Migration / Navigation

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
1. Seasonal
2. Sporadic
3. Dispersal

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 sun.

Examples (non-continental movements)
Pennant-winged 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 America.

Unique migrators
Bronzed 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.
Blackpoll 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’ called Zugunruhe.
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.

  1. Turns out that the young that were raised with the non-migratory birds, actually did migrate.
  2. 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 Britan.
• 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 the south-west.
• 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.

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 breeding grounds.

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 to fly.
  • 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 a day.
• The Homing Pigeon can fly 1080 km a day.
• 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).

Stop-Over Ecology
• Birds have evolved to fly to stop-over points in the middle of their migrations.
• People have studied just how far you can go depending on the amount of fat stored.

Arctic Tern:
• Flies 36,000 Kilometers a year!

Politics of Migration
• Stopover ecology. The quality of stopovers sites may be conservation hotspots.
• 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 ability hypothesis
• 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

  1. European Starling adults knew where they had to fly while the juveniles innately flew back to where they were supposed to.
  2. 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).
  3. Altering the magnetic Field: Birds use the vertical component of the magnetic field in order to orient themselves to the magnetic field.
  4. 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.
  5. SUN Orientation: On a sunny day, most birds can orient correctly. But on an overcast day the birds in a cage have no preferred direction.
  6. 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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