The Distribution of Birds

Every species of bird has its own range-that is, a particular part of the world in which all the members of the species normally live. Some birds have a broad range. The osprey and common barn owl, for example, live on every continent except Antarctica. However, no species of bird is found in every part of the world, and many species have an extremely limited range. For example, a species called the Whitehead's broadbill lives only in a small, mountainous area of northern Borneo.

Oceans and continents strongly influence the distribution of various species of birds. Most birds cannot make long ocean flights. Widely separated continents, such as Africa and North America, therefore have different kinds of birds. However, people have transported many species overseas, and some of these birds have become adapted to their new environment.

Climate also influences a bird's range. Most birds would starve during a long cold spell. For this reason, few birds live all year in regions with severe winters. However, many birds nest in such regions in summer and migrate to warmer climates for the winter. Birds that migrate have two ranges-a summer one and a winter one. They are summer residents in their summer range and winter residents in their winter one. Along their migration route, they are transients (temporary visitors). Birds that do not migrate are permanent residents.

More kinds of birds live in the tropics than anywhere else in the world. Tropical rain forests have more kinds of birds than any other habitat. Most birds of the tropics are permanent residents. However, some parts of the tropics have an annual dry season, and many of the birds migrate to moister parts of the tropics to avoid it. The tropics also have many winter residents that migrate from cool or cold climates. The temperate zones-that is, the parts of the world between the tropics and the polar regions-have fewer permanent residents than do the tropics. In the parts of the temperate zones nearest the polar regions, most of the birds are summer residents only. Few birds live all year in the polar regions. However, both the Arctic and the Antarctic have many residents during the summer.

The ranges of birds are further determined by the kinds of food and nesting places that are available. For example, fish-eating birds must live near bodies of water. Birds that nest in trees normally live only in wooded areas. Thus, most birds live not only in a particular region of the world but also in a particular type of environment, or habitat, within that region.

THE IMPORTANCE OF BIRDS

In nature. Each species of animal in a woodland, grassland, or other natural area depends on other living things in the environment for food. In a woodland, for example, some birds get their food mainly from plants. Others chiefly eat small animals, such as insects or earthworms. Birds and bird eggs, in turn, serve as food for such animals as foxes, raccoons, and snakes. The feeding relationships among all the animals in an environment help prevent any one species from becoming too numerous. Birds play a vital role in keeping this balance of nature.

Birds also serve other purposes in nature. Fruit-eating birds help spread seeds. The birds eat and digest the pulp of berries and other fruits but pass the seeds in their droppings. The seeds may sprout wherever the droppings fall. Hummingbirds pollinate certain flowers that produce nectar. Hummingbirds feed on nectar. As they visit flowers in search of it, they carry pollen from flower to flower. In these ways, birds help numerous kinds of plants reproduce and spread.

Many kinds of birds assist farmers by eating weed seeds, harmful insects, or other agricultural pests. Unlike birds that feed on fruits, seed-eating birds digest the seeds they eat. One bobwhite may rid a field of as many as 15,000 weed seeds a day. Many birds eat insects that damage farm crops. Some birds are especially helpful in keeping the number of certain kinds of insects under control. Robins and sparrows, for example, are highly effective against cabbageworms, tomato worms, and leaf beetles. Rats and mice can cause huge losses on farms by eating stored grain. Hawks and owls prey on these animals and so help limit such losses.

People consider a few kinds of birds to be pests. One such species, the common, or European, starling, was introduced into the northeast United States in the 1890's. The birds multiplied and spread rapidly. Today, starlings are so numerous in many North American cities that they have become a nuisance because of their noise and droppings. Moreover, starlings fight with certain native American birds, including bluebirds and swallows, over the tree holes in which they nest. Pigeons are also a nuisance in many cities because of their droppings. Flocks of starlings and pigeons leave masses of droppings on buildings where the birds have been roosting. The fungus Histoplasma capsulatum can grow on these droppings. The spores of the fungus may be carried in the air and cause the infectious disease histoplasmosis in people who inhale them. Some strains of influenza, another infectious disease, can be passed to human beings from infected domestic birds, including chickens and ducks. West Nile virus, which causes a flulike disease, is transmitted from infected birds to human beings and other animals through the bites of mosquitoes.

