Migration, as a topic, cuts across a lot of different disciplines. Birds
pose a great group to study this phenomenon.
We can look at movements of any animal. By migration,
we deal with seasonal movements. In other words, a bird is in different
places at different times of the year. This is in contrast to sporadic
movement. This kind of movement can be associated with the tracking
of food sources. This is mostly on a local scale. If the birds, however,
move to the same places every year to track food, this would be considered
migration. Hummingbirds do this in Costa Rica where they move from high
to low elevations. Another type of movement is considered dispersal.
For example, breeding dispersal is when the young move away from the nest
to utilize other areas. This is often called natal dispesal. BUT, when
we talk about migration, we are only talking about seasonal dispersal.
Types of Movements
What drives Migration?
• On a basic level, the spinning of the earth drives migration.
The earth is on a tilt so that the winter and summer have different amounts
of light. Thus, at different times of the year there are differential
amounts of productivity, depending on the time of the year.
• Birds will usually breed in areas that are high in productivity,
and then leave that area in the off-season.
• Sunlight is a function of the earth's tilt and location from the
Examples (non-continental movements)
Nightjars breed in southern Africa during the rainy season,
and then migrate across the Congo to its winter quarters in the Sudan
in time to enjoy the insect-rich rainy season there. The Nacunda
Nightjar in South America makes a corresponding migration between
Venezuala and Argentina.
Examples (large migrations)
• Golden Plover – North to south
Cuckoo – is from New Zealand that is a brood parasite
that never sees its parents. It flies from N.Z. 1900 Km to Australia and
then 1600 km to the Soloman Islands.
Warblers – from New England to Venezuala non-stop.
Spectacular Homing feats
• Unusual sensory systems – birds
use the earth’s magnetic field a lot. They also use the sun in a
way that allows them to track the sun’s movement.
• Ingenious experiments: We have discovered
some of the unique experiments where people have put birds on turn-tables
and moved them around.
Proximate and Ultimate Factors
- Proximate: physiological or mechanistic in nature.
HOW does it happen?
- Ultimate: factors shaped by nature's selective pressures:
Why does it happen?
- The problem with migration: If birds leave to avoid poor food resources,
how can natural selection (which does not for-see the future) shape migratory
behavior such that birds leave before food becomes scarce?).
Photoperiod (is invariant)
If birds focus on photoperiod early enough, they will be predicted
to prevail in the population. Thus they are being shaped by selection.
Origins of Migration:
To look at the origin of migration we are going to have to look at the
costs and advantages. It is of course, very energetically difficult to
migrate. Thus, there must be some sort of pay-off for moving to another
area for part of the year.
Another view of migration is that it's simply taking advantage of breeding
opportunities. Lots of tropical ornithologists state that many birds are
tropical birds that just leave the tropics to breed. In fact, many studies
have shown that these birds have territories in both the tropics and in
their temperate regions.
If you look at migration, not all ages and sexes will actually migrate
at the same time every year.
• The gain of arriving early is much greater for males than females.
It’s also much more of a gain for young males than old males.
Changes in migration of birds:
1. Serin Finches: mediteranean birds that
are non-migratory. Yet a population was established in northern Europe
that became migratory.
2. Fieldfares – Found in Greenland
where they are non-migratory versus in Europe where they are migratory.
3. Polymorphism in Robins: The Molting of
migrants of the European Robin occur earlier than resident birds. The
migrant birds also put on a lot of body mass at certain times of the year.
The migrants also show a type of ‘migration restlessness’
4. Gulls innate nature: Gull Cross-fostering
experiment. For birds, experiments have been done whereby the eggs of
one bird can be switched with that of others. For example, Herring gulls
are non-migratory, while Lesser Black-Backed gulls are migratory. People
have switched these eggs around and looked for a change.
- Turns out that the young that were raised with the non-migratory
birds, actually did migrate.
- The opposite, however, did move with the migratory parents because
they thought they were just following the parents.
Black-caps and Micro-Evolution of New Routes:
• People noticed that Black-caps started spending the winters in
• Before the 1950’s a few would pass through Britan from Germany
on their way to Spain.
• Banding birds has shown that these birds are coming from Germany.
Thus, they were basically flying west to get to England.
• It’s amazing that this actually happens, and it wasn’t
noticed until the 1950’s.
• So, researchers bred in captivity, males and females and got the
F1 offspring. These offspring were then put into a cage to see which way
they orient during the Zugenruhe. They showed that there is a
heritable basis for orientation in blackcaps.
• In south-west Germany, the Blackcaps would, however, orient to
• Thus, the point is that you can see micro-evolutionary change
in a short period of time within 50 years.
Arrival times of different birds are almost entirely accurate. They are
accurate enough that you can almost set your calendar to them leaving.
The Canada Goose will move to their breeding
grounds with the movement of warmer weather, northward.
Predators can set their clock as to when to breed knowing when the food
will be most abundant for the offspring.
INTERESTING PARTS: Fox-Sparrow
Populations of different regions may overwinter in different spots.
a. the Fox-Sparrow shows this well.
a. In the Seattle area, there is no migration.
b. In the Pacific Northwest, migrates past the Seatle Population .
c. The ones up in Alaska, migrate down to Lower California.
