foot. Overall, birds have fewer bones in their feet than mammals do. Like the tibiotarsus, the tarsometatarsus is found in birds but not mammals.
• Toes: Most birds have four toes on each foot. Most birds have one toe facing backwards, allowing the feet to grip. Some birds, such as woodpeckers, have two toes facing back and two facing forward. The toe bones in birds' feet, like those in mammals, are called phalanges. Finger bones are also called phalanges.

Pectoral girdle & wings

Diagram from Wikimedia Commons.

The pectoral girdle is the set of bones that supports the forelimbs (wings). In birds, it includes the furcula, scapula and coracoid:

• Furcula (wishbone):  The furcula apparently corresponds to the clavicles (collarbones) of mammals. In birds, these bones are fused into a single structure. The furcula is springy; when a bird flies, it flexes in and out, storing and returning energy like a spring. This spring action wouldn't be possible if birds had two separate clavicles instead of a single furcula. It makes sense to view this structure as an adaptation to flight, but many nonflying dinosaurs (including Tyrannosaurus rex!)  also had a furcula, so this fused structure didn't originate as an evolutionary response to flying. On the other hand, the furcula of T. rex and similar species was short and not structured to be springy. The furcula of birds did not originate as an adaptation to flight, but it has certainly been shaped by selection for efficient flying.
• Scapula: The scapula (shoulder blade) of birds is proportionally much smaller and flatter than that of most mammals. This means that the space for muscle attachments is limited, as is the leverage that these muscles can produce. Birds don't have large back muscles. As mentioned above, muscles in the chest area are used for both raising and lowering the wings. Do you think that the small, flat scapula of birds is an adaptation to flight? Take a moment to consider how you would answer this question.
• Coracoid: Forming a part of the pectoral girdle supporting the wings in birds, the coracoid connects the shoulder joint (head of the humerus) to the sternum. Thanks to the rigid coracoid, the bird's chest does not get flattened when the bird flaps its wings. In most mammals, the pectoral girdle consists only of the scapula and the clavicle; there is no coracoid bone.

The wing skeleton includes these bones:

• Humerus, radius, ulna: These bones are quite similar to their homologs in most mammals.
• Carpometacarpus: This bone is homologous to both the mammalian carpal and metacarpal bones. Overall, the number of "hand" and "finger" bones in birds is greatly reduced compared to most other reptiles or to mammals. The joint betweeen the carpometacarpus and the ulna (along with the radius) more or less corresponds to the wrist. In birds, this joint has a wide range of lateral movement, allowing birds to fold their wings when at rest.
• Phalanges: Phalanx is singular, phalanges is plural. Phalanges are finger bones, for birds or humans. Birds only have three "fingers"; their ancestors apparently had five, as do humans. The first one, roughly corresponding to a thumb, is called the alula. The alula bone supports a few feathers that can be moved separately from the others on the wing. (Incidentally, the word alula is Latin for "little wing." "Little Wing"  is also the name of a classic song by Jimi Hendrix.)

Study the diagram above and think about what it means for bird flight. 

• The main flight muscles are the pectorals, connecting the humerus to the sternum.
• There is very little muscle in the wings, making them light and relatively easy to move.
• The muscles for raising the wings (supracoracoideus) are also in the chest area. This is not the case for humans; you would use various muscles of your back to pull your arms backward. One thing this means for birds is that more muscle mass is concentrated in the ventral part of the chest, giving the body a lower center of gravity during flight.

Overall, it appears that the presence of the coracoid and furcula, along with the thin flat scapula, represent characteristics that birds inherited from their dinosaur ancestors. On the other hand, the deeply keeled sternum, springy shape of the furcula, and reduced number of bones in the wings all represent ways that birds are different from their nonflying ancestors. These features apparently evolved along with flight.

Great horned owl

Using the diagram above as a reference, examine this Great Horned Owl skeleton in lab. The Great Horned Owl is one of the more common owls in central California, and if you've heard an owl hooting in a dark forest in our area, it was probably one of these.