As a source of food and raw materials. People have always hunted birds for food. Some of the first birds used for food were ground-feeding birds, such as quails and turkeys, which were caught in traps and snares. Hunters captured pigeons, ducks, and other birds by placing nets where the birds normally flew. After the invention of guns, most people hunted large, meaty birds to save gunpowder and shot. The eggs of wild birds were also an important food for people in prehistoric times, and people in some parts of the world still eat such eggs. Because most birds' nests are hard to find, the eggs used for food have come chiefly from sea birds that nest in open places in large colonies (groups).

People eventually discovered that certain wild fowl could be domesticated (tamed). This discovery led to the development of poultry-that is, domesticated fowl that farmers raise for meat and eggs. Chickens are probably the oldest kinds of poultry. They were domesticated in Asia at least 3,000 years ago. Since then, farmers have developed other poultry, including ducks, geese, guineafowl, pheasants, and turkeys. Mallard ducks, geese, and pheasants were domesticated in Asia; guineafowl in Africa; and Muscovy ducks and turkeys in Mexico.

Today, chickens rank as the most widely raised poultry by far. Farmers throughout the world produce hundreds of millions of chickens annually for meat and eggs. Ducks and turkeys rank second and third in production worldwide. Ducks are raised for both meat and eggs. Turkeys are raised mainly for meat.

People use the feathers of certain birds to stuff pillows, mattresses, sleeping bags, coats, and quilting. Goose feathers are preferred because they are soft and springy. Manufacturers often mix goose feathers with down feathers, or down, to provide extra softness. Down feathers are small, fluffy feathers that some adult birds, especially water birds, have between their stiffer outer feathers. Most of the down used for stuffing comes from ducks and geese raised on farms.

People throughout the world use colorful bird feathers to decorate jewelry, clothing, and hats. Many countries forbid the use of feathers from wild birds. People may only use feathers from domesticated birds, such as turkeys, or from other birds raised in captivity, such as peacocks and pheasants.

Over the centuries, the droppings of ocean birds have formed huge deposits on certain islands where the birds nest in dry areas. This waste matter, which is called guano, provides an excellent source of nitrogen that people use to make fertilizer and explosives. The mining of guano for fertilizer was once an important industry in some countries.

As pets. People have long kept birds as pets. Favorite bird pets include canaries, parrots, finches, and parakeets called budgerigars, or "budgies." Budgies and parrots are especially popular because they can be trained to imitate human speech and even to whistle.

Most birds sold as pets have been raised in captivity. The birds are hatched in cages and sold to the public by pet stores. After years or centuries of captive breeding, some of these birds look much different from their wild ancestors. For example, wild budgerigars are green, but breeders have produced white, yellow, blue, and even violet budgerigars. In the past, most of the parakeets and parrots sold as pets were caught in the wild. Over the years, this practice wiped out some species. To help protect such birds, many countries have made it illegal for wild birds to be caged except in zoos. However, many wild parakeets and parrots are still captured and sold illegally throughout the world.

Bird is an animal with feathers

All birds have feathers, and they are the only living animals that have them. When people think of birds, they usually think first of their flying ability. All birds have wings. The fastest birds can reach speeds well over 100 miles (160 kilometers) per hour. No other animals can travel faster than birds. Yet not all birds can fly. For example, ostriches and penguins are flightless. Instead of flying, ostriches walk or run. They use their wings only for balance and to attract mates. Penguins swim. They use their wings as flippers.

People have always been fascinated by birds. Birds' marvelous flying ability makes them seem the freest of all animals. Many birds have gorgeous colors or sing sweet songs. The charms of birds have inspired poets, painters, and composers. Certain birds also serve as symbols. People have long regarded the owl as a symbol of wisdom and the dove as a symbol of peace. The eagle has long represented political and military might.

