Bird
Migration
The Indian sub-continent is host in winter
to a multitude of birds which have nested further north
in Asia, and migrate over the mountains to seek congenial
winter quarters. Some species, such as Rosy Starlings,
migrate in large flocks over quite narrow routes, but most
migration is less obvious, and often takes place at night.
Summer visitors are inland migrators, like Pittas and Golden
Orioles.
What stimulates a bird to migrate?
The urge to migrate at the appropriate seasons is evoked
by both external and internal stimuli. One of the primary
external stimuli is the variation in day length. The
internal stimulus seems to be provided by the breeding
conditions.
What determines the goal of a migratory journey?
Birds not only return to the same general locality for
breeding year after year, but often also to the identical
nesting site. Day flying migrants maintain their course
from the angle the sun makes with the earth at the appropriate
season, while nocturnal migrants are guided by the by
the major constellations of stars. Once the goal is roughly
reached there seems every likelihood that landmarks,
imprinted on the senses in some way as the result of
previous experience and association, may play their part
in guiding old birds to their former haunts with such
astonishing precision.
Velocity and Altitude of migratory flight
The
average cruising speed of ducks and geese has been
found to
be between 65 and 80 km.p.h. A bird’s
flying day (or night)ranges from 6 to 11 hours. Average
km known to be covered in a ‘hop’: Coot 250
km; stork 200 km ( 6 hrs); woodcock 400-480 km; plover
880 km(11hrs).
Non-stop flights of at least 3200 km across open sea
are undertaken by the Eastern Golden Plover which is
a winter visitor to India too. There are others, especially
among the shore birds or waders, that cover enormous
stretches without halting for rest or food. Except when
the birds have to cross lofty mountain barriers, they
fly chiefly under 400m and only rarely over 900 m above
the ground.
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