Natural History of Vertebrates
Lecture Notes
Chapter 15 - The Evolution of Birds

These notes are provided to help direct your study from the textbook. They are not designed to explain all aspects of the material in great detail; they are a supplement to the discussion in class and the textbook. If you were to study only these notes, you would not learn enough to do well in the course. These notes are also linked with the notes from Vertebrate Structure and Development (ZO 515).

List of Terms

Origin of Bird Flight

Two competing types of theories, arboreal versus cursorial
  • Arboreal Theory– ancestors were tree dwellers with a need to leap from branch to branch and then from tree to tree, wings started as an increased area for gliding and to slow falls. All present day flying vertebrates are arboreal. Initially flying could be a rather cheap, gravity-powered flight. Steps something like this
  • Terrestrial (cursorial) Theory - ancestor was a fast running, bipedal, insectivore. Morphology places the earliest birds as sister taxon to a terrestrial, fast, bipedal dinosaur. No evidence of an arboreal life style in the lineage leading to birds. Wings initially evolve to help snare insects and to control body position as it leaped after insects. The terrestrial theory is better accepted than the arboreal theory.
    Some fossil birds
  • Archaeopteryx - earliest known and most primitive bird, from the Jurassic of Europe. Lacks many features of modern birds and shares many characteristics with the deinychosaur.
  • Ambiortus - known from the early Cretaceous in Mongolia. certainly a flying bird with a well-developed keel on the sternum and an avian shoulder joint (coracoid, scapula, and furcula)
  • Enaliornis – from the early Cretaceous of England, diving bird, flightless. By the early Cretaceous there is already a wide diversity of birds.
  • most modern orders evolved by the Eocene (40 to 50 mybp). Most families had appeared in the Miocene. Most extant genera (some species) by the Pliocene.
  • Today there are about 9000 species, 28 orders, divided into 2 superorders (Table 15.1)


    Flight is very fascinating to humans and as birds are mostly diurnal, humans have studied bird flight for centuries.
    Largest flying bird is about 20 kg with a 7 meter wingspan, while the largest pterosaur is also about 20 kg (bot of these are extinct)
    Twenty kg is the maximum weight for a bird that can fly. Smaller birds can take off much easier than large birds, because of the weight to power ratio is more favorable for smaller birds. As a bird increases in size, the beat frequency of the wings decreases which leads to the reduction in relative power. If you double the weight a bird, you will increase the power that it can generate for flight by only 1.6 times, however it will need 2.25 times as much power for flight. As the power needed increases much faster with increase in body size than does the power generated, there comes a point in size increase where flight is not possible. For modern birds the largest bird is about 12 kg.

    Flightless birds do not have this weight restriction and can be much heavier, for example ostriches weigh 150 kg and the largest bird ever (elephant birds) weighed 450 kg. No birds are quadrupedal.


    In general, feathers grow in tracts called pterylae over the birds body. Unfeathered areas are called apteria (figure 15-7). There are some birds with a uniform distribution of feathers (penguins).

    A typical feather has a calamus, which is a short tube attached to the bird; a rachis, which is the main support in the middle of the feather; a vane, which is made up of barbs that are held together by barbules (Figure 15-8).

    There are several types of feathers
    Physics of flight

    There are several important characteristics of wings.
    Flapping Flight
    The shoulder joint of a bird involves the humerus, scapula, and coracoid. The muscles that power the wing are on the ventral surface. The muscles on the dorsal surface are very weak and are not used to power the wing. The downstroke is power by the pectoralis major. This muscle originates on the keel of the sternum and inserts on the ventral surface of the humerus. The upstroke is powered by the supracoracoideus. This muscle also originates on the keel of the sternum and is deep to the pectoralis major. A tendon runs from the supracoracoideus, though the foramen triosseum and inserts on the dorsal surface of the humerus.

    Strong fliers have as much as 20% of their body weight made up by breast muscle (pigeons), whereas some birds have the breast muscle make up only 10% of the body weight (owls). During takeoff birds usually strongly power both the upstroke and the downstroke.

