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In-Depth Information
but are highly suited to assisting in walking and running at great speeds.
Being flightless, the ostrich needs other tactics to avoid predators; its long
legs, powerful muscles, and strong toes enable it to sprint at speeds in excess
of 40 miles an hour, higher speeds than many quadrupeds can achieve.
Bird Animation
One typical feature of birds—in addition to their main feature, feathers—is
their ability to fly. The large majority of bird species do this very well. Many
of them use flight as the main mode of locomotion, but they also walk, run,
hop, jump, swim, climb, and even burrow with varying degrees of expertise.
Bird dynamics are determined in part by each bird's size as well as its
physiognomy.
Some birds, like the South American rhea, run very well; others seem to have
difficulty even walking. Emperor penguins are an example of the compromise
in design often seen in nature. Each year they walk, in large columns, long
distances from their breeding grounds on the mainland of Antarctica back
to the sea to feed. As they do so, at times they drop down onto their bellies
and slide along on the ice and snow, propelling themselves by their feet. On
the downward slopes this seems to be a more efficient and effective way of
locomotion. A different physiognomy would no doubt prepare them better
for this journey. Longer legs would clearly be better suited for it, though given
the distances involved, flight would be the best mode of travel. However, once
the penguins arrive at the coast and slip into the water through the gaps of
sea ice and begin to hunt for their diet of fish, they demonstrate that they are
more than suited to life underwater. There is a compromise here, and it is clear
that the most important factor in their survival is their capacity for swimming
at speed.
Principles of Flight
Several forces are associated with flight:
weight, lift, drag
, and
thrust
. Two of
these forces impede flight; the other two counter those impediments. Weight
is a vertical downward force experienced as a result of gravity that must be
overcome for a bird to become airborne. Any forward movement is a result of
thrust, which in turn induces drag. Drag is a horizontal force that comes into
play as an object moves through a particular medium such as air or liquid.
Flight becomes possible only if these forces are balanced appropriately, with
the force of lift being greater than the force of weight and thrust being greater
than drag. The amount of drag induced is a direct result of the size and shape
of the object; the more streamlined a shape, the more efficiently it will travel
through a medium.
Wings alone will provide lift only if there is forward motion or if the air moves
across the wing. Wings vary in length, width, thickness, and shape, with most
wings being asymmetrical and having a rounded, leading edge that tapers
