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Biology
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31-2 Birds
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What Is a Bird?
What characteristics do birds have in common?
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What Is a Bird?
What Is a Bird?
Birds are reptilelike animals that maintain a constant internal body temperature.
Birds have an outer covering of feathers; two legs that are covered with scales and are used for walking or perching; and front limbs modified into wings.
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What Is a Bird?
Feathers separate birds from all other living animals.
Feathers are made mostly of protein and develop from pits in the birds' skin.
Feathers help birds fly and also keep them warm.
The two main types of feathers are contour and down.
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What Is a Bird?
Feathers
Barb
Contour feather: Contour feathers provide the lifting force and balance needed for flight.
Birds have different types of feathers that vary in structure and function. An outer covering of feathers is the main characteristic that sets birds apart from other animals.
Barbule: The hooks on each barbule fit together, holding them flat.
Down feather: Down feathers trap air close to the body and keep the bird warm.
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Evolution of Birds
Evolution of Birds
Paleontologists agree that birds evolved from extinct reptiles.
Embryos of birds and reptiles develop within amniotic eggs.
Both excrete nitrogenous wastes as uric acid.
Bones that support the limbs, and other skeleton parts, are similar in both groups.
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Evolution of Birds
Archaeopteryx was the first birdlike fossil discovered.
Archaeopteryx looked like a dinosaur, but it had feathers.
It had teeth in its beak, a bony tail, and toes and claws on its wings.
It may be a transitional species between dinosaurs and birds.
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Evolution of Birds
Other fossil evidence leads some to hypothesize that birds and dinosaurs both evolved from an earlier common ancestor.
The origin of birds is still not completely resolved.
New fossils of ancient birds are being found all the time.
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Evolution of Birds
Evolution of Birds
Dinosaurs
Modern reptiles
Modern birds
Saurischia (lizard-hipped dinosaurs)
Ornithischia (bird-hipped dinosaurs)
Archaeopteryx
The diagram at the top shows the evolutionary tree of modern birds. None of the animals shown are direct ancestors of modern birds. But fossils such as Archaeopteryx show a mixture of characteristics of birds and dinosaurs. 
Ancestor of dinosaurs
Reptile ancestor
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11. Form, Function, and Flight
How are birds adapted for flight?
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12. Form, Function, and Flight
Birds have a number of adaptations that enable them to fly, including:
highly efficient digestive, respiratory, and circulatory systems
aerodynamic feathers and wings
strong, lightweight bones
strong chest muscles
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13. Form, Function, and Flight
Body Temperature Control 
Birds generate their own body heat and are called endotherms.
Endotherms have a high rate of metabolism.
Metabolism produces heat.
Feathers insulate a bird enough to conserve most of its metabolic energy, allowing it to keep warm.
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14. Form, Function, and Flight
Feeding 
Birds need to eat a lot of food to produce the heat energy they need to maintain metabolism.
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15. Form, Function, and Flight
Birds’ beaks, or bills, are adapted to the type of food they eat.
Insect-eating birds have short, fine bills that pick ants and insects off leaves and branches, or can catch flying insects.
Seed-eaters have short, thick bills.
Carnivorous birds shred their prey with strong hooked bills.
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16. Form, Function, and Flight
Long, thin bills gather nectar or probe mud for worms and shellfish.
Large, long bills pick fruit from branches.
Long, flat bills grasp fish.
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17. Form, Function, and Flight
Birds do not have teeth and cannot chew.
Many birds have specialized structures to help digest food.
The crop is a structure at the lower end of the esophagus in which food is stored and moistened.
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18. Form, Function, and Flight
Esophagus
Crop
When a bird eats, food moves down the esophagus and is stored in the crop.
Birds have a number of adaptations that enable them to fly, including an efficient digestive system.
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19. Form, Function, and Flight
In some birds the crop has a second function.
During nesting season, the crop produces a substance rich in protein and fat.
Parents regurgitate this to feed their newly hatched young.
This provides young birds with materials they need to grow.
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20. Form, Function, and Flight
First chamber of stomach
Gizzard
Moistened food passes to the stomach, a two-part chamber.
Birds have a number of adaptations that enable them to fly, including an efficient digestive system.
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21. Form, Function, and Flight
First chamber of stomach
Gizzard
The first chamber secretes acid and enzymes.
Birds have a number of adaptations that enable them to fly, including an efficient digestive system.
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22. Form, Function, and Flight
First chamber of stomach
Gizzard
The partially digested food moves to the second chamber, the gizzard.
Birds have a number of adaptations that enable them to fly, including an efficient digestive system. Trace the path of food through the digestive system.
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23. Form, Function, and Flight
Birds that eat meat or fish have an expandable area in which large amounts of soft food can be stored.
Birds that eat insects or seeds have a muscular organ called the gizzard that helps in the mechanical breakdown of food.
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24. Form, Function, and Flight
The muscular walls of the gizzard squeeze the contents, while small stones grind the food.
Birds have a number of adaptations that enable them to fly, including an efficient digestive system.
Gizzard
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25. Form, Function, and Flight
Birds have a number of adaptations that enable them to fly, including an efficient digestive system.
Large intestine
Small intestine
As digestion continues, the food moves through the intestines.
