Homing and migration: long-distance orientation
Homing and migration compared
- Homing is the ability to return to a specific location — a nest, burrow or breeding site — from unfamiliar ground.
- Migration is a regular, directed movement of a population between two areas, usually seasonal and usually two-way.
| Homing | Migration | |
|---|---|---|
| Destination | One specific site | A region or habitat |
| Timing | Whenever displaced | Regular, usually seasonal |
| Who moves | An individual | Most of the population |
| Trigger | Displacement from the site | Usually an internal rhythm cued by day length |
- Both are orientation in space, and both require the animal to know which way to go over distances far beyond direct sensory range.
How animals navigate
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Long-distance orientation needs two distinct pieces of information, and confusing them is a common error:
- A compass tells the animal which direction is which.
- A map tells the animal where it currently is relative to its goal.
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A compass alone is not enough. An animal displaced sideways and given only a compass would travel in the right direction but arrive in the wrong place.
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Compass mechanisms:
- Sun compass — the sun's bearing, corrected for time of day using an internal clock. Requires daylight and a clear sky.
- Star compass — the rotational centre of the night sky, which stays fixed while stars wheel around it. Used by night-migrating birds.
- Magnetic compass — the Earth's magnetic field, detected by magnetoreceptors. Works in cloud, fog and darkness, so it is the reliable fallback.
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Map mechanisms:
- Magnetic map — field intensity and inclination vary predictably with latitude, so an animal can infer position.
- Olfactory map — learned chemical signatures. Salmon and eels identify their natal stream by its dissolved chemistry.
- Landmark memory — visual features learned on an outward journey, used near the destination.
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Most well-studied migrants use several systems together, with a hierarchy: a primary system used when available, and backups when it is not. This redundancy is itself the adaptation, since any single cue can fail.
Migration in New Zealand species
- Kuaka (bar-tailed godwit).
- Flies non-stop from Alaska to New Zealand each spring — the longest known non-stop flight of any bird, over roughly 11,000 km in about 8–9 days.
- Before departure it doubles its body mass in fat and shrinks its gut and liver, since digestive tissue is dead weight on a flight with no feeding.
- It waits for favourable following winds, which cuts the energy cost of the crossing substantially.
- Tuna (longfin eel).
- Spends decades in NZ fresh water, then migrates to spawning grounds in the tropical Pacific, breeds once, and dies.
- The larvae drift back on ocean currents, taking months to years, then use an olfactory map to locate fresh water.
- Tītī (sooty shearwater).
- Makes a figure-of-eight circuit of the Pacific each year, exploiting seasonally productive feeding areas in both hemispheres.
Selective advantage
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Migration is enormously costly — energy, predation risk, and mortality en route — so the benefit must be very large.
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Exploiting seasonal resource peaks.
- The Arctic summer offers near-continuous daylight and a brief explosion of insect abundance. Breeding there gives chicks far more feeding hours per day than a temperate site.
- Faster chick growth means more chicks fledged per breeding attempt.
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Escaping seasonal scarcity.
- The same site is uninhabitable in winter. Migrating to New Zealand means access to food year-round, rather than surviving one lean season.
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Reduced predation and parasite load.
- High-latitude breeding sites carry fewer predators and parasites than tropical or temperate ones, so more eggs and chicks survive.
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Homing to a proven site.
- Returning to a site where the animal has already bred successfully is safer than assessing a new one, because the site is known to have adequate food and shelter.
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The cost–benefit balance is what Excellence answers turn on. Migration persists only where the extra offspring produced at the destination outweigh the mortality suffered en route. Where that balance shifts — through habitat loss at either end, or a mismatch between arrival timing and the food peak — migration can become disadvantageous, and populations decline.
Worked Example
Worked Example
Young kuaka are tracked on their first southward migration from Alaska.
- Birds that have never made the journey arrive in New Zealand successfully.
- They depart after the adults, and travel without adult company.
- Birds tested in an orientation cage under total cloud cover still orient south.
- When a magnetic coil reverses the field around the cage, the birds reorient north.
Explain what these results show about how the birds navigate, and why the response provides a selective advantage.
Answer:
What the results show.
Each result rules something out, so we take them in turn.
- Juveniles succeed without ever having made the journey. The route cannot be learned from experience, so the information must be innate — genetically determined and inherited.
- They travel without adults. The route cannot be learned by following experienced birds either. This rules out cultural transmission and confirms an inherited programme.
- They orient correctly under total cloud. Sun and star compasses require a visible sky, so neither can be the system operating here. A non-celestial cue must be available.
- Reversing the magnetic field reverses their heading. This is the decisive result: the birds' chosen direction follows the field, so they are using a magnetic compass.
Together these show the birds possess an innate magnetic compass and an inherited directional preference — a heading to fly and, presumably, a duration to fly it. Note what has not been shown: the experiment tests a compass, not a map. It shows the birds can determine direction, not that they know where they are.
Why the response provides a selective advantage.
The migration itself is advantageous because it lets kuaka use two seasonal resource peaks instead of one.
- Breeding in the Arctic summer provides near-continuous daylight and a brief peak of insect abundance, giving far more chick-feeding hours per day than a temperate site. Chicks grow faster, so more fledge per attempt.
- Wintering in New Zealand provides productive intertidal feeding while the Arctic is frozen and uninhabitable, so adults survive to breed again the following year.
The magnetic compass specifically is advantageous because of reliability. The route crosses roughly 11,000 km of open ocean in 8–9 days with no opportunity to land, and the birds fly through cloud, at night, and across regions with no landmarks. A celestial compass alone would fail whenever the sky was obscured, and even a few hours off course over that distance would mean missing New Zealand entirely — with no land within range, that is certain death. A magnetic cue is available continuously, in any weather, day or night.
That the system is innate is itself the adaptation. A juvenile gets exactly one attempt with no opportunity to learn from failure, so a route that had to be learned could not be inherited and could not spread. Because the heading is genetically determined, birds carrying alleles for an accurate heading survive to breed and pass those alleles on, while birds with inaccurate headings die at sea and remove theirs from the population — selection acting directly and lethally on the navigation programme every single generation.