Look at the date on the tag around this owl's leg — this specimen has been in our lab for a long time.

Bat wing vs. bird wing

We have a specimen of a bat wing skeleton and also a whole skeleton of a bat. Compare the specimen of a bat's wing skeleton to the bird wings that you've already observed.

Although many of the same bones are present in all three of these vertebrate forelimbs, there are significant differences among them. In this picture, you can see that the proportions vary among species; the human arm has a relatively long humerus (upper arm) and short hand compared to the bird. However, the opposite might be true if we had chosen different mammal and bird species — for example, a deer (short humerus, long foot) and an albatross (long humerus, short "hand").

Looking at specific bones, you can see that the bird has some distinctive features. The number of carpals, metacarpals, and phalanges is reduced, and so is the size of these bones. Also, the carpometacarpus is formed by the fusion of carpals and metacarpals in the embryo. Rather than using numerous bones to control wing shape (as bats do), birds use a small number of bones, supplemented by a large number of feathers.

A comparison of bird wings and bat wings is an example of both homology and analogy. These wings, along with the arms of humans or the forelegs of cats, are examples of vertebrate forelimbs. It's clear that all these are homologous structures. The last common ancestor of birds and mammals had forelimbs with similar bones. In each of these species, the forelimbs develop the same way in the embryo, using homologous genes to control limb development. So as forelimbs, the wings of bats and birds are homologous. On the other hand, as wings they are analogous. The most recent common ancestor of bats and birds had forelimbs, but not wings. Wings evolved separately in bats and birds, so the wings of bats should be considered analogous to the wings of birds.

Bats also have keeled sternums

Take a close look at the whole bat skeleton. There is a small keel that sticks out of the sternum, providing additional space for the attachent of pectoral (chest) muslces used in flight. You can see a keeled sternum on three kinds of specimens that we have in lab: bats, birds, and moles.

Bird Skull

Bird skulls are dramatically different from those of other reptiles, as well as mammals.

Beak. The most obvious thing that tells you you're looking at the skull of a bird and not a mammal is the beak. Some mammals (dolphins, for example) may also have a beaklike shape, but birds are distinctive in having a hard coating of keratin (the same protein found in hair, feathers, and scales) on the outside.

Birds don't have teeth. The bird clade lost its ancestral reptilian teeth as it adapted to flight; the skull is much lighter without them. The lack of teeth does pose a problem, though: how do birds chew their food? The answer is that birds must swallow whole whatever piece of food they can pick up or tear apart with their beaks. They can't chew their food before they swallow it, but they can chew it after they swallow it. Birds have a gizzard for chewing their food after they've swallowed it.

Eye ring. Birds have a bony, movable ring surrounding the eye; mammals lack this.

Salt glands. Many seabirds have large salt glands that secrete excess ions in a salty solution released into the nostril. The salt glands (also called nasal gland or supraorbital glands) are not part of the skull, but their location is visible on the outer surface of the skull, just above the eye.

Jaw hinge and palate. For more on these features, see Skulls: reptiles vs. mammals on this Bio 6A site.

Summary

Birds are the last surviving dinosaurs. Birds have many characteristics that are unique among living vertebrates, and birds are uniquely adapted to flying. However, most of the unique characteristics of birds did not originally evolve as adaptations to flight; instead, these characteristics evolved in birds' dinosaur ancestors, long before the evolution of flight in birds. The form and function of bird skeletons reflects both the birds' dinosaur ancestry and the evolutionary demands of flight.

Some important characteristics that are shared by birds and extinct dinosaurs:

• Furcula. While nonflying dinosaurs had a furcula, it did not have the springy construction found in living birds.
• Pneumatic bones. Air-filled bones may be stiff and lightweight for flying, but they originally evolved in dinosaurs that didn't fly. As far as I know, it's not clear why the air spaces these bones should be connected to the respiratory air sacs.
• Feathers. Birds are the only living animals with feathers, and feathers are essential for the mechanics of bird flight. However, many nonflying dinosaurs also had feathers.

