Zoology Chapter 19

Chapter 19 Study Questions

1. Explain the significance of the discovery of Archaeopteryx. Why did this fossil demonstrate beyond reasonable doubt that birds are grouped phylogenetically with dinosaurs?

The fossil was approximately the size of a crow, with a skull like that of modern birds except that the beak-‐like jaws bore small bony teeth set in sockets like those of dinosaurs. The skeleton was reptilian, with a long bony tail, clawed fingers, and abdominal ribs. It might have been classified as a theropod dinosaur except that it carried an imprint of feathers, which only birds possess. Archaeopteryx lithographica means "ancient wing inscribed in stone". The fossil was an especially fortunate discovery because it demonstrated beyond reasonable doubt the phylogenetic relatedness of birds and theropod dinosaurs.

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2. The special adaptation of birds contribute to two essentials for flight: more power and less weight. Explain how each of the following contributes to one or both of these two essentials: feathers, skeleton system, circulatory system, respiratory system, excretory system, and reproductive system.

Feathers: Feathers are very lightweight, and yet they possess remarkable toughness and tensile strength.

Skeleton: A major structural requirement for flight is a light, yet sturdy skeleton. The bones of modern birds are phenomenally light, delicate, and laced with air cavities. Pneumatized bones are nevertheless strong. Bones of the forelimbs are highly modified for flight. They are reduced in number, and several are fused together.

Muscular System: Locomotor muscles of wings are relatively massive to meet demands of flight. The largest of these is the pectoralis, which depresses the wings in flight. Its antagonist is the supracoracoideus muscle, which raises the wing. Surprisingly, this later muscle is not located on the backbone but is positioned under the prectoralis on the breast and is attached via a tendon in a "rope-‐and-‐pulley" arrangement.

Digestive System: Birds pocess an energy-‐rich diet rapidly and thoroughly with efficient digestive equipment. Some birds can digest a mouse in 3 hours and berries pass completely through the digestive tract in just 30 minutes.

Circulatory System: Birds have a four-‐chambered heart that is large with strong ventricular walls. Bird have a complete separation of respiratory and systematic circulation. Their heartbeat is extremely fast with an inverse relationship between heart-‐rate and body weight. All these features add to a light and efficient system.


Respiratory System: The respiratory system of birds is marvelously adapted for meeting the high metabolic demands of flight. In birds, the finest branches of the bronchi, rather than ending in sac-‐like alveoli as in mammals, are tube-‐like parabronchi through which air flows continuously. Also unique is the extensive system of nine interconnecting air sacs that are located in pairs in the thorax and abdomen and even extend by tiny tubes into the centers of the long bones. It takes two respiratory cycles for a single breath of air to pass through the respiratory system. The advantage of such a system is that an almost continuous stream of oxygenated air is passed through a system of richly vascularized parabronchi.

Excretory System: Birds excrete their nitrogenous wastes as uric acid rather than urea. Because of uric acid's low solubility, a bird can excrete 1 gram of uric acid in only 1.5 to 3mL of water; whereas mammals would require 60mL per 1 gram of urea.

Reproductive System: In female birds, only the left ovary and oviduct develop; those on the right dwindle to vestigial structures. This loss of one ovary is another adaptation of birds for reducing weight for flight.

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3. How do marine birds rid themselves of excess salt?

Marine birds use extra-‐renal mechanisms to excrete salts gained from the food they eat and the seawater they drink. Excess salt is removed from their blood by special salt glands, one located above each eye. Each gland consists of several lobes arranged in parallel. These glands are capable of excreting a highly concentrated solution of sodium chloride -‐ up to twice the concentration of seawater. Salt is excreted into many radially arranged tubules, and then flows into a central canal that leads into the nose. The salt solution runs out the internal or external nostrils, giving gulls and other sea birds a perpetually runny nose.

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4. In what ways are a bird's ears and eyes specialized for the demands of flight?

Birds have good hearing and superb vision, the keenest in the animal kingdom. The cochlea is much shorter than that in mammals. Yet birds can hear roughly the same range of sound frequencies as humans. Although smaller, a bird's ear far surpasses that of humans in both capacity to distinguish different intensities and to respond to rapid fluctuations in pitch.

Bird's eye resembles that of other vertebrates in gross structure, but is larger, less spherical, and almost immobile. Their light-‐sensitive retina is generously equipped with rods (dim light


vision) and cones (color vision). The extraordinary keen vision of hawks is attributed to the extreme density of cone cells in the foveae: 1.5 million per fovea compared to 0.2 million for humans. This gives a hawk the ability to see clearly a crouching rabbit (prey and survival for the bird) 2 kilometers away. The extra density of rods in an owl's eye give it the ability to see in dim light more than 10 times that of a human; which make night hunting possible.

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5. Explain how a bird wing is designed to provide lift. What design features help prevent stalling at low flight speeds?

A bird's wing is an airfoil subject to recognized laws of aerodynamics. It is streamlined in cross section, with a slightly concave lower surface (camber) and with small, tight-‐fitting feathers where the leading edge meets the air. Air slips smoothly over the wing, creating lift with minimum drag. Some lift is produced by positive pressure against the undersurface of the wing, but on the upper side, where the airstream must travel farther and faster over a convex surface, negative pressure is created that provides more than two-‐thirds of the total lift.

