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Birds spend much of their time
feeding and searching for food. Most birds cannot
store large reserves of food internally, because
the extra weight would prevent them from flying.
Small birds need to eat even more frequently than
large ones, because they have a greater surface
area in proportion to their weight and therefore
lose their body heat more quickly. Some extremely
small birds, such as hummingbirds, have so little
food in reserve that they enter a state resembling
hibernation during the night and rely on the warmth
of the sun to energize them in the morning.
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Depending on the species,
birds eat insects, fish, meat, seeds, nectar,
and fruit. Most birds are either carnivorous,
meaning they eat other animals, or herbivorous,
meaning they eat plant material. Many birds,
including crows and gulls, are omnivorous,
eating almost anything. Many herbivorous
birds feed protein-rich animal material
to their growing young. Some bird species
have highly specialized diets, such as the
Everglade kite, which feeds exclusively
on snails.
Two unusual internal organs
help birds to process food. The gizzard,
which is part of a bird's stomach, has thick
muscular walls with hard inner ridges. It
is capable of crushing large seeds and even
shellfish. Some seed-eating birds swallow
small stones so that the gizzard will grind
food more efficiently. Birds that feed on
nectar and soft fruit have poorly developed
gizzards.
Most birds have a crop-a saclike extension
of the esophagus, the tubular organ through
which food passes after leaving the mouth.
Some birds store food in their crops and
transport it to the place where they sleep.
Others use the crop to carry food that they
will later regurgitate to their offspring.
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The bills of birds are modified in ways
that help birds obtain and handle food. Nectar-feeders,
such as hummingbirds, have long thin bills, which
they insert into flowers, and specialized tubular
or brushlike tongues, through which they draw
up nectar. Meat-eating birds, including hawks,
owls, and shrikes, have strong, hooked bills that
can tear flesh. Many fish-eating birds, such as
merganser ducks, have toothlike ridges on their
bills that help them to hold their slippery prey.
The thick bills and strong jaw muscles of various
finches and sparrows are ideal for crushing seeds.
Woodpeckers use their bills as chisels, working
into dead or living wood to find insect larvae
and excavate nest cavities.
At least two species of birds
use tools in obtaining food. One is the woodpecker
finch, which uses twigs or leaf stalks to extract
insects from narrow crevices in trees. The other
is the Egyptian vulture, which picks up large
stones in its bill and throws them at ostrich
eggs to crack them open.
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