especially during the breeding season.
Their songs show that they are ready to
mate. Songbirds may also make a variety
of calls to communicate with their mates
and young. Many songbirds sing to
announce that a certain territory is
theirs. They sing at different spots along
the edges of their land to try to prevent
other birds from taking over it. In many
species only the male sings complicated
songs.
Different types of songbird eat different
foods, including insects, fruits, and
seeds. Many types are known for building
complex nests.
#More to explore
Bird • Blackbird • Bluebird • Canary
• Cardinal • Crow • Mockingbird
• Nightingale • Robin • Sparrow
Songhai Empire
The Songhai Empire controlled trade in
much of western Africa during the
1400s and 1500s. The empire was cen-
The grasshopper warbler is a songbird of
Europe and Asia.
132 Songbird BRITANNICA STUDENT ENCYCLOPEDIA
tered in what is now central Mali. It
eventually extended west to the Atlantic
coast and east into what are now Niger
and Nigeria. Songhai grew rich trading
gold and salt up and down the Niger
River and across the desert lands of the
Sahara.
Songhai people had settled in the city of
Gao in about AD 800. The Mali Empire
took over Gao in 1325. After 50 years
the Songhai won back their independence.
A great Songhai warrior named
Sonni !Ali took power in 1464. He built
the Songhai Empire by conquering Timbuktu,
Jenne, and other nearby cities.
Most Songhai people raised herds of
animals for a living. However, many
Songhai lived in big cities. All the cities
were centers of trade on the Niger River.
Gao was the capital. It had about
100,000 people. Timbuktu had at least
80,000 people. It was the site of a
famous university. While most of Songhai’s
herders continued to follow traditional
religions, Islam became the
religion of the cities.
The empire prospered until the late
1500s. Then an army from Morocco, a
kingdom on Africa’s northwestern coast,
swept down over the Sahara. By 1591
the Moroccans had easily captured the
cities because they had better weapons
than the Songhai had. The people outside
the cities continued to fight the
Moroccans, but they could not bring
back the empire.
#More to explore
Mali • Mali Empire
Sorghum
Sorghum plants belong to the grass family,
as do corn and other grains. Sorghum
is one of the major grains grown
in Africa. Farmers also grow sorghum in
Hundreds of seeds are bunched
together on a sorghum plant.
BRITANNICA STUDENT ENCYCLOPEDIA Sorghum 133
the Americas, Asia, southern Europe,
and Australia.
Sorghum plants often grow to a height
of 2 to 8 feet (0.5 to 2.5 meters). Some
may grow to be 15 feet (4.5 meters) tall.
Clusters of flowers grow at the top of
each sorghum plant. The clusters produce
800 to 3,000 seeds.
Farmers grow and harvest sorghum for
these starchy seeds. The seeds are a cereal
grain like wheat, rye, oats, and rice.
People grind them to make cakes,
breads, and porridge, which is a dish
similar to oatmeal. People also use sorghum
plants to feed animals, to make
hay, and to make brooms and brushes.
In some types of sorghum the stems
contain sweet juices. In the southern
United States and in southern Africa
farmers grind sorghum stems to collect
the juices. They then boil the juices to
make a thick, sweet syrup, also called
sorghum.
#More to explore
Grain • Grass • Seed
Soto, Hernando
de
#see De Soto, Hernando.
Sound
A sound is anything that can be heard.
Music, the barking of a dog, the wailing
of a siren, and the voice of a friend are
all sounds.
What Makes Sound
For a sound to be made and heard, three
things need to happen. First, an object
vibrates. A vibrating object makes tiny,
very fast back-and-forth movements. For
example, when a musician strums guitar
strings, they vibrate. The vibration
moves the surrounding air and produces
waves of sound.
Second, the sound waves pass into a
medium. A medium is any substance
through which the waves can travel.
Sound waves may travel through many
mediums. These include air, water, and
solid objects. Sound waves pass through
mediums in all directions.
Third, some kind of receiver, such as the
ear of a person, picks up the sound
waves. The ear changes the sound waves
into signals that travel to the brain. The
brain understands these signals as sound.
Speed of Sound
The speed of sound varies depending on
what medium it is traveling through. In
air at a temperature of 70° F (21° C)
sound travels at a speed of 1,129 feet
(344 meters) per second. This means
Sorghum
grows better in
hot and dry
conditions
than most
other grains.
Sound waves travel through air from a
source to a receiver.
134 Soto, Hernando de BRITANNICA STUDENT ENCYCLOPEDIA
waves to loudspeakers to reproduce the
original sound.
Recording devices can preserve sound in
a number of formats. The earliest formats
were phonograph records and magnetic
tape. A phonograph record has a
groove with patterns cut into it that
represent the waves of sound. Magnetic
tape represents sound with wavelike
patterns of magnetized particles.
Most sound-recording devices today are
digital devices. They store sound as a
long series of numbers that describe the
