School › Animal migration › Ages 15–16
Fuel, speed and navigation on the move
The physiology of migration, record journeys analysed with speed and energy calculations, the classic experiments on sun, star and magnetic compasses, and how tracking devices produce the evidence.
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Migration is triggered mainly by changing day length, along with temperature and food; hormone changes make caged birds restless and face the direction they would migrate (Zugunruhe). Most birds migrate at night.[1]
Fat is the most energy-dense fuel: it gives more than twice the energy of carbohydrate or protein per gram. Small songbirds can be up to 50% fat before their first big flight, and a godwit's mass can more than double.[3][7]
In 2007 a bar-tailed godwit called E7 flew about 7,250 miles (about 11,700 km) nonstop from Alaska to New Zealand in about 8 days, averaging nearly 35 miles per hour (about 56 km/h) without food or water. A 2005 study had already argued for an 11,000 km nonstop flight from re-sightings of marked birds, fat loads and flight models.[7][8]
Classic experiments tested the compasses. Kramer (1951) moved the apparent sun with mirrors and starlings changed direction; the sun compass needs an internal clock. Emlen (1967) found that young indigo buntings learn north from the rotation of the stars. Robins changed direction when the magnetic field was turned; birds seem to sense the field's axis and dip angle, not its polarity.[2]
Tracking devices give the evidence for individual routes, but they must be light: a rule of thumb is no more than 3% of the animal's body mass. E7's satellite transmitter weighed about 25 g. Before such devices, much was learned from large-scale ringing schemes.[7][3]
Mammals and insects migrate too: humpback whales can swim from Alaska to Hawaiʻi, about 3,000 miles, in as few as 28 days, and monarch butterflies need 3 to 4 generations to return north after one generation flies up to 3,000 miles south.[5][4]
Try it: Mapping a magnetic field: what a magnetic compass can and cannot tell
You need
- Bar magnet (poles marked)
- Plotting compasses (at least 4)
- A3 paper and pencil
- A magnetic compass for the room
- Protractor
- Optional: a magnetometer app on a phone or a dip needle
Safety
Keep magnets away from phones, bank cards and computers. Do not let strong magnets snap together near fingers. Handle a dip needle gently.
What to do
- Put the bar magnet in the middle of the paper and draw round it. Keep other magnets and steel objects away.
- Place a plotting compass near the north pole and mark the point its needle points to. Move the compass to that point and repeat, building up a field line from pole to pole. Draw at least six field lines.
- Mark the direction of the field with arrows. Then turn the magnet round (swap the poles) and check how the arrows change.
- Discuss: if an animal could sense only the line (axis) of the field and how steeply it dips, but not which end is north, what could it still work out? (Hint: compare the steepness of field lines near the poles of the magnet and at its middle.)
- Optional: use a dip needle or a magnetometer app to measure the angle of the Earth's field to the horizontal in your classroom.
- Evaluate: list sources of error (nearby steel, the compass's own size, reading the needle angle).
What happens, and why?
A compass needle lines up with the magnetic field. Experiments with robins suggest that birds sense the axis of the field and its dip angle rather than its polarity, so turning the magnet round would not change what such an animal senses about the axis. Birds may also estimate their latitude from the strength of the field.[2]
Play: What kind of evidence?
Sort each piece of evidence by the method that produced it.
Your best score is kept only on this device. Nothing is sent anywhere.
Quiz
Other ages: Ages 6–7 · Ages 10–11