Migration In Birds ( A small description)

Migration Of Birds


Introduction:-
The word migration has come from the latin word migra which means going from

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one place to another.
In birds, migration means two-way journey from the 'home' to the new
places to the 'home'. This movement occurs during the particular period of the year
and the birds usually follow the same route. There is a sort of 'internal biological
clock' which regulates the phenomenon.

Defination:-
According to L.Thomas (1926)- Bird migration may be describe as- "Changes of
habitat perictically recurring and alternating in direction, which tend to secure
optimum environmental conditions at all times."

OR

"Bird migration is a more or less regular extensive movements of birds between
their breeding regions and their wintering region."-- (Weffy, 1962)

Types Of Migration:-
All birds do not migrate but all species are subject to periodical movement of
varying extent. The birds which live in northern part of the hemisphere have greatest
migratory power. Migration may be (i) Latitudinal and (ii) Altitudinal.

(i)Latitudinal Migration:- The latitudinal migration usually means the movement from
northcto south, and vice versa. Most birds live in the land masses of northern
temperate and subarctic zones where they get facilities for nesting and feeding
during summer . They move towards south during winter.
Some trophical birds migrate during rainy season to outer trophics to
breed and return to the centarl trophics in dry season. Many marine birds also make
considerable migration. Puffinus (Great Shear Water) breeds on small islands and
migrates as far as Greenland in May and returns after few months.

(ii)Altitudinal Migration:- The altitudinal migration occurs in mountainous region.
Many birds inhabiting the mountain peaks migrate to low lands during winter.
Golden Plover starts from Arctic tundra and goes up to the plains of Argentina
covering a distance of 11,250 km. Birds migrate either in flocks or in pairs. Swallows
and Stroks migrate a distance of 9,650 km from northern Europe to South Africa and
travels to Great Britain, Africa, India and Ceylon thus travelling a distance of 9,650 km.

Factors Controlling Migration
Most species of birds migrate more or less on schedule and follow the routes in a
regular fashion. the actual causative factors determining the course and direction
of migration are not clearly known.
The following factors may be related to the problems of migration-

1) Insect and Gonadal Changes:-
It is widely accepted thet the impulse to migrate in birds is possibly instinctive and
the migration towards the breeding ground is associated with gonadal changes.

2) Scarcity of Food and Day Length:-
Other factors viz. scarcity of food, shortening of day light and increase of cold
are believed to stimulate migration. Migration in birds depends upon two important
factors- stimulus and guidance.
Scarcity of food and fall of day light are believed to produce endocrinal
changes which initiate bird migration.

3) Celestial Bodies:-
The guidance during migration is chiefly instructive but various factors are believed
to act as guide. Matthews (1955) and Saucer (1657) have emphasised the position of
the sun (during day time) or stars (during night time) that helps the birds to navigate
along definite route.
Expeimental evidences by creating artificial planetarium have shown
that the shifting of position of the sun changes causes a change of the migratory
pathways. heavy cloud or fog interferes migration during night.

4) Magnetic Intensity of teh earth and hand marks:-
The magnetic intensity; east-west and north-south gradients of gravity; lines of
magnetic force obvious land marks like costal lines, rivers, mountain ranges etc
which influence the migration.

5) Seasonal Migration:-
The north-to-south migration of birds take place under stimulus from the internal
condition of the gonads which are affected by seasonal variation.

6) Light:-
The experiment of Rawan with Juncos (Summer visitor to Canada) have
established that light plays important role in the development of gonads, which has
indirect role on migration. If the gonads undergo regression, the urge for migration
is not left. So the seasonal changes in illumination appear to be a crucial factor
determining migration.

Despite all these suggestion, it is not clear how birds through
successive generations follow the same route and reach the same spot. The
intinctive behaviours like migration, breeding, moulting or phasic occurances in the
annual cycle which are possibly controlled by the endocrine system. In all migratory
birds, accumlation of aft takes place for extra fuel during prolonged flight in migration


Source- Biology of Animal Vol-II (book)