Birds are a class of vertebrate animals, meaning animals with backbones. Birds are bipedal, meaning that they walk on two feet. They are also endothermic, meaning that they are warm-blooded. Like mammals they use the food they eat and oxygen from the air to create energy to warm their bodies, even an cold days. This is unlike amphibians, reptiles, and fish whose body temperatures change with the surrounding temperature. Birds lay eggs as amphibians and reptiles do. However, bird eggs have hard shells, while amphibian and reptile eggs usually have soft shells. Unlike other animals, birds have feathers. Probably the most important difference between birds and other animals is that birds all have wings, even though there are a few of them (such as penguins and ostriches) that cannot fly. Birds are found all over the world, on every continent. There are about 10,000 living species of birds in the world today and they range in size from a 2-inch (5 cm) hummingbird to a 9-foot (2.75 m) ostrich. Birds play a very important role in life on this planet - on land, on the sea, and, of course, in the air.
BIRD SOUNDS
Birds have a muscular chamber in their throats called a syrinx in which they create sounds. In mammals and humans there is a larynx, often called a “voice box” with structures often called “vocal cords.” Air from the lungs causes the vocal cords or folds to vibrate creating the sound of the voice. In birds, air moving through the syrinx causes the walls of the syrinx to vibrate creating a resonating chamber of sound. This allows birds to make a variety of interesting sounds. Many songbirds make beautiful music. Some birds create sounds that are almost like striking a piece of metal. Parrots and mynas can imitate the sound of human speech or the sound of a motor running. Since the syrinx is located where the trachea (air tube) forks to the two lungs some birds can even produce two sounds at the same time!
SMELLING, SEEING, HEARING
Most birds have a poor sense of smell, with a few exceptions like the kiwi. However, birds have excellent vision and they are able to see ultraviolet light as well as the colors that humans can see. Birds that dive into the water often have flexible lenses that allow them to see well in the air or the water. Birds have a special eyelid that moves horizontally called a nictitating membrane. This third eyelid lubricates the eye and protects it and even acts as a contact lens in some aquatic birds. Most birds cannot move their eyes; they move their heads to change their field of view. Most birds have eyes on the sides of their heads giving them a wide field of view. Some birds, such as owls, have eyes on the front of their heads giving them stereoscopic vision to help them estimate distances to their prey. Having eyes on the front of its head like a human being, has led to the idea that an owl is wise. Birds have ear openings on the sides of their heads, but no visible ear structure. However some birds, including some owls, have tufts of feathers which look like ears.
FEATHER GROOMING
Birds lose their feathers and gain new ones in a process called molting. Most birds molt once a year, some two times a year, and others only once every few years. Feathers needed for flying are usually not all lost at the same time, but are replaced one at a time. However some ducks and geese lose all their flight feathers at once making them temporarily grounded. Woodpeckers need their tail feathers to give support as they climb and peck on trees. For that reason they lose their tail feathers only one or two at a time so that they can continue to have a working tail for climbing. Birds groom their feathers by preening them with their beaks. In this way they brush away dirt and apply wax from their saliva to keep their feathers smooth and flexible and to kill bacteria.
BIRD FOOD
Various birds eat a variety of foods including nectar, fruits, seeds, insects, worms, fish, rodents, and dead animals. Birds may pick berries from bushes, catch insects in the air, snatch live animals from the ground, or even dive underwater for their food. Some birds, such as gulls, even steal their food from other birds. Birds which feed on nectar, such as hummingbirds, are important for pollinating the flowers they feed from. Other birds who eat fruit are important for spreading the seeds of the plant which supplies the fruit. Birds which eat insects help to keep the insect populations under control.
GETTING A DRINK
Birds need water and most of them drink by scooping water in their beaks then raising their heads to let the water run down their throat. A few birds are able to suck up water through their beaks without tilting their heads back. Seabirds are able to drink seawater and then flush the salt out through their nostrils. Some desert birds exist only on the moisture they get from their food.
DIFFERENT WAYS TO SLEEP
When birds sleep it's called roosting. Birds often roost in trees for safety; even birds that spend most of their time on the ground, like quails. Some birds sleep with their heads over their backs and their bills tucked under their wings. Some birds sit down to sleep, while others sleep standing up sometimes on two legs and sometimes on one leg. Perching birds, that is birds that sit on tree limbs, have a tendon locking system that keeps their feet wrapped around the limb while they are asleep. Some parrots even hang upside down while they sleep. Birds often use vigilant sleep in which they periodically open an eye to look for dangers so they can escape quickly. One hemisphere of the bird's brain may sleep while the other half is awake to watch for predators. Birds sometimes sleep in groups, called flocks, and those on the outside of the group will use vigilant sleep to watch for dangers. When the danger is spotted the whole group is quickly alerted and they fly away. These birds will periodically trade places so that different birds take over the guard duty while the others are able to sleep more soundly. Roosting in groups can also help birds to stay warm in cold weather. Birds called swift's are apparently able to sleep while they're flying. They had an autopilot feature even before people had airplanes.
