How Does Animal Migration Work? Navigating Without a Map
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
A young bird that has never left its birthplace flies alone to a wintering ground thousands of kilometres away, arrives in roughly the right region, and returns the following spring. It does this without being shown the route, which means the information must be carried internally, and unpicking what that information is has taken a century of experiments that remain among the most inventive in biology.
Why animals move at all
Migration is expensive and risky, so it persists only where staying put is worse. The usual driver is seasonality in food: high latitudes offer an enormous flush of insects and plant growth in summer and almost nothing in winter, so a bird that breeds in the north and winters in the tropics exploits the best of both at the cost of the journey. Other drivers include breeding requirements, since many fish and amphibians need specific water conditions to reproduce, and avoiding predators or parasites concentrated in one place. The forms vary: latitudinal migration between north and south; altitudinal migration up and down a mountain, which achieves a similar change in climate over a few kilometres; and nomadic movement tracking irregular resources such as rainfall in arid regions. The scale can be extraordinary, with the Arctic tern travelling between polar summers and covering the greatest annual distance of any animal, and some migrations occupy several generations, as in the monarch butterfly, where no individual completes the round trip.
The compasses
Animals use several independent direction-finding systems and switch between them as conditions allow:
- •A sun compass, which requires an internal clock to correct for the sun's movement across the day, demonstrated by shifting an animal's day-night cycle and watching its chosen direction rotate by a predictable amount
- •A star compass, learned in young birds from the rotation of the night sky around the celestial pole, shown by raising birds in a planetarium and rotating the artificial sky, after which they orient to the false pole
- •A magnetic compass, which in birds detects the angle of the field lines relative to the ground rather than polarity, so reversing the field's direction does not reverse the bird's heading while changing its inclination does
- •Polarised light patterns at sunrise and sunset, which indicate direction even under partial cloud and appear to be used for calibrating the other systems
- •Learned landmarks, coastlines and rivers, which matter most near the start and end of a journey
- •Smell, established in homing pigeons and increasingly implicated in seabirds that locate distant feeding grounds by odour plumes over the ocean
Knowing where you are
A compass gives direction and not position, and the harder question is how a displaced animal knows which way to correct. Classic experiments displaced migrating starlings sideways across Europe and found that adults corrected course towards the correct destination while first-year birds continued on their original heading, which indicates that inexperienced birds run a simple programme, a direction and a duration, while experienced ones have acquired a map. What the map is made of remains partly open, with evidence pointing to gradients in the magnetic field, since both its intensity and its inclination vary predictably with latitude and can in principle give coordinates, alongside olfactory landscapes and familiar landmarks. The magnetic sense itself has two leading candidate mechanisms: magnetite particles that align with the field and could be sensed mechanically, and a light-dependent chemical reaction in cryptochrome proteins in the eye whose outcome depends on the field's orientation, which would mean birds see the field. Neither is fully settled, which is unusual for a sense that has been studied this long.
The physical preparation and the threats
Migration requires an internal reorganisation that begins weeks before departure. Birds enter hyperphagia, feeding heavily and doubling body mass in fat, which is the densest fuel available; several species also shrink their digestive organs before departure and rebuild them on arrival, since carrying an unused gut costs energy. Restlessness at night appears in caged migratory birds at the right season and in the right direction, a phenomenon called Zugunruhe that made much of the compass research possible. Timing is controlled by an internal annual rhythm entrained by day length, which is reliable and unresponsive to weather. That last point is why climate change causes trouble: day length is unchanged while the timing of spring insect peaks has advanced, so long-distance migrants increasingly arrive after the food peak they evolved to exploit, a mismatch documented with measurable declines. Other pressures include habitat loss at stopover sites, which are disproportionately important because a single refuelling location may serve a whole population, collisions with buildings and wind turbines, artificial light that disorients night migrants, and hunting along traditional routes.
The takeaway
Migration pays when the cost of the journey is less than the cost of staying, usually because food is seasonal. Animals navigate with several compasses at once: the sun corrected by an internal clock, a star pattern learned from the sky's rotation, a magnetic sense reading the angle of field lines, polarised light, landmarks and smell. Young birds appear to run a fixed direction and duration while experienced adults hold a map and can correct for displacement. Day length still triggers departure while spring food peaks have moved earlier.