    Each bird has a set wing beat (number of strokes per second). In general, large birds have a slower wing beat than smaller birds. To increase speed, a bird increases the amplitude of the stroke, but not the beat. Breathing is timed with the up and down stroke of the wings.

    There are four basic wing types (figure 15-15)

    Birds have a four-chambered heart and blood flows through each chamber in sequence as in a human heart. Blood can not be shunted from the pulmonary circulation to the systemic circulation.

    Air flow through an avian lung is unique in that air flow is unidirectional and not tidal as in other vertebrates The system consists of a bronchus, parabronchial lungs (lungs around the bronchus) and relatively large air sacs, which are responsible for moving the air (figures 15-18 and 15-19). Gas exchange occurs in the lung, not in the air sacs. The air sacs cause the air to flow through the lung during both inspiration and exhalation. The flow through the lung is unidirectional and opposite (countercurrent configuration) with the blood moving through the capillaries (figure 15-20). Thus a bird is able to extract almost all of the oxygen from the air that it takes in.

    Because of this very high efficiency, some birds routinely fly at 30,000 feet (at air pressure that would be fatal to humans due to the lack of oxygen). The air sacs also serve to cool the interior of the bird during flight and prevents overheating.


    The digestive tract of birds has some interesting adaptation for holding food and for mechanical digestion crop is basically a pocket in the esophagus. The function is to hold food, for example a blue jay gathering seeds at a bird feeder. It puts the seeds in the crop and then carries them to a cache for the seeds.

    stomach has two parts
    gizzard is the first part. Has a thick lining and is very muscular. Birds eat small bits of gravel and the gizzard grinds the food with the gravel. The function is for mechanical digestion of the food and basically replaces the chewing as birds do not have teeth.
    proventriculus is used for chemical digestion, secrets acid and enzymes for the digestion process.

    small intestine is used for further chemical digestion and absorption of nutrients
    large intestine is for storage of wastes and water absorption
    cloaca is the very last section and receives waste from the excretory system as well. In birds, the feces is composed of urate salts from the excretory system (the white stuff) and indigestible matter from the food (the dark stuff).

    Terrestrial locomotion Birds have two sets of limbs that serve very different modes of locomotion. We have covered flight, which is done by the forelimbs. Terrestrial locomotion is done by the hindlimbs and consists of walking/running To increase running speed, we generally see Birds can not follow this trend as far as mammals as birds have a balancing problem that mammals do not have.

    hopping This is a specialization in which both feet move together. This is typical of smaller birds (songbirds), however in larger birds hopping becomes energetically unfavorable (for example among the corvids, blue jays hop but ravens walk).

    perching Usually birds have three toes forward and one back (called ansiodactylus) which is typical of passerines which perch on limbs, however some birds such as woodpeckers have two toes forward and two toes backward (called zygodactylus) for perching on vertical trunks of trees. In perching birds there is a tendon that runs from the femur across the knee and ankle joints to insert on the plantar surface of the toes. When the leg is bent, the tendon is pulled in such a way that the toes curl. The tendon also has a rough surface so that it "locks" in place. Thus, a bird does not need to expend energy to "hang on" to a limb, even in a high wind. The bird must straighten its leg to be able to let go.

    wading In general wading birds have long legs in relation to their body. Birds with longer legs can forage for food in deeper water (for example herons or egrets).

    Some birds have specialized adaptation for special circumstances. Some birds have highly feathered feet which aids in insulation in cold environments and helps to support the weight of the bird on snow (ptarmigans). Some birds have very long toes for walking on unstable surfaces, such as lily pads. In some birds the feet have been modified for clinging to a wall that they are not very capable of walking or even climbing (chimney swifts).

    Because of the structure of the feet and positioning of the legs, swimming birds are poor at walking, Most waddle like ducks and many only rarely come on land (grebes). Birds that dive have some other adaptation

    Last updated on 13 April 2003
    Provide comments to Dwight Moore at
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