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Migration lecture overview

What migration is and what it is NOT

Who migrates?

How do animals migrate?

– physiological, navigational, & other behavioral challenges

How does a species become migratory?

Why do some animals migrate (while others do not)?

– learned or innate?

– proximate reasons (or, knowing when it’s the right time to go)

– ultimate reasons (the evolutionary answers)

If we have time: a case study

Migration vs. Nomadism

Unpredictable seasonal movements

Example: crossbills

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Migration vs. Dispersal:Permanent movement away from a former home range to a new one

Typically young animals

Often sex-biased

Benefits:• Genetic benefits

• Reduce intra-specific competition

• Avoid extinction in unpredictable environments

Costs:• Energetic cost of moving

• Uncertainty & risk

Migration:

Definition depends on taxon

• Genes, rotifers, in population dynamics: = dispersal

• Insects (e.g., locusts): usually refers to one-waymovements, return trip, no specific temporal pattern (≈nomadism or dispersal)

• Birds, whales, fish, etc:

Movements of all or most of the individuals in a population between spatially separated breeding and non-breeding home ranges occurring on predictable cyclical time scales

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Who migrates?

Marine mammals (whales)

Terrestrial mammals (wildebeest)

Aerial mammals (bats)

Sea turtles

Insects (monarchs)

Birds of course…

Temporal patterns of migration

Dial: e.g. pelagic zooplankton

Annual:

• Same individual (birds)

• Different individuals

– Multiple generations in 1 year (many insect spp.)

– Each individual returns once in lifetime (> 1 yr), but some individuals migrate every year

Milkweed BugLygaeusequestris

Atlantic SalmonSalmo salar

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Spatial patterns of migration: migratorydistance – huge variation between (& sometimes within) species

Spatial patterns of migration: differences among sex & age groups in distance migrated

Nebel et al, 2003

Distance migrated (kms)

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Spatial patterns of migration: migratory routes—sometime complex decision-making & navigation required

How do animals migrate?

Example of cognitive adaptations:

Clocks and maps

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How do animals migrate?

Examples of physiological adaptations:

Freshwater vs. saltwater

Examples of physiological adaptations:

Fat storage and metabolism

• Pre-migratory fattening—hyperfagia. Involves switching to alternative metabolic pathways

• Being fat is dangerous and costly but necessary: migrants may increase normal body weight to 130% of normal or more with fat.

• Great knots reduce all body organs other than brains and lungs during sustained migratory flight. Reduction of BMR to 46% of normal!

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How do animals migrate?

Example of morphological adaptations:

Wing length

• Migrants have pointier, longer wings that aid reduce maneuverability but reduce cost of long flights

Innate or learned?• Evidence for both

• Phylogenetic patterns

• Learned components even in innate migrations

PasseriformesAnseriformes,

Gruiformes

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Evidence for genetic control of migratory behavior in birds: naïve birds, Emlen funnels

• Hand-raised chicks from egg

• Exposed to normal night sky

• At correct time, birds exhibited zugenruhe

• Oriented the “correct” direction for the right amount of time, then changed direction as if following migratory route

Evidence for genetic control of migratory behavior in birds:

breeding experiments

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Evidence for learned migratory behavior in birds:

loss of migration

• Endangered whooping cranes have lost cultural memory and have to be taught to migrate by humans

How fast does migration evolve?: FAST

phylogenetic evidence

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ecological evidence: Eastern House Finches

• Native to Western North America

• Introduced to Long Island 1940

• Source population 80% sedentary

ecological evidence:

European Blackcaps

• Black-caps have established novel non-breeding grounds and new migratory pathways in response to increased use of bird feeders in England since 1940’s

Both ecological and evolutionary evidence points to turning on and off of genetic program

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How does migratory behavior originate? population dynamics

• Original population of resident individuals

residents

How does migratory behavior originate? population dynamics

• Original population of resident individuals

• Some individs move to adjacent habitat on boundary of their range to exploit some seasonally abundance resource

residents

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How does migratory behavior originate? population dynamics

• Original population of resident individuals

• Some individs move to adjacent habitat on boundary of their range to exploit some seasonally abundance resource