There are about 9,700 species (kinds) of birds. The smallest bird is the bee hummingbird, which grows only about 2 inches (5 centimeters) long. The largest living bird is the ostrich, which may grow up to 8 feet (2.4 meters) tall. The largest bird that ever lived was the elephant bird, which died out hundreds of years ago. It weighed about 1,000 pounds (450 kilograms). Birds inhabit all parts of the world, from the polar regions to the tropics. They are found in forests, deserts, and cities; on grasslands, farmlands, mountaintops, and islands; and even in caves. Some birds, including albatrosses and certain ducks, always live near water. Most such birds can swim. Other birds, especially those in the tropics, stay in the same general area throughout life. Even in the Arctic and the Antarctic, some hardy birds stay the year around. But many birds of cool or cold regions migrate each year to warm areas to avoid winter, when food is hard to find. In spring, they fly home again to nest.

All birds hatch from eggs. Among most kinds of birds, the female lays her eggs in a nest built by herself or her mate or by both of them. The majority of birds have one mate at a time, with whom they raise one or two sets of babies a year. Some birds keep the same mate for life. Others choose a new mate every year. Most baby birds remain in the nest for several weeks or months after hatching. Their parents feed and protect them until they can care for themselves. Other kinds of baby birds, including chickens and ducks, become active and able to walk and feed themselves soon after hatching. Most birds leave their parents after only a few months.

 

Birds belong to the large group of animals called vertebrates. Vertebrates are animals with a backbone. The group also includes fish, reptiles, and mammals. Birds have two forelimbs and two hindlimbs, as do cats, frogs, lizards, and many other vertebrates. But in birds, the forelimbs are wings rather than arms or front legs. Like mammals, and unlike amphibians and reptiles, birds are warm-blooded-that is, their body temperature always remains about the same, even if the temperature of their surroundings changes. Unlike most other vertebrates, living birds lack teeth. Instead, they have a hard bill, or beak, which they use in getting food and for self-defense. A number of the earliest birds possessed teeth, but these species no longer exist.

Many birds have great value to people. Such birds as chickens and turkeys provide meat and eggs for food. Some kinds of birds help farmers by eating insects that attack their crops. Others eat farmers' grain and fruit. But in general, birds do much more good than harm.

Since the 1600's, about 80 kinds of birds have died out. People have killed off most of these species by overhunting them and by destroying their environment. Today, most countries have laws to protect birds and help prevent any more kinds from dying out.

This article discusses the importance of birds, their distribution throughout the world, how they live, and how they raise a family. The article also describes bird migration, the bodies of birds, bird study and protection, and the evolution of birds.

How botanists classify fruits

Fruit, the seed-bearing structure of a flowering plant, develops from the ovaries of the flowers. An ovary is a hollow structure near the base of a flower. It may hold one seed or more, depending on the species of the plant.

The wall of an ovary of mature fruit, in which the seed is fully developed, has three layers. The outer layer is called the exocarp, the middle layer is known as the mesocarp, and the inner layer is the endocarp. The three layers together are called the pericarp.

Botanists classify fruits into two main groups: (1) simple fruits and (2) compound fruits. A simple fruit develops from a single ovary, and a compound fruit develops from two or more ovaries.

Simple fruits are by far the largest group of fruits. They are divided into two types, depending on whether their pericarp is fleshy or dry.

Fleshy simple fruits include most of the seed-bearing structures that are commonly called fruits. The three main kinds are: (1) berries, (2) drupes, and (3) pomes.

Berries have an entirely fleshy pericarp. Botanists classify bananas, blueberries, grapes, green peppers, muskmelons, oranges, tomatoes, and watermelons as berries. Some berries, including watermelons and muskmelons, have a hard rind. Such fruits are called pepos. Other berries, including the citrus fruits, have a leathery rind. They are called hesperidiums. Raspberries, strawberries, and most of the other fruits commonly known as berries are actually compound fruits.

Drupes have an exocarp that forms a thin skin. The endocarp develops into a stone or pit, and only the mesocarp is fleshy. Such fruits include apricots, cherries, peaches, and plums.

Pomes are fleshy fruits with a paperlike core. Apples and pears are pomes.

Dry simple fruits include the pods of the bean plant, the milkweed, the pea plant, and the locust tree; the grains of the corn, rice, and wheat plants; and nuts. Botanists regard nuts as single-seed fruits with a hard pericarp called a shell. The seed is the edible part. Acorns, chestnuts, and hazelnuts are true nuts. But many so-called nuts are classed otherwise by botanists. For example, almonds are the seeds of drupes.