BIRD REPRODUCTION
Birds reproduce by laying eggs, usually in a nest that they have constructed. Various kinds of birds have their own designs and locations for their nests. A few birds, like emperor penguins, have no nest at all. Incubation time for the eggs to hatch can be anywhere from 10 days to 80 days. In some bird species the female keeps the eggs warm, in some species the male does the job, and in still others the parents trade-off incubation duties. There are some birds, referred to as parasite birds, which lay their eggs in the nests of other birds and leave the other birds to raise their young. The brown-headed cowbird is a common example and the cuckoo is a well-known example. In most species of birds the parents have to take care of the chicks for a period of time. However, there are a few, like the albatross, where the chicks leave the nest on their own and need no help from their parents. Other young birds, like the great frigatebird, are fed by their parents for more than a year.
THE BAD AND THE GOOD
Birds can sometimes spread diseases, like avian influenza commonly called bird flu, and they can be a hazard for airplanes. They can also cause damage to crops. But the good things that birds do, far outweigh the bad. Birds such as chickens and turkeys provide an important source of food for humans, as do chicken eggs. Birds, such as parrots or parakeets, have been kept as pets, falcons have helped humans in hunting, and pigeons have been used to carry important messages in difficult situations. Many people enjoy feeding wild birds as well as watching them and listening to them.
INCREDIBLE MIGRATION
Many species of birds migrate, some traveling close to home, but many covering great distances. Often birds fly for thousands of miles across deserts and oceans without stopping to rest. Arctic terns average 44,000 miles in a year. These migrating birds can travel for thousands of miles and return to the exact spot they left the season before. Sometimes, as in the case of the arctic tern, the migration does not so much benefit the bird itself so much as other animals or plants in the areas they visit. Migrations like this can can only be explained by design, not accident.
DESIGNED TO FLY
In many ways birds are designed for flying. Of course, the wings and feathers make it possible for them to fly, but that is not all. Birds also have a lightweight skeleton that is very strong. The bones of a bird are lighter because they are hollow with air-filled cavities. Those cavities are connected to the bird's respiratory system creating one of the most complex breathing systems of any animal. When a bird inhales, 75% of the fresh air does not go into the lungs but flows into an air sac that connects with the air spaces in the bones. The other one fourth of the air goes into the lungs. When the bird exhales, air flows out of the lungs and the air stored in the air sac is forced into the lungs. In this way a bird receives fresh air when it breathes in and when it breathes out. This provides more oxygen for the bird's high metabolism rate required in order for it to fly. Flying takes lots of energy, and the need for energy requires more oxygen.
EATING IN A HURRY
The digestive system of birds is also adapted for flight. Birds are able to swallow their food quickly and then fly away. They don't chew, because they don't have teeth. They have an area where food is stored before digestion called a crop. It's a muscular pouch that is basically an enlarged portion of the esophagus, the tube that carries food from the mouth to the stomach. Instead of a stomach, birds have a muscular organ called a gizzard. Birds swallow small stones which go into the gizzard with the food and the gizzard uses the stones to grind the food. This makes up for the fact that the birds don't have teeth. Some birds, like vultures, will gorge themselves on food when it is available and then sit around half asleep while their food digests.
AIRFRAME
In addition to having a light-weight skeleton with hollow bones, birds also have other special skeletal features for flying. The design of the bones in their necks (the cervical vertebrae) is different in various kinds of birds, but it is especially flexible. However, in the lower part of the back the last few bones are fused together with the pelvis. This reduces flexing in the lower back area which is not necessary for birds and aids in their ability to fly. Also the sternum, or breastbone, has a special shape like the keel of a sailboat to the allow strong flight muscles to be attached. Compare this to the breastbone of a human, which is basically flat. Bird skeletons also have wings instead of arms or front legs as other animals have.
BIRD BRAIN
The brain and nervous system of a bird is also specially designed for flight. In relation to the size of the bird the nervous system is relatively large. The largest part of the brain is the part that controls flight functions. Sometimes people call another person is a “bird brain” meaning that person is not very smart. Actually birds are among the more intelligent animals. Some of them can be trained to follow commands. Some can even “talk” by imitating human voice sounds. Their intelligence and talents have made birds very useful to people throughout history. People learned to fly by studying birds. Their amazing abilities, along with their beauty, testify to an intelligent Creator.