• They return to previous areas when resources run out�partial migration of single species

residents

How does migratory behavior originate? population dynamics

• Original population of resident individuals

• Some individs move to adjacent habitat on boundary of their range to exploit some seasonally abundance resource

• They return to previous areas when resources run out�partial migration of single species

• Competition with residents eventually drives the 2 strategies apart, spatially & genetically�2 distinct populations +/or eventually species

residents

migrants

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How does migratory behavior originate? population dynamics

• Original population of resident individuals

• Some individs move to adjacent habitat on boundary of their range to exploit some seasonally abundance resource

• They return to previous areas when resources run out�partial migration of single species

• Competition with residents eventually drives the 2 strategies apart, spatially & genetically�2 distinct populations +/or eventually species

• Ranges of 2 groups evolve independently

residents

migrants

winter summer

How does migratory behavior originate?

Pre-adaptations?

Living in a very seasonal environment or relying on very seasonal resources?

Year-round territoriality not hugely advantageous?

Genetic predisposition?

Having somewhere to go?

…others….

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Proximate causes (cues) of migration

Exogenous:

• Daylength/photoperiod

• Temperature

• Direct cues: e.g. changes in resource abundance

Endogenous:

• Circannual rhythms

• Hormones

• Physiological status

“why go south?” OR “why go north?”

• Reflects historical biases and ideas about ancestral range of migratory organisms

• Should attempt to explain both directions of movement

• Ultimate and proximate causes probably usually DIFFERENT for the two directions of migratory movements

• Goal: explain differences among individuals and species in terms of costs and benefits of different strategies

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Ultimate causes of migration

Seasonality of resource or seasonality of need

Food-based hypotheses:Why leave the breeding area?

• High seasonality of habitat means that there are times when absolute food abundance is too low to survive

• Inter- or intra-specific competition drives resources to low levels. Migrants are the ‘loosers’.

Why leave the non-breeding area?• Seasonally high resource abundance means that birds that migrate to exploit peak can produce more offspring

• Year-round ‘stability’ of tropical habitats means that migrants can take advantage of year-round food availability during non-breeding season

Food and the costs of migration:• Enormous energetic costs of migration mean migrants need much higher food intake rates

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Weather and physiological toleranceWhy leave the breeding area?

• Extreme cold or severe storms exceed the absolute range of conditions individuals can survive in

Why leave the non-breeding area?• Long daylength means that parents can forage for longer, feed more chicks, achieve higher growth rates and fledge bigger, healthier and more young.

• Moderate climates in non-breeding areas increase survival of adults

Weather and the costs of migration:• Bad storms can cause delays in migration or blow individuals off course

• May have to migrate over areas of climatic unpredictability without knowledge of area that residents have

Predation risk

Why leave the breeding area?

Why leave the non-breeding area?

• High nest predation risk in tropical areas means that higher per-nest chance of success in temperate areas than tropical areas

Predation and the costs of migration:

• Higher risk of adult predation when aggregated during migratory flights and in unknown areas

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Others: Bugs (cariboo)

Snow depth (moose)

Water levels & water salinity

Parasites?

Pathogens?

How do you test of hypotheses explaining evolution or migratory behavior?

Some problems:

– Animals (by definition) don’t stay put!

– Following them around is hard

– Very hard to invent effective manipulations

– Behaviors repeat on annual time scales

FEW EMPIRICAL TESTS OF ANY HYPOTHESES REGARDING ULTIMATE CAUSES OF MIGRATION

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How do you study the causes of migration?

Comparative method

Correlative predictions

Case study:

Why do some (but not all) birds migrate altitudinally in the tropics?

Breeding

Non-breeding

Patterns of altitudinal migration

Cloud forest

Premontane forest

Lowland forest

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Why migrate?

Food

Predation

Weather

Migrants more frugivorous than their non-migrant counterparts

…but not more specialized on certain fruit types

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Food

Does the timing and location of white-ruffed manakin migration make sense if they are moving up and down hill to track food resources?

Manakins not migrating because of protein needs and resources at breeding areas

Fruit could still explain their migration

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Why migrate?

Food

Predation

Weather

Most nest predation at the elevation where white-ruffed manakins breed