Bird migration is the periodic movement of birds from one area to another and back again. There are benefits to migration such as avoiding harsh winter weather and finding abundant food sources. However, migration also carries risks and costs like exposure to predators, exhaustion, and dangerous weather. Different types of migration include seasonal north-south movements, elevational shifts, and latitudinal coastal movements. Not all birds migrate - some enter dormancy, remain on territories year-round, or show behavioral polymorphism where some populations migrate and others do not. Evolution of migration is still conjectural but may have originated from ice age displacements or from exploiting new empty habitats. Migration requires physiological preparation like molting, migratory restlessness, and putting on

1. The Riddle of
Bird
Migration
 Definition: periodic mass
movement of all or part of
population from one area to
another and then back
Some other types of
Movements
Nomadism
Post-natal dispersal
Wanderings for
unoccupied home ranges
or territories to claim
Irruptions (e.g., Snowy
Owl
Aggregations at
superabundant food source

2. Why
Migrate?
• Migration as
Opportunity
– Nesting
– Food
• Migration as
Escape
– From weather
– From lack of
food

3. What are the Benefits of Migration?
• Avoid harsh and dangerous
winter climates
• Avoid lack of food during winter
– Especially insectivorous birds
such as wood warblers
– Especially fruit-eating birds
such as waxwings
• Avoid lack of cover during
winter
• Migrate from area of sparse or
no food to area of relatively
abundant food
• So migration enhances
survivorship of many
populations

4. What are the Costs of Migration?
• The Price of Migration
– Migration Risks
• 100 million NA waterfowl migrate
in fall, 40 million return in spring
• Risks attend all long distance
migrations
– Exposure to predators
– Exposure to exhaustion
– Exposure to harsh, dangerous
weather conditions during
middle of migration
• Storms, contrary winds blow many
species off course
• Especially risky for over water or
over desert migrants
• Eleonora’s Falcon breeds on
Mediterranean areas in fall to take
advantage of migrants.

5. What are the Types
of Migration?
• Seasonal North/South
– Primary pattern
– Except that in southern
hemisphere migration
occurs as a South/North
movement
• Elevational Migrations
– Summer/winter shift from
high elevations to low
elevations
• Latitudinal
– Movement from interior
areas to shore areas

6. Everyone
doesn’t
Migrate:
Alternatives
• Enter dormancy
or torpor
• Remain on
territory
throughout the
year
– Thereby first
in spring to
claim mate

7. How did migration evolve?
• Still Highly Conjectural
• Northern Birds South: Ice Age Theory
– 100,000 years plus of Ice Ages
– Separated by Interglacial Periods
– Northern Birds displaced southward with each new ice age,
northward with deglaciation
• Tropical Birds North
– Open, empty habitats (competition-free habitats)
– Hummingbirds, flycatchers, wood warblers are all moving
northwards
• Gradualism: Generational North with the spring

8. Migration Evolution as a balance between
Advantages/Disadvantages---Survivorship

9. Is Migration is Evolving Now?
 European Robins and Ringed Plover
 Partial Migration of population
(some subpopulations remain
throughout year
 Others migrate every year
 Still others migrate or don’t
migrate
Remain on territory during
mild winters
Migrate during harsh winters
Called Behavioral
Polymorphism (i.e., the
population is polymorphic
with respect to behavior

10. How do Birds
Prepare
for
Migration?
• Molting
– Postnuptual molt
– New feather
growth needed
for long
migratory flights
• Migratory
restlessness
– Termed
Zugunruhe
• Putting on weight

11. Migration Fat
• Fat yields 2x energy
• Birds add to 20% body
mass, mostly in fat
• Fat deposits in adipose
tissue of skin, in muscles,
organs
• Spreads as a thin layer
just under skin
• Also provides insulation
during high altitude
migration flights

12. Some Patterns of Migration I
• Red Phalarope
• Nests in tundra
• Winters at Sea

13. Self Study Topics for Migration
• Use the internet to find information sites
about bird migration
• Select your favorite bird and review its
migration patterns with respect to:
– How far does it migrate?
– What route does it take to migrate?
– What factors influence its migration?
– How does it navigate (find its way) during
migration?