Bird migration

موضوع: Bird migration الخميس مارس 12, 2009 8:32 am

Many birds populations migrate long distances along a flyway.
The most common pattern involves flying north to breed in the temperate
or Arctic summer and returning to wintering grounds in warmer regions
to the south.
The primary advantage of migration is energetic. The longer days of the northern summer provide greater opportunities for breeding birds to feed their young. The extended daylight hours allow diurnal birds to produce larger clutches
than related non-migratory species that remain in the tropics
year-round. 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, energetic costs, and other
risks of the migration. 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 also
make them prone to parasites and pathogens requiring a heightened
immune response.^[5]
Within a species not all populations may be migratory and this is
termed as partial migration. Partial migration is very common in the
southern continents; in Australia, 44% of non-passerine birds and 32%
of passerine species were partially migratory.^[6]
In some species the population at higher latitudes tend to be migratory
and will often winter at lower latitude past the latitudes where other
populations may be sedentary, with suitable wintering habitats already
occupied, and this is termed as leap-frog migration.^[7]
Within a population, there can also be different patterns of timing and
migration based on the age groups and sex. Only the female Chaffinches in Scandinavia migrate with the males staying resident. This has given rise to its specific name coelebs, a bachelor.
Most migrations begin with the birds starting off in a broad front.
In some cases the migration may involve narrow belts of migration that
are established as traditional routes termed as flyways.
These routes typically follow mountain ranges or coastlines, 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 learnt to varying degrees. The routes taken on forward and return migration are often different.^[5]
Many of the larger birds fly in flocks. Flying in flocks helps in
reducing the energy needed. Many large birds fly in a V-formation and
individual energy savings have been estimated in the range 12–20 %.^[8]^[9] Red Knots Calidris canutus and Dunlins Calidris alpina were found in radar studies to fly 5 km per hour faster in flocks than when they were flying solitarily.^[5]
The altitude at which birds fly during migration varies. An expedition to Mt. Everest found skeletons of Pintail and Black-tailed Godwit at 5000 m (16,400 ft) on the Khumbu Glacier.^[10] Bar-headed Geese
have been seen flying over the highest peaks of the Himalayas above
8000 m (29000 ft) even when low passes of 3000 m (10000 ft) were nearby.^[11] Seabirds fly low over water but gain altitude when crossing land and the reverse pattern is seen in landbirds.^[12]^[13]
However most bird migration is in the range of 150 m (500 ft) to 600 m
(2000 ft). Bird hit records from the United States show most collisions
below 600 m (2000 ft) and almost none above 1800 m (6000 ft).^[14]
Most species of Penguin perform regular migrations by swimming. These routes can cover over 1000km. Blue Grouse Dendragapus obscurus
perform altitudinal migration mostly by walking. Emus in Australia have
been observed to undertake long-distance movements on foot during
droughts

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