Natural History of Vertebrates
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
- ancestral ground-dwelling quadrupedal reptile
- bipedal ground dweller
- bipedal arboreal dweller
- leaping between trees
- parachuting then gliding
- active powered flight
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.
- ancestor is a quadrupedal biped
- facultative biped
- obligate biped as forelimbs becomes useless for walking
- elongation of forelimbs and increase in scales (feathers) for snaring insects (maybe also for insulation)
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
- contour feathers have a large vane and are found in the wing and tails (figure 15-8)
- down feathers the rachis is shorter than the longest barb and there are no barbules, thus the feather seems very disorganized. This feather is used for insulation and covers the bod of the bird.
- semiplumes are intermediate between a contour feather and a down feather (figure 15-10). The base of the feather is like a down feather but the rachis is longer than the longest barb. Provides shaping and insulation.
- powder down feather these produce an extremely fine powder as they break up. The powder helps to keep the feather dry.
- bristles have a stiff rachis and no barbs (figure 15-10). Generally used for protection around the eyes, nose, mouth and they are also tactile sense organs.
- filoplumes are very fine feathers with a few barbs at the distal end (figure 15-10). They are sensory in nature and provide information about the position of the flight feathers.
Physics of flight
There are several important characteristics of wings.
- Aspect ratio is the length of the wing divided by the width. Thus a long thin wing has a high aspect ratio. Increased aspect ratio leads to increased lift
- Wing loading is the mass of the bird divided by the wing area. Heavier birds, in general, have higher wing loadings than lighter birds (Table 15-2)
- Angle of attack is the angle that the wing is tilted above horizontal as is moves into the air.
If the angle of attack is too high, turbulence results across the wing and the lift decreases to the point that it can not keep the bird in the air. At this point the bird stalls. The angle at which this occurs is called the stalling angle.
- Camber is the degree to which the ventral surface of the wing is concave. The more camber the more lift that is developed at low speed.
- Induced drag is the result of airflow from beneath wing around the distal tip to the upper surface of the wing. (picture an airplane landing on a dusty runway).
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)
- elliptical wings highly maneuverable wing, relatively slow speed, commonly found in woodland species, constantly flapping to produce lift.
low aspect ratio (length to width ratio is small)
slotting in the outer primaries
- high-speed wing fast flying birds such as pigeons and falcons
moderately high aspect ratio
tapered to a point at the distal end
very little camber (almost flat on the ventral surface)
no slots in the outer primaries
- dynamic soaring only possible in areas where winds are high and persistent, such as in mid latitudes over the oceans. The bird glides downwind while increasing speed from the wind, as it nears the surface of the ocean, it turns back into the wind and begins to rise, as the bird begins to stall, it turns and glides back down the wind.
very high aspect ratio (18:1)
long narrow, flat wing
no slots in the outer primaries
- slotted high-lift wing These birds engage in static soaring where they ride thermals or other air currents while gliding. Highly maneuverable wing, with high lift at slow speed.
intermediate aspect ratio
marked slotting in the outer primaries (the slotting reduces induced drag and provides lift at slow speed with each feather acting as an airfoil)
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).
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.
- an increase in the length of the distal elements of the leg
- a decrease in the surface area of te foot contact with the substrate
- a decrease in the number of toes (for example, ostriches have 2 toes)
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).
- modified hind limb
- webbing or lobing between the toes
- legs positioned toward the back of the bird
- muscle mass for the limbs is more streamlined into the body
- wide body for stability when floating
- dense plumage for buoyancy and insulation
- preen gland that produces oil that waterproofs the feathers
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
- modified the wings for propulsion
- decrease volume of air sacs to decrease buoyancy
- decrease air space in bones
- decrease the amount of air trapped in the feathers
- penguins swallow small stones for ballast
Last updated on 13 April 2003
Provide comments to Dwight Moore at firstname.lastname@example.org
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