Graphics Reference
In-Depth Information
provides an anchor for the strong wing muscles, and their bones are no longer
honeycombed but are more solid and filled with marrow, much in the same
way as mammal bones.
Beaks
The beak is a very versatile tool, one that is shaped and determined by the
nature of the bird's diet. For example, the curved and sharp beak of a raptor is
ideal for ripping flesh from bone.
Some beaks, thin, short, and pointed, are suited to a diet of insects; blunter
and stronger ones are suited to a diet of seeds; fish are best hunted with a
spear-like bill. Some beaks act as tools. The sturdy beak of the woodpecker
allows it to hammer holes in wood to reach insects or, in the case of a
sapsucker, to chisel out shallow wells in the bark of pine trees to tap the
supply of sap rising through a tree in spring. A sturdy beak is also useful
in excavating nest sites. The length of a beak may be also be critical and
determined by a particular kind of diet. A curlew's beak is long and curved,
allowing it to reach food deep under the sand, and nectar is best probed for
from the depths of flowers by very long and slender beaks like that of the
hummingbird.
Not all beaks that are intended to do similar tasks exhibit the same design
solutions or the same shapes. Filter feeders such as flamingos, shoveler
ducks, and avocets have different-shaped beaks that reflect the manner
in which they gather their food by filtering. They all sort out very small
crustaceans and other material from the water, but shovelers do it by
filtering from the surface of the water with their broad, flat bill as they
swim along. Flamingos, being tall, hold their heads down on the ends of
their long necks and seem to filter the water from an inverted position with
their large, curved beaks. Avocets have a far more slender and delicate
upturned beak that they sweep through the water in a sideways motion,
gathering any food particles that the beak strikes on its passage through
the water.
As we have already seen, the shapes and sizes of finches' beaks are good
indicators of their specialist diets, even for birds within a single family. The
crossbill is one such finch; it is the only bird that has developed the ability to
achieve lateral movement in the beak by twisting the upper and lower parts
to access the seeds inside pinecones, on which it feeds.
Legs and Feet
The variety in bird leg length and the nature of the bird foot reflect
environmental and behavioral changes and adaptations. They provide
support, perching, walking and running, climbing and clinging, swimming,
grasping, hunting—even the manipulation of primitive tools. Birds
with long legs are able to wade into water, giving them access to other
environments to exploit for food sources. Short legs are ideal for birds used
to living among dense hedgerows, as are feet that have developed gripping