Compound fruits consist of a cluster of ripened ovaries. There are two main types of compound fruits, aggregate fruits and multiple fruits. Aggregate fruits develop from single flowers, each of which has many ovaries. Blackberries and raspberries are aggregate fruits. The strawberry is a special type of aggregate fruit. Each "seed" in a strawberry is actually a complete fruit. The flesh surrounding the seeds develops from the base of the flower rather than from the ovaries. Multiple fruits develop from a cluster of flowers on a single stem. Figs, mulberries, and pineapples are multiple fruits.

Fruit on the Tree

Fruit is the part of a flowering plant that contains the plant's seeds. In this sense, fruits include acorns, cucumbers, tomatoes, and wheat grains. However, the word fruit commonly refers to the juicy, sweet or tart kinds that people enjoy as desserts or snacks. The word comes from the Latin word frui, meaning enjoy. Popular fruits include apples, bananas, grapes, oranges, peaches, pears, and strawberries.

Many fruits are nutritious as well as appetizing. For example, oranges and strawberries contain large amounts of vitamin C. Most fruits have a high sugar content, and so they provide quick energy. Fruits alone cannot provide a balanced diet, however, because the majority of them supply little protein.

The world's fruit growers raise millions of tons of fruit annually. Fruit growing is a branch of horticulture, a field of agriculture that also includes the raising of nuts, vegetables, flowers, and landscape crops. Most nuts are actually fruits, as are the edible portion of such vegetables as cucumbers, green peppers, and tomatoes. To prevent confusion, horticulturists define a fruit as an edible seed-bearing structure that (1) consists of fleshy tissue and (2) is produced by a perennial. A perennial is a plant that lives for more than two years without being replanted. The horticultural definition of a fruit excludes nuts and vegetables. Nuts are firm rather than fleshy. Most vegetables are annuals-that is, the plants live for only one season.

In some cases, the horticultural definition of a fruit conflicts with the definition used by botanists and with common usage. For example, watermelons and muskmelons are fruits, and most people regard them as such. But they grow on vines that must be replanted annually, and so horticulturists regard melons as vegetables. Rhubarb is sometimes considered a fruit because of its use as a dessert. But people eat the rhubarb leafstalk, not the seed-bearing structure. Therefore, horticulturists classify rhubarb as a vegetable.


                    How horticulturists classify fruits

Most of the fruits that are widely raised in North America were originally brought from other regions. For example, apples, cherries, and pears originated in Europe and western Asia. Apricots and peaches first came from China, and lemons and oranges from China and Southeast Asia. All these fruits are now grown in any part of the world that has a favorable climate.

Most fruit plants require considerable amounts of moisture. Dates and olives are among the few fruits that can be grown in dry regions without irrigation.

Horticulturists classify fruits into three groups, based on temperature requirements for growth: (1) temperate fruits, (2) subtropical fruits, and (3) tropical fruits.

Temperate fruits must have an annual cold season to grow properly. They are raised chiefly in the Temperate Zones, the regions between the tropics and the polar areas. Most temperate fruits are grown in Europe and North America, but Asia and Australia also have major producing areas.

The principal temperate fruits are apples, apricots, cherries, peaches, pears, and plums. In addition, most small fruits, which grow on plants smaller than trees, are raised mainly in the Temperate Zones. They include blueberries, cranberries, grapes, raspberries, and strawberries.

Subtropical fruits require warm or mild temperatures throughout the year but can survive an occasional light frost. They are grown chiefly in subtropical regions.

The most widely grown subtropical fruits are the citrus group, which includes grapefruit, lemons, limes, and oranges. Oranges, the leading citrus crop, are grown throughout the subtropics, from southern Japan to southern Europe. In the United States, Florida produces by far the most oranges. Other subtropical fruits include dates, figs, olives, and avocados.

Tropical fruits are raised mainly in the tropics and cannot stand even a light frost. Bananas and pineapples, the best-known tropical fruits, are grown throughout the tropics, and much of each crop is exported. Other tropical fruits include acerolas, cherimoyas, litchis, mangoes, mangosteens, and papayas.