When the angle of attach becomes too steep, turbulence appears on the upper surface, lift is destroyed, and stalling occurs. Stalling can be delayed or prevented by the presence of a wing slot along the leading edge so that a layer of rapid moving air is directed across the upper wing surface. In birds, two kinds of wing slots have developed: alula, or a group of small feathers on the thumb which provides a mid-‐wing slot and slotting between the primary feathers, which provides a wing-‐tip slot.

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6. Describe four basic forms of bird wings. How does wing shape correlate with flight speed and maneuverability?

Elliptical Wings: This type has a low-‐aspect ratio (ratio of length to average width). Elliptical wings have both alula and slotting between the primary feathers. These wings have high maneuverability and low relative flight speed.

High-‐Aspect Ratio Wings: This type obviously has high aspect ratio and lack wing-‐tip slotting. This type is efficient for high-‐speed flight, but cannot easily keep the bird airborne at low speeds. The fastest of the birds have this type of wing.

Dynamic Soaring Wings: These are high-‐aspect ratio wings shaped like those of sailplanes. These long narrow wings lack slots. Dynamic soaring only can be done over seas with strong, reliable winds, and it exploits different wind speeds near the ocean surface.


High-‐Lift Wings: Hawks, vultures, eagles, owls, and ospreys are all predators that carry heavy loads. These are wings that have slotting, alulas, and pronounced camber, all of which promote high lift at low speed.

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7. What are the advantages of seasonal migration for birds?

Moving between southern wintering regions and northern summer breeding regions with long summer days and an abundance of insects provides parents with ample food to rear their young. Predators of birds are not so abundant in the far North, and a brief once-‐a-‐year appearance of vulnerable young birds does not encourage buildup of predator populations. Migration also vastly increases the amount of space available for breeding and reduces aggressive territorial behavior. Finally, migration favors homeostasis -‐-‐ the balance of internal physiological processes -‐-‐ by allowing birds to avoid climatic extremes.

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8. Describe different navigational resources birds may use in long distance migration.

Most birds navigate chiefly by sight. Birds recognize topographical landmarks and follow familiar migratory routes. This is assisted by flock migration, during which the navagational resources and experience of older birds can be pooled. Birds have a highly accurate sense of time. Numerous studies support a hypothesis that birds can navigate by the earth's magnetic field. Deposits of a magnetic substance called magnetite have been discovered in the beaks of pigeons.

It has been demonstrated convincingly that birds can navigate by celestial cues: the sun by day and the stars by night. Birds maintain compass direction by referring to the sun, regardless of the time of day. The process is called sun-‐compass navigation. Other studies strongly suggest that some birds are able to detect and navigate by the North Star axis around which the constellations appear to rotate.

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9. What are some advantages of social aggregation among birds?

Togetherness offers advantages: mutual protection from enemies, greater ease in finding mates, less opportunity for individual straying during migration, and mass huddling for protection against low night temperatures during migration. Certain species, such as pelicans, use cooperative behavior to feed. At no time are the highly organized social interactions of


birds more evident than during the breeding season, as they establish territorial claims, select mates, build nests, incubate and hatch eggs, and rear their young.

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10. More than 90% of all bird species are monogamous. Explain why monogamy is much more common among birds than among mammals.

Monogamy is rare in most animal groups, but is common in birds. A few bird species, such as swans and geese, choose partners for life and often remain together throughout the year. Seasonal monogamy is more common as the great majority of migrant birds pair only during breeding season.

One reason monogamy is much more common among birds than among mammals is that male and female birds are equally adept at most aspects of parental care. Female and male birds can alternate care of the nest and young, which permits one parent to be at the nest at all times. This constant attendance to the nest may be particularly important in species that would experience high loss of eggs or young to predators or rival birds if a nest were left unguarded.

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11. Briefly, describe an example of polygyny among birds.

Polygyny ("many females") is polygamy in which a male has more than one female mate. In many species of grouse, males gather in a collective display ground, or lek, which is divided into individual territories, each defended by a displaying male. Females appear to choose the dominant male for mating because, presumably, social rank correlates with genetic quality.

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12. Define altricial and precocial as they relate to birds.

Newly hatched birds are of two types: altricial and precocial . Altricial young which are naked and unable to walk or see at birth, remain in the nest for a week or more. Precocial young (quail, fowl, ducks, and most water birds) are covered with down when hatched and can run or swim as soon as their plumage is dry. The young of both types require care from parents for some time after hatching.

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13. Offer some examples of how human activities have affected bird populations.

Both starlings and house sparrows have been introduced by man into numerous countries has made them the two most abundant bird species on earth. Man direct action has caused the extinction of several species like hunting the passenger pigeon to extinction. Other man activities have contributed to the extinction or near extinction of bird species.

Intensification of agriculture => deprived ground-‐nesting birds of areas to nest

Excessive Fragmentation of Forests => increased exposure of nests to nest predators

Rapid Loss of tropical forest => deprives migrating song birds of wintering homes.

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Zoology Chapter 19