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26. Form, Function, and Flight
Birds have a number of adaptations that enable them to fly, including an efficient digestive system.
Undigested food is expelled through the cloaca
Cloaca
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27. Form, Function, and Flight
Respiration  
Birds have a highly-efficient way of taking in oxygen and eliminating carbon dioxide.
Air enters air sacs.
It flows through the lungs where gas exchange takes place.
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28. Form, Function, and Flight
Air flows in a single direction.
The one-way flow of air:
constantly exposes the lungs to oxygen-rich air.
maintains a high metabolic rate.
provides efficient extraction of oxygen, which enables birds to fly at high altitudes where the air is thin.
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29. Form, Function, and Flight
Circulation  
Birds have four-chambered hearts and two circulatory loops.
There is complete separation of oxygen-rich and oxygen-poor blood.
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30. Form, Function, and Flight
Oxygen-poor blood from the body is pumped to the lungs.
Oxygen-rich blood returns from the lungs and is pumped to the rest of the body.
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31. Form, Function, and Flight
Bird Heart
Domestic pigeon
Heart
To keep blood moving rapidly, a bird’s heart beats quickly—from 150 to more than 1000 beats per minute! 
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32. Form, Function, and Flight
Excretion  
Excretion in birds is similar to that of most living reptiles.
Nitrogenous wastes are removed from the blood by the kidneys, converted to uric acid, and deposited in the cloaca.
Most of the water is reabsorbed, leaving uric acid crystals in a white, pasty form.
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33. Form, Function, and Flight
Response  
Birds have well-developed sense organs, which are adaptations that enable them to coordinate the movements required for flight.
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34. Form, Function, and Flight
Bird Brain
Compared to reptiles, birds have an enlarged cerebellum that coordinates the movements of wings and legs. 
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35. Form, Function, and Flight
Birds’ brains can quickly interpret and respond to signals.
The cerebrum controls behavior and is large.
The cerebellum coordinates the movement of the wings and legs; it is larger in birds than in reptiles.
The medulla oblongata coordinates basic body processes.
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36. Form, Function, and Flight
Birds have well-developed eyes which allow them to see color very well.
Most bird species can hear quite well.
Taste and smell are not well developed in most birds.
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37. Form, Function, and Flight
Movement  
Some birds, such as ostriches and penguins cannot fly.
Most birds, however, can fly.
The skeletal and muscular systems of flying birds exhibit adaptations that enable flight.
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38. Form, Function, and Flight
Skeletal System of a Bird
Skull
Vertebra
Pelvic girdle
Collarbone (wishbone)
Strut
Tailbone
Like most of its anatomy, a bird’s skeleton is well adapted for flight, providing a sturdy attachment point for muscles. The long bones are exceptionally strong and light because of cross-bracing and air spaces. In strong flying birds, such as pigeons, the chest muscles may account for as much as 30 percent of the animal’s mass. 
Pectoral griddle
Air space
Sternum
Rib cage
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39. Form, Function, and Flight
In flying birds, many large bones are fused together, making the skeleton rigid.
These form a frame that anchors the muscles used for flight.
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40. Form, Function, and Flight
Bones are strengthened by struts.
Air spaces make bones lightweight. . 
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41. Form, Function, and Flight
Birds have large chest muscles that power the upward and downward wing strokes necessary for flight.
Muscles attach to a keel that runs down the front of an enlarged breastbone, or sternum.
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42. Form, Function, and Flight
Reproduction   
Both male and female reproductive tracts open into the cloaca.
Mating birds press their cloacas together to transfer sperm from male to female.
Some male birds have a penis that transfers sperm to the female.
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43. Form, Function, and Flight
Bird lay amniotic eggs that have hard outer shells.
Most birds incubate their eggs until the eggs hatch.
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44. Form, Function, and Flight
When a chick is ready to hatch, it makes a hole in the shell with a small tooth on its bill.
Once the bird has hatched, it rests for a while and lets its feathers dry.
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Groups of Birds
Groups of Birds
There are nearly 30 different orders of birds.
The largest order of birds is the passerines, or perching birds.
Other groups of birds include: pelicans, parrots, birds of prey, cavity-nesting birds, herons, and ostriches.
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Ecology of Birds
Ecology of Birds
Birds interact with ecosystems and humans in many ways.
Hummingbirds pollinate flowers.
Fruit-eating birds disperse seeds in their droppings.
Insect-eating birds catch insects, controlling populations.
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Ecology of Birds
Many birds migrate long distances, usually seasonally.
Some species use stars and other celestial bodies as guides.
Others use a combination of landmarks and cues from Earth’s magnetic field.
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31-2
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31-2
A bird with a short, thick bill probably eats
fish.
seeds.
insects.
fleshy fruit.
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31-2
Archaeopteryx has characteristics of both
modern birds and ancient birds.
amphibians and reptiles.
reptiles and modern birds.
amphibians and modern birds.
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31-2
Which of the following bird adaptations is NOT associated with flight?
bones with many hollow air spaces
air sacs in addition to lungs
gizzard
contour feathers
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31-2
The largest order of birds is the
perching birds.
birds of prey.
pelicans and relatives.
penguins.
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31-2
Which of the following birds assists in pollinating flowering plants?
pelican
hummingbird
raptor
heron
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54. END OF SECTION