                    Growing fruit

Almost all species of fruits grow on plants that have a woody stem. Such plants are trees, bushes, or woody vines. Fruits that grow on trees include apples, cherries, lemons, limes, oranges, and peaches. Most small fruits grow on bushes, but grapes come from woody vines. Bananas and strawberries grow on plants that have a nonwoody stem.

 

Fruit crops, unlike most other crops, are not grown from seeds. Plants grown from seeds may vary in many ways from generation to generation. But growers strive to produce plants that will bear fruits of uniform type, appearance, and quality. Such fruits bring the highest prices when marketed. Fruit plants produce fruits of uniform quality if grown vegetatively-that is, from certain parts of desirable plants, such as stems, buds, and roots. The part that is grown develops new tissues and new parts identical to those of the parent plant.

Fruit plants are produced vegetatively in three main ways: (1) by grafting, (2) from cuttings, and (3) from specialized plant structures. Most fruit trees are reproduced by grafting. In this process, a bud or piece of stem from one tree is joined to a rootstock from another. A rootstock is a root or a root plus its stem. The resulting tree will have most of the same characteristics as the tree from which the bud or stem was taken. However, the rootstock may determine such characteristics as the size and productivity of the new tree.

Some fruit plants are produced from cuttings or from specialized structures. Most cuttings are pieces of stem that grow roots when placed in water or moist soil. Specialized structures called runners are used to grow strawberry plants. Runners are long, slender shoots that mature strawberry plants send out along the ground. A runner placed in soil develops into a new plant.

Some fruit growers produce their own plants vegetatively. However, most growers buy their plants from nurseries that specialize in producing them.

The branch of horticulture that deals with fruit growing is called pomology. Pomologists have developed highly efficient methods of planting and caring for fruit crops, and most fruit farms use these techniques.

There are three main steps in growing fruit: (1) planting, (2) caring for the crop, and (3) harvesting.

Planting. Fruit crops are perennials, and so they do not have to be replanted annually as do most other crops. After the original planting, a fruit farmer need only replace plants that become unproductive. Many fruit plants remain productive for 30 to 50 years or even longer. In mild climates, farmers generally plant trees, bushes, and vines in fall. In cold climates, planting usually takes place in spring.

Most bushes are planted from 3 to 5 feet (0.9 to 1.5 meters) apart in rows that are 6 to 10 feet (1.8 to 3 meters) apart. Rows of grapevines are spaced about 10 feet (3 meters) apart. In the past, farmers almost always grew full-sized fruit trees. In most cases, the trees were planted from 20 to 40 feet (6 to 12 meters) apart to allow room for growth. Today, many farmers prefer to grow dwarf trees, which are planted close together. The branches of each tree may grow up a supporting framework called a trellis. The trellis enables all the fruit to receive maximum sunlight, and so the crop ripens better and faster than it otherwise would. Fruit is also easier to harvest from dwarf trees than from full-sized trees.

Caring for the crop. Most fruit growers use special machinery to fertilize, cultivate, and otherwise care for their crops. Fruit crops must be fertilized at least once a year. Some fertilizers are applied to the soil, and others are sprayed on the plants. Many fruit growers cultivate the soil around young fruit plants periodically. This practice helps control weeds and thus encourages crop growth. Most fruit crops grown in extremely dry regions must be irrigated. Farmers use various methods, such as ditches and sprinklers, to distribute irrigation water.

In many cases, the branches of a young fruit tree must be trained so that the tree develops a uniform shape and a sturdy structure. Training may involve propping the trunk or tying the branches, or it may consist entirely of pruning. Pruning strengthens a plant by ridding it of unproductive branches. Nearly all fruit plants have to be pruned at least once annually. In addition, most farmers remove some of the crop from the trees during the early stages of the fruit's growth. This practice, called thinning, helps increase the size of the remaining fruit.

The majority of fruit growers use chemical pesticides to protect their crops against diseases and insect pests. Most pesticides are sprayed or dusted on crops by tractor-driven machinery or specially equipped light airplanes or helicopters. Plant breeders have also developed varieties of fruit plants that resist certain diseases and harmful insects.

