How Do Birds Fuel a Migration? Eating Until the Body Rebuilds Itself

A migrating bird faces an energy budget that admits no error. A non-stop crossing of a sea or desert may take several days of continuous flapping with no opportunity to feed or drink, and the fuel for it must be carried, which means the bird has to be heavy enough at departure and not so heavy that it cannot fly efficiently. Fat is the fuel of choice because it stores roughly twice the energy per unit mass of protein or carbohydrate and carries no associated water, which is decisive when every gram must be lifted. Small songbirds routinely depart at close to double their lean weight, and some shorebirds making the longest recorded non-stop flights depart carrying more than half their mass as fuel. The margin on arrival is frequently very small.

The preparation involves more than eating, and several changes are reversible and repeated each season:

• •Hyperphagia, a period of greatly increased feeding driven by seasonal physiological change
• •Rapid fat deposition in specific locations, visible as a bulge that observers can score by eye
• •Enlargement of the digestive organs during the feeding phase to process the intake
• •Shrinking of those same organs before departure, since they are dead weight in flight
• •Enlargement of the flight muscles and the heart
• •Changes in blood chemistry and in the muscles' capacity to burn fat directly

Most migrations are made in stages, and the places where birds pause to refuel are as critical as the breeding and wintering grounds. A stopover has to supply enough food to rebuild reserves within days, which means abundance rather than mere presence, and the birds arriving are frequently in poor condition and vulnerable. Certain sites concentrate enormous numbers because the geography funnels the route, including coastal wetlands at the ends of sea crossings, oases beyond deserts and estuaries with rich invertebrate mudflats. The dependence makes those sites disproportionately important for conservation, since losing one can affect a whole population regardless of how well the other ends of the route are protected, and several documented population declines have been traced to the loss or degradation of a single staging area. The timing matters as much as the place, since the food supply must peak when the birds arrive.

A multi-day flight raises a question the fuel budget does not answer, which is what happens to sleep. Several lines of evidence suggest that migrating birds sleep very little during the crossing and recover afterwards, and recordings from birds in flight have shown brief episodes of one hemisphere sleeping while the other stays awake, an arrangement also used by some birds on water and by marine mammals. Species that normally migrate show reduced sleep during the migratory season even when held in captivity and unable to fly, without the impairment that equivalent deprivation causes in other conditions, which suggests a genuine seasonal change in what the brain requires rather than simply enduring a deficit. How complete the recovery is and what the costs are remain open, and the phenomenon is studied partly for what it might reveal about why sleep is needed at all.

The evidence comes from methods that have improved dramatically. Ringing birds and recovering them elsewhere established the routes over a century and still provides the backbone of the data, though recoveries are rare. Weighing birds caught at stopovers established the rate of refuelling directly. Radar reveals departures and the altitudes and densities of nocturnal movements that nobody can see. Miniaturised tracking devices now follow individual small birds across whole journeys, which produced the documentation of non-stop flights previously thought impossible and revised several accepted routes. Stable isotope analysis of feathers indicates where a bird grew them, linking populations across the annual cycle. And the physiological changes have been measured directly by imaging the organs of live birds, which established the organ shrinkage that earlier work had only inferred.

Fat is the fuel because it stores about twice the energy per gram of the alternatives and carries no water, and small birds depart at close to double their lean weight. Digestive organs enlarge during feeding and shrink before departure, since they are dead weight aloft. Stopover sites must supply enough food to rebuild reserves in days, which makes a few locations critical to whole populations.