What is an adaptation?

Campbell defines an adaptation as "an inherited characteristic of an organism that enhances its survival and reproduction in a specific environment." Under this definition, avian structures such as the fucula, pneumatic bones, and feathers are important adaptations for flying animals. On the other hand, these structures did not arise originally as adaptations for flight; instead, dinosaurs already had a set of characters that made them well suited to flight, long before the origin of birds.

Sample test questions

1. Describe some important structural and functional differences between bird skeletons and mammal skeletons, with respect to the structures listed below. As you answer this question, give the names of the specific bones involved, explain how they are different, and relate the differences to both the ancestry and functional characteristics.

2. What is this bone? You should be ready to identify these, and compare them to equivalent features on mammal skeletons:

3. Compare and contrast the vertebral column of a bird and a cat, in terms of these regions: cervical, thoracic, lumbar, sacral, caudal. (See the cat skeleton or Skeleton Lab Introduction on the skeleton lab introduction page for a diagram.)
4. Are the pneumatic bones of birds an adaptation to flight? Explain.
5. Describe several ways that the skeletons of birds are similar to those of extinct dinosaurs, but different from mammals.
6. Describe several ways that the skeletons of birds are distinctly different from those of extinct, non-birdlike dinosaurs.
7. What is a keeled sternum? Why is it important? Which specimens that you have seen in lab have this feature?

References & Further Reading

You don't need to read any of these references do do well in Bio 6A. I'm including this extensive list to provide a record of the sources I used in making this lab activity and to offer some suggested reading for anyone who wants to dig a little deeper on any particular topic related to this lab.

Bird skeletons: basic references

Pigeon skeleton from Udo Savalli at ASU.

Ornithology (bird biology) in general

Ornithology 554/754 at Eastern Kentucky University with Gary Ritchison. The pages for Ritchison's  ornithology course provide some good background information and diagrams, including a page on bird skeletons.

Bird evolution

Great Transitions: The Origin of Birds. For an overview of the origin of birds, this video from HHMI BioInteractive is the best place to start.

How Dinosaurs Shrank and Became Birds. Singer, 2015. Quanta. Excellent exploration of the evolutionary processes behind this amazing transition.

Benton MJ (2010) Studying Function and Behavior in the Fossil Record. PLoS Biol 8(3): e1000321. This brief, nontechnical article discusses the ways that paleontologists attempt to reconstruct the biology of extinct species.

Are Birds Really Dinosaurs? Part of DinoBuzz at UC Museum of Paleontology. This article describes some of the skeletal features that link birds to a specific group of dinosaurs. Unfortunately, the article is a little dated; it's missing some important recent fossil evidence of dinosaur feathers and hollow bones.

Benton, Michael J., 2014. How birds became birds. Science 1 August 2014: 345 (6196), 508-509. Birds evolved from much larger dinosaurs, so one of the most striking trends in bird evolution was miniaturization. This article by Benton is a Perspective, a brief, nontechnical summary of the context and findings of some recent research. For a deeper look, see Sustained miniaturization and anatomical innovation in the dinosaurian ancestors of birds, by Michael S.Y. Lee (2014), Science, Vol. 345 no. 6196 pp. 562-566.

From dinosaurs to birds: a tail of evolution Dana J. Rashid et al. 2014, EvoDevo 2014, 5:25. This article from EvoDevo (a journal dedicated to the study of evolution and development) discusses the possible genetic changes that might have led to one key step in the evolution of birds from their dinosaur ancestors: the shortening of the tail and the fusion of the last few caudal (tail) vertebrae. This is an example of a research paper that connects the worlds of Bio 6A (form & function) and Bio 6B (molecular biology).

Bird Evolution. Julia Clarke & Kevin Middleton, 2006. Current Biology, 16(10): pR350-R354. An excellent review of bird evolution. This article is brief and readable. The authors explain why birds should be considered as dinosaurs, and has a cladogram showing important events in the evolution of birds.

