Why do birds migrate?
Prashant Mahajan, senior Education Officer, BNHS, Mumbai
The answer to why some birds migrate while others don't is not as simple as
it seems. Ornithologists have studied the phenomenon for many years and still have not come up with an accurate explanation.
Birds mostly migrate long distances in order to escape from extreme cold, diminishing food supply and rapidly shortening days. Birds do not get sufficient time to search
for food because of shortening days in their breeding grounds. This motivates them to migrate. These are only few of the several theories about migration but none is able to provide a fully satisfactory explanation. Their stay in winter is confined to only
a few weeks after which they set out again on the return trip to their nesting grounds.
More is known about where and how the birds migrate, and on which routes. Answers to these questions have been found with the aid of high-powered radar equipment, aircraft
and short wave transmitters. Today there are bird ringing stations in all parts of the world which mark individual birds by putting a ring around the bird's leg with an identification number and the address of the respective country's ringing scheme. Recovered
rings are sent to the respective ringing country's ringing scheme's headquarters that can then plot maps of migration. These are added and updated every year on the basis of information received from other ringing schemes through the world.
This makes it possible to determine the route, speed of flight and winter destination of some species. The Bombay Natural History Society has studied Bird Migration at various
places in India. Birds were ringed at Bharatpur, Chilka, Harika, Karera etc. The birds ringed were reported at Moscow, Novosibirsk and shores of the Caspian Sea.
How do they find their way?
It is amazing that birds always find their way back to their nesting ground after traveling from as far away as 1000 kms. It would seem, then, that birds must have an innate
sense of long-distance orientation. But what leads them to their goal? This aspect has been studied by many scientists for years. One theory states that birds are directly influenced by the earth's magnetic field and they are able to distinguish the various
points of the compass. However modern instruments reveal that the earth's magnetic field has only slight influence on birds. The other theory based on observation and experiment states that birds navigate by means of light or by the position of the sun, the
moon and stars. This becomes possible even when the sky is overcast, but not in very foggy condition.
Travel Per day:
In most birds a migratory journey is not made non-stop as many people once believed. Some birds travel about 100 km a day while others cover as much as 300 to 600 kms a day.
On the way they may stop, rest and obtain food. In bad weather, especially if it is foggy, they even remain in one place for several days.
- Speed of travel: The speed of a bird on migration varies according to the species but is always less than that of which it is capable over short distances. Some birds fly
50 Kms per hour whereas few do 74 kms per hour.
- Flight Formation: Some species migrate in flocks, but some make the flight alone or in small groups. Some migrate by day; others travel by night and few both by day and
night. The birds traveling in flocks form their own formations such as V shape or slanting lines, as the ones in front disturb the air for those behind. The birds sometimes take turns as leader e.g. Grey-lag Geese and Demoiselle Cranes.
- Altitude: Birds travel at various altitudes. Small birds usually fly upto 100 meters and most raptors at a height of about 1000 meters. When they want to cross high mountains
they can fly even higher.
- Total Distance travelled: The distances some birds travel, when migrating, are almost unbelievable. Many birds fly as far as ten thousands kilometers twice a year, in winter
and in summer. The Arctic Tern migrates twice each year from Arctic Winter to Antarctic Summer and back again covering distances of over 17,000 kms each way!