Aves: Migration | Zoology Optional Notes for UPSC PDF Download

Bird Migration: Nature's Annual Odyssey

Understanding Migration

• The term "migration" finds its roots in the Latin word "migrare," signifying movement between spatial units.
• In the avian realm, migration involves a cyclical journey between specific breeding and non-breeding grounds, aligning with favorable conditions.
• Cahn (1935) defines migration as the periodic movement of animals from one place to another.
• For birds, migration constitutes a round trip, from breeding and nesting locations to feeding and resting spots, following the same route annually.

Kinds of Migration

1. Return Migration: Birds migrate back to previously known places, revisiting locations from prior journeys.
2. Exploratory Migration: Birds venture into completely unknown spaces, though they retain the ability to return.
3. Removal Migration: Migration to a comparable spatial unit without returning to the original location.

Types of Migration Based on Plane of Movement

1. Horizontal Migration: Occurs perpendicular to the Earth's gravitational force.

   • Latitudinal Migration:From north to south (or vice versa), such as the journey of the Cuckoo covering 7250 km.

   • Longitudinal Migration: Takes place in an East-West direction, often to avoid continental winters.

2. Vertical Migration: Movement parallel to the Earth's gravitational pull.

3. Altitudinal Migration: Combines horizontal and vertical components, involving flights over mountains and hills and Birds migrate from plains to Himalayan slopes, ascending thousands of feet during summer and descending in winter.

Types of Migration Based on Time of Flight

1. Diurnal Migration

   • Large birds, like crows and swallows, fly during the day, often in flocks.
   • Some forage during flight, while others, like swallows, capture insects mid-air.

2. Nocturnal Migration

   • Majority of small birds, including sparrows and warblers, prefer night flights for safety from predators.
   • Geese and ducks may fly both day and night during migration.

Types of Migration Based on Reasons

1. Climatic Migration

   • Responds to changes in the environmental climate, e.g., North-South migration of ducks and geese.

2. Alimental Migration

   • Occurs due to food shortages at any time of the year.

3. Gamatic Migration

   • Necessary for specific reproductive processes.
   • Most birds engage in this type of migration.

Types Based on Seasons

1. Summer Visitors: Birds arriving in spring from the south to breed, leaving for the south in autumn (e.g., Swifts, Swallows).
2. Winter Visitors: Migrate Southward in winter and return North in spring (e.g., Fieldfare, Snow Bunting).
3. Birds of Passage: Short visits in spring and autumn while en route to colder or warmer countries (e.g., Sandpipers).

Problems of Navigation (Way Finding)

• Birds use landmarks like mountains, rivers, and valleys for route identification.
• Navigation involves responses to the Earth's magnetic field, guidance by the sun, moon, and stars, and learning through experience.
• Migration is an inherent trait passed down through hereditary accumulation.

Causes of Migration

• The primary stimulus for migration remains unclear, with factors such as environmental, physiological, and endocrine influences proposed by various researchers.
• Decline in temperature, changes in food availability, and hormonal regulation are considered potential triggers.

Advantages of Migration

• Birds benefit from migration by accessing better breeding and feeding grounds.
• Migratory patterns enable birds to enjoy longer daylight hours for food gathering.
• Escape from unfavorable conditions is a key advantage of migration, allowing birds to thrive in more suitable environments.