X. Xu et al., “An integrative approach to understanding bird origins,” Science 346, 6215 (12 December 2014). This recent review provides a detailed look at recent research on bird origins. From the abstract: "Recent discoveries of spectacular dinosaur fossils overwhelmingly support the hypothesis that birds are descended from maniraptoran theropod dinosaurs, and furthermore, demonstrate that distinctive bird characteristics such as feathers, flight, endothermic physiology, unique strategies for reproduction and growth, and a novel pulmonary system originated among Mesozoic terrestrial dinosaurs. The transition from ground-living to flight-capable theropod dinosaurs now probably represents one of the best-documented major evolutionary transitions in life history." Free abstract.

S. L. Brusatte et al., “Gradual Assembly of Avian Body Plan Culminated in Rapid Rates of Evolution across the Dinosaur-Bird Transition,” Current Biology 24, 20 (20 October 2014). Pdf available here. From the abstract: "there was no great jump between nonbirds and birds in morphospace, but once the avian body plan was gradually assembled, birds experienced an early burst of rapid anatomical evolution."

Resolving the Flap over Bird Wrists, Robin Meadows, 2014. PLOS Biology. The number of bones in birds' wrists is much smaller than in their dinosaur ancestors. How did this structure, which is essential for bird flight, evolve? As summarized in this article, the answer to this question depends on both paleontology and developmental biology. This article is a nontechnical summary of a research article, New Developmental Evidence Clarifies the Evolution of Wrist Bones in the Dinosaur–Bird Transition, by Botelho et al. (2014).

T. Wogan, “Flight may have evolved multiple times in birds,” Science (2 July 2014).

The evolution of feathers

Feathers are one of the defining characteristics of birds, and they play a key role in flight. However, recent paleontological studies show that feathers evolved before flight. What did these feathers do in the non-flying ancestors of birds?

C. Foth, H. Tischlinger, and O. W. M. Rauhut, “New specimen of Archaeopteryx provides insights into the evolution of pennaceous feathers,” Nature 511, 7507 (3 July 2014). The authors suggest that they may have played an important part in mating displays, as they do in modern birds. See also New fossil shows Archaeopteryx sported 'feathered trousers' on Phys.org for a nontechnical summary of this article.

“Feathers that didn’t fly,” Science 345, 6192 (4 July 2014).

M. Balter, “Earliest dinosaurs may have sported feathers,” News from Science (24 July 2014).

P. Godefroit et al., “A Jurassic ornithischian dinosaur from Siberia with both feathers and scales,” Science 345, 6195 (25 July 2014).

Body size

Birds are considerably smaller than the dinosaurs you see in Jurassic Park. The bird lineage evolved very rapidly both in size and in other aspects; the reduction of body size may have

University of Southampton, “Shrinking dinosaurs evolved into flying birds,” ScienceDaily (31 July 2014).

E. Singer, How Dinosaurs Shrank and Became Birds. Quanta, 2015. (This article was reprinted on Scientific American).

M. Balter, “How Birds Survived the Dinosaur Apocalypse,” News from Science (6 May 2014). Free.

M. S. Y. Lee et al., “Sustained miniaturization and anatomical innovation in the dinosaurian ancestors of birds,” Science 345, 6196 (1 August 2014).

R. B. J. Benson et al., “Rates of Dinosaur Body Mass Evolution Indicate 170 Million Years of Sustained Ecological Innovation on the Avian Stem Lineage,” PLOS Biology (6 May 2014).

Pneumatic bones in birds and other dinosaurs

Many of the bones in birds' skeletons contain air spaces, which are not found in the bones of other living vertebrates. The hollow bones of birds were traditionally considered to be an adaptation to flying, but recent fossil studies have shown that some of the nonflying dinosaur ancestors of birds also had hollow bones.

Respiratory evolution in archosaurs. Brocklehurst et al., 2020. The "unique" features of avian gas exchange and ventilation aren't all unique to birds. The system of immobile lungs with one-way air flow and air sacs for ventilation evolved step by step in the archosaurs, a group that includes crocodiles, velociraptors, and birds.

