Devaka K Weerakoon
Field Ornithology Group of Sri Lanka
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• Some of the animals in our surroundings are present throughout the year
• There are some birds that appear during a certain time and disappear again after a while which is repeated faithfully year after year
• Sometimes during certain months of the year we see large number of butterflies fly in the same direction
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• These observations have captured mans imagination raising questions such as
– Why do animals undertake such journeys?
– How do they find the way?
• In fact study of migration dates back to
Aristotle (384 - 322 BC)
• In his book Historia Animalium he wrote that some creatures move south during winter to avoid the cold and return back during the spring. He also recognized that some migrate short distance while other travel a long way
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• Migration gave rise to lot of myths as well
• Aristotle proposed that some birds hibernate during the winter
• Aristotle was also responsible for theory of transmutation where he proposed some birds change into others (red start – Eurasian robin) with the arrival of the winter
• Barnacle/ Brent geese (Anser bernicla – the goose from barnacles) arose from Barnacles
(Lepas antifera - the goose bearer)
• In 1955 Olaus Magnus, wrote that swallows spend winter under water
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• It is not possible to give a simple definition for the term migration
• Nature exist in infinite variety as does the phenomenon we refer to as migration making it impossible to capture it within a single common definition
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Barn Swallow
(Hirundo rustica)
• Breeds in the north and winters in the southern hemisphere
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Monarch
(Danaus plexippus)
• Found in North America
• Heads south to winter in Central America
• In spring it flies north again
• Few makes it back to where they started
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Atlantic Salmon
(Salmo salar)
• Spawns in rivers of North America and
Europe
• As they develop they drift down to the sea
• When they become adults they swim back to freshwater to breed
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• Tiny microscopic animals that are at the bottom of marine food chains
• In summer they occupy the surface layer feeding on phytoplankton and breeding
• In winter they descend to deeper colder water and does not feed at all
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There are Commonalities and
Clear Differences
• All four are clear examples of migration and yet there are many differences
• Barn swallow – Seasonal movement over a long distance
• Monarch – Seasonal movement over long distance but does not complete the trip
• Salmon – Once in a life time journey over a long distance
• Zooplankton – Seasonal movement over short distance
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Why Do Animals Migrate?
• To avoid cold weather
• To make use of an seasonal abundance of a food source
• To Breed or spawn
• To disperse or avoid crowding
• Retreat to a place of safety
• Slow down the metabolism
• Avoid predators during early life stages
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• Zooplankton
• Flatworms, star fish, lobsters, crabs
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• Zooplankton
• Flatworms, star fish, lobsters, crabs
• Insects Butterflies, locusts, aphids, social insects
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• Zooplankton
• Flatworms, star fish, lobsters, crabs
• Insects Butterflies, locusts, aphids, social insects
• Fish – Salmon (Anadromous move from marine to freshwater), eel (catadromous – from freshwater to sea), move within marine habitat
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• Zooplankton
• Flatworms, star fish, lobsters, crabs
• Insects Butterflies, locusts, aphids, social insects
• Fish – Salmon (Anadromous move from marine to freshwater), eel (catadromous – from freshwater to sea), move within marine habitat
• Amphibians and reptiles
• Birds
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• Zooplankton
• Flatworms, star fish, lobsters, crabs
• Insects Butterflies, locusts, aphids, social insects
• Fish – Salmon (Anadromous move from marine to freshwater), eel (catadromous – from freshwater to sea), move within marine habitat
• Amphibians and reptiles
• Birds
• Mammals
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Human Migration
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• Complete migration - Cyclic event where the migrant travels along distance and returns to the original location
• Partial migration – Short distance migration
• Re-migration – migration cycle completed by off spring
• Removal migration – One way movement
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• North South Movements
• East–west journeys
• Circuits of the land and ocean
• Trips up and down mountains
• Vertical movements through the water column of seas and lakes
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Where do they migrate and from to?
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How do they Find the way?
• Navigation vs Orientation
• Navigation – Finding the way to a known location through unknown territory
• Orientation – Finding the way to a known location through familiar territory
• Navigation is done by
•Sun, moon and stars
•Routes learned from other birds
•Detecting the earths magnetic field
•Land marks – visual, olfactory, auditory
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How do we study this Phenomenon?
SIGHTING
Observations using binoculars or a spotting scope
BANDING
US Fish and Wildlife Service Bird
Banding Laboratory
RADAR (NOAA)
Several years ago, researchers at
Cape May counted 14 million birds in one night
RADIO TELEMETRY
Carl Safina followed a single radio-tagged albatross throughout its range
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Diurnal migrants
• Most birds which rely on gliding or flap
& glide flight are diurnal migrants
• The chief benefit of daytime flight is thermals – rising currents of warm air that provide lift
Nocturnal migrants
• Many birds which use powered flight are nocturnal migrants
• Night flying helps birds avoid predators
• The air is calmer and more stable at night
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Where do birds spend their time while in Sri Lanka
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Main Migratory Routes
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Major “fly-ways” to the country
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Principal Entry Points
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Major routes of dispersal within the country
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• The longest insect migration is the monarch butterfly. It travels up to 4,750km in the autumn.
• The largest migrant is the blue whale. It reaches a length of 24-27m.
• The longest recorded journey in water is a leatherback turtle, which travelled 20,558km in
647 days.
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Bar-tailed Godwits have the longest known nonstop flight of any migrant, flying 11,000 km from Alaska to their New
Zealand non-breeding areas.
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• The highest migration is the bar-headed goose that can reach up to 9,000m altitude.
• The Arctic Tern has the longest-distance migration of any bird, from its Arctic breeding grounds to the Antarctic non-breeding areas, a sea journey of over 22,000 km (14,000 mi).
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• Habitat destruction – loss of resting areas
• Hunting
• Climate change
• Changes in landscape
• Dam construction
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