The Golden Plover shows that the route from the breeding to the winter
quarters are not always the same as the route from the winter to the summer
Ancestral migration routes
• Pectoral Sand-Pipers: Many birds
follow ancestral routes to winter quarters. The Pectoral sand-pipers for
instance, has extended its range from Northern N. America have extended
its range to Northern Siberia. But, instead of flying strait south they
follow the ancestral breeding route, back into North America and then
south to S. America.
• Arctic Warbler: Finland and Norway
across Siberia to Alaska
o These birds generally migrate down to SE Asia.
o The birds are originally from Sibera. So the birds from Finland fly
all the way to Siberia and then to SE Asia.
o They do this instead of flying south to the area that would be very
close to their range.
Fourty-eight-hour records of the rate of activity of a first year male
White-crowned sparrow, show that in April, the bird is inactive at night,
but that in May there is much greater activity at night (this is when
the bird migrates).
- The amount of Zugenruhe increases with the distance that they have
- Species that have different populations (migratory and non-migratory).
The hybrids have intermediate amounts of Zugenruhe.
- People have used radar to track bird migrations.
- Black Gulls, fly way out over the Ocean to Bermuda before they fly
south to South America. It's believed they are tracking the winds.
How far to birds fly?
• Laysan Albatros can fly 510 kilometers
• The Homing Pigeon can fly 1080 km
• The Ruddy Turnstone can fly 1045
km a day.
Building fat for migrations
• Birds will get fat before they migrate.
• As a migratory species they can have 5 different levels of fat.
• Ruby throated hummingbird puts on 2/3’s of its body weight
to fly across the Atlantic.
• Fat is used as a fuel for migration because it has a lot of calories
per gram of weight. The metabolism of fat also uses a lot more water than
the other forms of fuel (carbohydrates or protein).
• Birds have evolved to fly to stop-over points in the middle of
• People have studied just how far you can go depending on the amount
of fat stored.
• Flies 36,000 Kilometers a year!
Politics of Migration
• Stopover ecology. The quality of stopovers sites may be conservation
• Migratory birds do not recognize national boundaries. Conservation
applies both to breeding and non-breeding sites.
Orientation in Birds
Problems in Animal Movement: There are many challenges that birds have
in navigation. A few of the main problems are listed here.
• The bird must know where it is
• It must know the direction of its goal
• It must be able to maintain a course in that direction
• It must know when to stop.
• Broad evidence in birds: philopatry in migration, homing experiments.
Homing Ability Hypothesis: There are three sets of homing
• No external cues are used: idea is that its just random searching
cues. Another idea is that there is some sort of inertial guidance (ie.
They remember every twist and turn).This hypothesis also does not account
for the first flight of young birds.
Single environmental Cue:
o Visual Orientation : Birds can recognize certain landmarks,
etc. Birds like seabirds seem to be lost until they find the ocean and
then make a beeline for it.
o Magnetic – During solar flares, when the earth’s
magnetic field is screwed up, causes confusion in birds.
o Celestial (sun, stars) – Its been shown that
birds have used stars to navigate their path. This also means that they
must have some sort of internal clock
o True Navigation
Expermients used to determing homing ability
- European Starling adults knew where they had to fly while the juveniles
innately flew back to where they were supposed to.
- INK-Pad / Blotter wheel: The classic way migratory
birds have been studied is to look at bird migratory orientation. Shows
that the birds are oriented to different directions in the winter and
summer (N and S).
- Altering the magnetic Field: Birds use the vertical
component of the magnetic field in order to orient themselves to the
- Magnets on a bird: Turns out that on an overcast
day, the birds that have the brass on their necks could orient ok, but
birds that have a magnet on their neck, they can not orient correctly.
Another way that orientations can be messed up is to wrap coils of wire
around the birds head. These electric coils (Helmholtz coils) will cause
a magnetic field to occur and thus it shifts the birds orientation.
If the magnetic field is reversed, they fly in the exact opposite direction.
- SUN Orientation: On a sunny day, most birds can orient
correctly. But on an overcast day the birds in a cage have no preferred
- If birds are using a magnetic compass they must be able to reset or
calibrate their magnetic compass every few days.
Orentation or Navigation: We’ve learned that birds
will orient themselves to where the sun is. However, how do you determine
if the sun is used for navigation? This can be done with some sort of
clock shifting. If you clock-shift them by 6 hours, this is basically
7,000 miles to the west. So, when you release birds by clock-shifting,
the Navigation theory says that they should realize this and fly west.
- Under the Sun compass hypothesis, you’d imagine that non clock-shifted
birds are released they should all fly to their goal.
- If you release clock-shifted birds, using the sun-compass hypothesis,
then you predict that birds fly in a predicted wrong direction. This
is exactly what happened.
Night Orientation: Using the Zugenruhe, birds with blotting
paper can show predicted paths of orientation while manipulating it with
a planetaria. By flipping the planetaria, they can flip the birds orientation.
The birds, however, are not focusing on a particular star, but are focusing
on the rotation of the night sky.
By Rob Nelson
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