Sudden spring frosts can endanger fruit crops in temperate or subtropical regions. Farmers use water distributed by sprinklers to protect small-fruit crops from frosts. Water releases heat as it freezes. If it is sprinkled onto the crops continuously, it keeps the tender flowers and young fruits from freezing. Farmers use heaters to protect tree crops from spring frosts.

Harvesting. Most fruits ripen rapidly after reaching their mature size. Harvesting occurs during different stages of the growth process, depending on the type of fruit and its intended use. For example, gooseberries and cherries used in making artificial coloring are harvested when immature. Apples, bananas, peaches, and pears are usually harvested when mature, but before they ripen. Berries and most fruit picked at home orchards are harvested during the ripening stage. Most fruits taste best when they are allowed to ripen on the plant. Citrus fruits do not go through a distinct ripening process and may be harvested over a long period of time after they mature. Fruits are bruised more easily than most other crops, and so they must be harvested with greater care. Most are picked by hand. However, the increasing cost of hand labor has encouraged the use of fruit-harvesting machines. Some of these machines have arms that shake the fruit loose from the plants. The loosened fruit drops onto outstretched cloths. Other mechanical pickers have fingers that "comb" fruit from the plants.

                    Marketing fruit

The United States is the leading fruit-producing country in the world. It raises more than 10 percent of all the apples, pineapples, and plums; about 20 percent of the lemons, oranges, peaches, and strawberries; and about 45 percent of the grapefruit. California is the nation's chief fruit-growing state. Other leading states include Florida, Michigan, New York, Oregon, and Washington.

Most fruit scheduled to be sold fresh is taken from the orchard or field by truck and delivered to a packing house. Many large fruit farms have their own packing facilities. Commercial packing houses are centrally located in fruit-growing regions. Most large packing houses are fully mechanized. Machines wash the fruit, sort it according to size and quality, and pack each batch into containers. The fruit is then shipped to market or stored for future delivery. Railroads and trucks carry most overland shipments of fruit. Most overseas shipments travel by ocean freighter.

Fruits can be stored for varying lengths of time under controlled conditions. Temperate tree fruits must be stored at temperatures near freezing. Some kinds of apples can be kept fresh for about a year under such conditions. On the other hand, most small fruits remain fresh only a few days or weeks in cold storage. Tropical and subtropical fruits can be stored for a few weeks or months under temperature-controlled conditions. The temperatures, though cool, must be well above freezing. The amount of oxygen ordinarily present in the air promotes spoilage of fruit. The storage time for all fruits can be lengthened by reducing the oxygen supply.

Much fruit is shipped directly from farms to food processors. Processing plants preserve fruit by such methods as canning, drying, and freezing.



                    Developing new varieties of fruit

Over the centuries, fruits have been improved by constant selection of the most desirable plants. In selection, plants grown from seed are examined for various desirable qualities. Individual plants are singled out for high productivity or for the superior color, texture, or flavor of their fruits. If the desirable characteristics of the selected plant are reproduced when the plant is grown vegetatively, the selection may be classed as a new cultivated variety. Cultivated varieties are also known as cultivars.

Occasionally, an individual plant develops an unexpected characteristic due to a mutation, a random genetic change. For example, an apple tree may suddenly start to bear fruit of a more intense red color. Horticulturists refer to such a mutation as a sport. Growers have used sports to develop many improved varieties of fruit. The trees of Delicious apples originally produced pale-colored, striped fruits. Some branches on individual trees began to bear solid-red apples. By grafting these branches onto appropriate rootstocks, growers produced the attractively colored types of Delicious apples available today.

Horticultural plant breeders use a technique called crossing or hybridization to improve fruits. In this process, pollen is taken from a plant that has been selected for a particular desirable trait. The pollen is placed in the flower of a plant selected for another desirable quality. Some of the plants grown from the resulting seed may have the desirable characteristics of both parents. Occasionally, one of these plants may prove worthy of being named as a new variety. In many cases, hybridization and selection are repeated over many generations to create a new variety. Hybridization is a highly useful technique because it enables breeders to produce varieties with combinations of more and more desirable qualities. In the future, desirable characteristics may be transferred using techniques of genetic engineering that remove genes from one plant and insert them into another.