Study of Bird Migration

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  • Topic: Bird migration, Bird, Cuckoo
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  • Published : December 3, 2012
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A flock of Barnacle Geese during
autumn migration
Example of long distance bird
migration routes.
Bird migration is the regular seasonal
journey undertaken by many species of
birds. Bird movements include those
made in response to changes in food
availability, habitat, or weather.
Sometimes, journeys are not termed
"true migration" because they are
irregular (nomadism, invasions,
irruptions) or in only one direction
(dispersal, movement of young away
from natal area). Migration is marked
by its annual seasonality. [1]
In contrast, birds that are non-migratory
are said to be resident or sedentary.
Approximately 1800 of the world's
10,000 bird species are long-distance
migrants. [ citation needed]
General patterns
Flocks of birds assembling before
migration southwards (probably
Sturnus vulgaris)
Migrating waders in Roebuck Bay,
Western Australia
Many bird populations migrate long
distances along a flyway . The most
common pattern involves flying north in
the spring to breed in the temperate or
Arctic summer and returning in the
autumn to wintering grounds in warmer
regions to the south. Of course, in the
Southern Hemisphere the directions are
reversed, but there is less land area in
the far South to support long-distance
migration.
The primary motivation for migration
appears to be food; for example, some
hummingbirds choose not to migrate if
fed through the winter. Also, the longer
days of the northern summer provide
extended time for breeding birds to feed
their young. This helps diurnal birds to
produce larger clutches than related
non-migratory species that remain in
the tropics. As the days shorten in
autumn, the birds return to warmer
regions where the available food supply
varies little with the season.
These advantages offset the high stress,
physical exertion costs, and other risks
of the migration such as predation.
Predation can be heightened during
migration: the Eleonora's Falcon , which
breeds on Mediterranean islands, has a
very late breeding season, coordinated
with the autumn passage of southbound
passerine migrants, which it feeds to its
young. A similar strategy is adopted by
the Greater Noctule bat , which preys on
nocturnal passerine migrants. [2][3][4]
The higher concentrations of migrating
birds at stopover sites make them prone
to parasites and pathogens, which
require a heightened immune
response. [5]
Within a species not all populations
may be migratory; this is known as
"partial migration". Partial migration is
very common in the southern
continents; in Australia, 44% of non-
passerine birds and 32% of passerine
species are partially migratory. [6] In
some species, the population at higher
latitudes tends to be migratory and will
often winter at lower latitude. The
migrating birds bypass the latitudes
where other populations may be
sedentary, where suitable wintering
habitats may already be occupied.
This is an example of leap-frog
migration . [7] Many fully migratory
species show leap-frog migration (birds
that nest at higher latitudes spend the
winter at lower latitudes), and many
show the alternative, "chain migration"
where populations 'slide' more evenly
North and South without reversing order.
Within a population, it is common for
different ages and/or sexes to have
different patterns of timing and
distance. Only the female Common
Chaffinches in Scandinavia migrate,
with the males staying resident. This
has given rise to the latter's specific
name of coelebs , a bachelor.
Most migrations begin with the birds
starting off in a broad front. Often, this
front narrows into one or more preferred
routes termed flyways . These routes
typically follow mountain ranges or
coastlines, sometimes rivers, and may
take advantage of updrafts and other
wind patterns or avoid geographical
barriers such as large stretches of open
water. The specific routes may be
genetically programmed or learned to...
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