Graphics Reference
In-Depth Information
FIG 4.61 The wing action of a
hummingbird describes a figure of
eight, creating lift on both the forward
and backward thrusts.
Hummingbirds need to hover to reach their food supply and therefore
maintain lift in calm conditions by their particular way of flying. Instead of the
simple upward and downward actions demonstrated by most birds, they also
use forward and backward strokes in a figure-eight configuration, generating
lift on both the downbeat and upbeat of the wings.
Some birds such as kestrels achieve a hovering gait by facing into the wind
and, through very delicate wing movements, simply allow the wind itself to
create the necessary lift.
Groups, Flocking, and Formation Flying
Flocking and living in groups present birds with some very distinct advantages
for species in which competition for food is not an issue. Starlings gather
together in huge numbers to roost, a tactic that offers greater safety against
predation. Flocks may vary from small family groups to large flocks, often
made up of mixed species. This is often seen on the shoreline of estuaries,
mudflats, and beaches and can be seen in European winters as small
passerines gather to forage for food. Other birds group together for warmth.
African sparrows build large communal nests not just for breeding purposes
but for protection against the cold night air. Penguins form huge colonies that
huddle together for protection against the extreme conditions of Antarctica,
each taking a turn on the outer edge of the flock to take the brunt of the
weather while those in the center of the group enjoy the higher temperatures
afforded by the bodies of their fellow penguins.
The general rules for a flocking model involve alignment, separation,
and cohesion. These three rules work together to ensure that collisions
are avoided. Separation ensures that overcrowding is avoided, alignment
ensures that the bird and its immediate neighbors face in the same direction,
and cohesion ensures that any change in movement, direction, or speed
is coordinated throughout the flock. The alignment, movement, and
synchronization of flight in a flock of birds are known as alilomimasis .
Shifting masses of birds do not have a leader in the flock; though they often
seem to be moving in a rather haphazard way, the individuals in the flock
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