MIGRATION IN BIRDS

MIGRATION IN BIRDS

Bird migrationis the regular seasonal movement, often north and south along betweenbreedingandwintering grounds.

Thewordmigrationcomes from the Latin word,migratuswhich means"to change.

Migration is the movement of an animal from one region, or habitat, to another. This happens at regular periods of time, and during a particular season. Birds migrate twice a year, once in spring and again in fall.

Animals migrate in order to breed, grow, find food or avoid cold weather.

Migration is movement from one habitat to another to get advantage of favourable conditions , it is traditional, instinctive and of regular occurance at regular intervals every year .

There are 4 kinds ofmigratingbirds:

Permanent residentsare non-migrating birds who remain in their home area all year round.

Summer residentsare migratory birds that move north in the spring, nest during the summer, and return south in the fall.

Winter residentsare migratory birds who fly south for the winter.

Transientsare migratory species who nest farther north but who winter farther south. We only see transients as they are passing through and have a stop over (birds of passage) Ex: Cormorants.

Usually birds migrate to the Northern hemisphere in spring to breed and return to Southern hemisphere in autumn to pass winter.Thus, birds migrate to secure optimum conditions for breeding and feeding, utilizing trophic niches of both the hemispheres .Of around 8000 species of birds half are migratory . Birds are best adapted for migration because they can fly long distances ,seas ,mountains etc are not obstacles and they can maintain their body temperature

Types of Bird Migration

I Altitudinal / Vertical Migration

Thisis a short-distancemigrationfrom lower altitudes to higher altitudes and back.It is commonly thought to happen in response to climate and food availability These migrations can occur both during reproductive and non-reproductive seasons. Birds living at high altitude descend to lower valleys in winter to escape intense cold ,they return again to higher altitude with advent of summer. Ex: Grebes and coots of Andes in Argentina ,willows in Siberia & Himalayan white caped redstart

Coots GrebesHimalayan white caped redstart II Latitudinal / Equatorial Migration

North to South or South to north migrationMost common migration5000 million or more birds fly 3200 km south to grassland, forests & lakes of central/southern Africa , Asia. Ex. Shore bird Arctic tern fly 17,600km.American Golden plover to ArgentinaSiberian bird to Himalayas

Birds fly from north to south or vice-versa, linking the equatorial regions of each hemisphere with its temperate and cold areas. Latitudinal migration can be seen in several birds e.g. Golden plover of America which spends nine months of winter, 8000 miles south in Argentina

Arctic TernMigration route of Arctic Tern

American Golden PloverMigration route of American Golden Plover III Longitudinal Migration

Similar to latitudinal migration, this type of movement is a change between different longitudes from east to west or west to east. This is a common type of migration in Europe, where geographic features encourage birds to move longitudinally rather than latitudinally. This migration takes place in East-West direction or vice versa Ex Evening gross beak that nest in northern Michigan (U.S.A) spend the winter in New England. Starlings move from East Europe or Asia to the Atlantic coast

European StarlingMigratory flock of starlings

Make daily movementIV Daily Migration Many birds make daily movements from their nesting/breeding site to feeding area. The movement maybe in response to environmental forces like light, darkness, temperature, humidity, food availability Ex crow, sparrow etc

V Local Migration Heavy flood, excess heat or cold flowering of certain plants, ripening of seeds etc force birds to leave the area for some time, they then return to their nesting sitesy VI Cyclic Migration Some migrations are seasonal ,do not occur at regular intervals Ex Snowy Owl of USA migrates once in 3-5 years

VII Moult Migration Males and juvenile Ducks migrate short distances for moulting ,leaving behind females and young birds

VIII Vagrant / Irregular Migration After breeding the adult and the young stray away from home to disperse in all direction over many miles in search of food and safety Ex Great blue heron, cuckoos , thrushes.

Great blue heronCuckoosThrushesIX Partial Migration In some cases all the birds of a flock of migratory birds do not leave the native land ,some birds stay on at the breeding place Ex May blue jays of USA and Canada

X Nocturnal Migration Smaller birds fly by night ,feed and rest during the day .They get protection from their enemies in the dark while flying Ex Sparrows, thrushes , warblers

SparrowThrushesWarblersirsXI Diurnal Migration Large birds are day fliers .They stop for feeding at suitable places Ex Pelicans, swifts, hawks, pigeon Some birds fly both day and night while migrating Ex Geese, ducks

PelicanSwiftHawk Causes of migration

Seasonal Food ScarcityAvoiding PredatorsBreeding Conditions

Certain species of birds may migrate to areas that have the best environment for breeding. For example, some birds migrate north to the Arctic in the summer to breed when food is plentiful and the daylight hours are abundant. These conditions are ideal for breeding, as birds can maintain high energy levels and avoid predators who will have died out in the harsh winter

Internal factors due to physiological changes in the gonads and reproductive hormones which occur with change in the seasonPhoto periodism, length of day affects pituitary and pineal body in the brain which in turn prepares birds for migration by initiating restlessnessFat deposition Substantial sub-cutaneous and visceral fat deposition causes migratory restlessnessTheories to explain bird migration

Northern ancestral home theory :According to this theory the original home of birds was in the Northern hemisphere. During Pleistocene period glaciation took place in north forcing the birds to migrate to south pole to protect themselves from intense cold. When glaciation receded the birds returned to their original home .By constant repetition of this movement generation after generation ,birds developed migratory insinct. This theory is not accepted Southern ancestral home theory : According to this theory the ancestral home of birds was in the South pole (tropics).When there was over -crowding in this region some birds migrated to the north pole and returned to south as soon as nesting was over This theory is not accepted

22 Continental drift theory

North and South continuous land mass was seperated by continental drift causing seperation of birds ,the birds still migrate to their original homes. This theory is not accepted Hypothesis of periodic response

It is understood that tropical birds inhabited colder northern part where plenty of food available.But they had to fly southward when cold increased and food became scarce.(Woodburry )

Length of the day is important factor which acts as a stimulus for migration .Increasing daylight stimulated gonadial activity ,this acts as a stimulus for migration by releasing gonado trophic hormones by hypothalamus/pituitary gland

Increased day length causes northward migration and decreased day length southward migration . No single factor is the cause for migration. Various external and internal factors is the guiding factor for migration Bird ringingorbird banding is the attachment of small, individually numbered metal or plastic tag to the leg or wing of a wildbirdto enable individual identification. This helps in keeping track of the movements of the bird and their life history. It is common to take measurementsand examine condition of , subcutaneous fat, age indications , sex and location during capture for ringing. The subsequent recapture or recovery of the bird can provide information onmigration, longevity, mortality, population studies,territoriality, feeding behaviour, and other aspects that are studied byornithologists.

Bird banding Radio collaring technique

When a bird cannot be followed visually, it may be equipped with a miniature radio-transmitter weighing about 2-3grams,which is stuck with an adhesive to the skin or inserted under the skin at the back of the bird between the wings or hung around the neck.

The transmitter is then monitored by a receiver and the movement of the bird is plotted on a map. The transmitter carried by the bird sends signals which maybe located by means of stationary direction finding aerials or by vehicles and aeroplanes. Even satellites and GPS is used for tracking.

Hordate Zoology Credits

WikipediaChordate Zoology by Jordan& P.S.Verma (edition -2004)