Matt Wedel: Hunting the inflatable dinosaur. UC Museum of Paleontology. This article describes research on dinosaur fossils which showed that dinosaurs had a system of hollow bones and air sacs, similar to modern birds.

Evidence for Avian Intrathoracic Air Sacs in a New Predatory Dinosaur from Argentina. Sereno PC, Martinez RN, Wilson JA, Varricchio DJ, Alcober OA, et al. (2008). PLoS ONE 3(9): e3303. doi:10.1371/journal.pone.0003303. The authors describe a recently discovered fossil of a large dinosaur (Aerosteon) that had air sacs in its bones, similar to those found in birds. They suggest that the air sacs and one-way lung ventilation of birds, often described as adaptations to flight, apparently evolved long before the origin of birds and vertebrate flight.

Aerosteon riocoloradensis: A Very Cool Dinosaur from Argentina. Greg Laden's Blog on Science Blogs.com. Discusses the significance of Aerosteon's breathing mechanisms, as detailed in the research article above.

Dumont, Elizabeth, 2010. Bone density and the lightweight skeletons of birds. Proceedings of the Royal Society B 277 (1691): 2193-2198. Birds have hollow bones. The traditional explanation for this is that it makes the bones lighter, making flight easier. However, the skeleton of a small bird weighs about the same as the skeleton of a mammal of the same body mass. Hollow bones don't result in a lighter skeleton because the bone tissue of birds is more dense than that of mammals. For a nontechnical summary of this article, see Bird bones may be hollow, but they are also heavy from ScienceDaily, 23 March 2010.

Birds running and flying

Birds on the run: what makes ostriches so fast? Nina Schaller, 2011. Science in School.

Activity of three muscles associated with the uncinate processes of the giant Canada goose Branta canadensis maximus. Codd, J.R. et al. 2005. Journal of Experimental Biology 208: 849-857. Researchers stuck electrodes into the muscles attached to the ribs of geese, then convinced the geese to run on treadmills. They found that some of the muscles attached to the uncinate processes of the ribs are used mainly for trunk stabilization, while others are used primarily for breathing.

Muscle function in avian flight: achieving power and control. Andrew Biewener, 2011. Philosophical Transactions of the Royal Society 366 (1570): 1496-1506. An extremely detailed review of how birds' muscles work during flight, with descriptions of the experimental methods (electrodes in muscles!) used to figure it out. This article is far too detailed for most people, but it does include an excellent diagram of flight muscles and how they're connected to the skeleton.

The Supracoracoideus – An Ingenious Adaptation For Flight by Ron Dudley. A simple description of some of the flight muscles and how they interact with the skeleton. The surprising thing is that muscles in the chest are used for both lowering and raising the wings. If you could fly, you'd probably use chest muscles for lowering your wings and back muscles for raising them.

Bird skeleton diversity and identification

Avian Osteology at the Royal BC Museum. Osteology is the study of bones. If you find a bird bone and want to figure out what species it's from, this site will help you.

Seabird osteology. Contains a few detailed images comparing bones from different seabird species. See General Topography and Nomenclature for some extreme detail.

Skullsite. Images of skulls and other bones from a wide range of species.

Books

Campbell Biology, 10th ed. See chapter 34 for an overview of vertebrate evolution, with a brief look at birds. See fig. 34.24, A phylogeny of amniotes, for a cladogram showing bird relationships.

Kardong, Kenneth, 2012. Vertebrates: Comparative Anatomy, Function, Evolution (6th Edition). This big, expensive textbook is an excellent resource. It is the main source I used in making these pages.

Birkhead, Tim, 2012. Bird Sense: What It's Like to Be a Bird. This book is about the senses of birds, not their skeletons. However, it provides a fascinating look at how birds live. There is one section that relates to the skeletons you study in today's lab: hearing, and particularly echolocation, work a little differently in birds compared to bats, as a result of their differing middle ear bones. Easy reading. I highly recommend this book.