Nature

What Migration Actually Costs a Bird

Nature Editorial TeamStaff Writer · 7 min read · Oct 27, 2023

In This Article

Long-distance migration pushes birds to physiological extremes. Learn about the energy demands, risks, and remarkable adaptations that make it possible.

Key Takeaways

Birds can burn up to 0.5% of their body mass per hour during sustained migratory flight.
Many species shrink and then regrow digestive organs to reduce unnecessary weight mid-journey.
Fat is the primary fuel for migration — some birds double their body weight before departure.
Predation risk increases sharply at stopover sites, where birds are temporarily grounded and vulnerable.
Environmental threats like habitat loss at stopover sites can collapse an entire migratory route.

Fueling a Journey of Thousands of Miles

Migration is one of the most energetically demanding activities in the animal kingdom. To understand what it truly costs a bird, start with fuel. Before setting off, birds enter a state called hyperphagia — a period of intense, compulsive feeding that can last days or weeks. During this phase, fat deposits accumulate rapidly beneath the skin and around organs, sometimes doubling a bird's normal body weight.

Fat is not just convenient; it is remarkably efficient. Gram for gram, fat stores more than twice the energy of carbohydrates or protein, making it the ideal aviation fuel for a creature where every extra gram adds drag. A Ruby-throated Hummingbird weighing roughly 3 grams at departure may cross the entire Gulf of Mexico — approximately 950 kilometers — on a single fuel load, arriving measurably lighter on the other side.

~11,000 km

Non-stop flight distance of Bar-tailed Godwit

Satellite tracking studies have recorded Bar-tailed Godwits flying non-stop from Alaska to New Zealand — the longest known non-stop bird migration.

Up to 50%

Body mass lost on a single long crossing

Research on trans-Gulf and trans-Saharan migrants has documented individual birds losing up to half their departure body weight on a single non-stop leg.

0.3–0.5%

Body mass burned per hour of flight

Studies using doubly labeled water and flight range equations estimate small passerines burn roughly 0.3–0.5% of their body mass per hour during sustained migratory flight.

But fat alone does not explain the full picture. Toward the end of extreme non-stop flights, birds begin catabolizing muscle protein to sustain output. The pectoral muscles — the powerhouse of flapping flight — may actually shrink slightly during the journey, while the heart and remaining flight muscles are preserved as long as possible.

The Radical Biology of Preparation

Perhaps the most striking adaptation birds make before long migrations is organ remodeling. Studies of shorebirds such as the Bar-tailed Godwit have documented the near-complete shrinkage of the stomach, intestines, liver, and kidneys prior to departure. These organs are metabolically expensive to maintain, and jettisoning their mass frees up capacity for additional fat and muscle.

Upon arrival at a stopover site, digestion must resume rapidly — and the digestive system regrows to near-normal size within days. This capacity for reversible organ atrophy and regeneration, called phenotypic flexibility, is a hallmark of long-distance migratory birds and has no close parallel in most other vertebrate groups.

How Birdwatchers Can Help Migrants

Providing native plantings and water sources in yards and gardens offers meaningful refueling resources for migrants passing through. Keeping domestic cats indoors during peak spring and autumn migration periods — typically April–May and August–October in North America — can also reduce predation on birds that arrive exhausted and temporarily ground-bound.

Compared to the strategy of remaining dormant through winter, active migration carries a completely different set of physiological trade-offs. Migration versus hibernation represents two fundamentally distinct biological solutions to the same seasonal challenge, each with distinct costs and benefits shaped by millions of years of evolution.

Risks That Cannot Be Stored Away

Energy expenditure is only one dimension of migratory cost. Birds in transit face compounding risks that cannot be offset by preparation alone. Predation is significantly elevated at stopover sites, where birds congregating near food sources also attract predators — raptors, domestic cats, and snakes among them. A bird arriving depleted after a long crossing has reduced maneuverability and slower reaction times, making it especially vulnerable.

Weather poses its own threats. Storms can force birds off course, strand them over open water, or deposit them far outside their normal range. Cold snaps mid-migration can eliminate insect food sources that arriving birds depend on for rapid refueling.

Habitat degradation has emerged as one of the most consequential modern threats. When stopover sites are developed, drained, or invaded by non-native plant species that support fewer insects, an entire leg of a migratory route can effectively collapse. Research tracking marked individuals has shown that poor refueling conditions at a single stopover can cascade into delayed arrivals on breeding grounds, reduced reproductive success, and elevated mortality — a chain of consequences that follows from a single disrupted rest stop.

Understanding what migration truly costs a bird reframes how we think about the landscapes birds pass through. The breeding ground and the wintering ground are obvious focal points for conservation, but the invisible thread of stopover sites connecting them is equally critical — and often more fragile.

Frequently Asked Questions

How much weight does a bird lose during migration?

Weight loss varies by species and route length, but some small songbirds lose 25–50% of their pre-departure body mass during a single long crossing. Birds that cross the Sahara Desert or the Gulf of Mexico non-stop can arrive critically lean. Refueling at stopover sites is essential for survival and continued travel.

What do birds use as fuel during long flights?

Fat is the primary fuel source — it yields more than twice the energy per gram compared to protein or carbohydrates. Before departure, birds enter a phase called hyperphagia, dramatically increasing food intake to build fat reserves. Some protein from muscle tissue is also metabolized, particularly toward the end of very long flights.

Why is stopover habitat so important for migratory birds?

Stopover sites are locations where birds rest and refuel between flight legs. Without adequate food and safe resting conditions at these sites, birds cannot complete their journeys. Loss or degradation of stopover habitat — through development, agriculture, or invasive plants — can interrupt migration routes and reduce survival rates across entire populations.

Do birds sleep during long migratory flights?

Evidence from EEG studies suggests some species can engage in unihemispheric slow-wave sleep — resting one half of the brain at a time — while remaining airborne. Certain shorebirds and swifts appear capable of brief sleep episodes in flight, though the full extent of in-flight sleep is still an active area of research.

How do birds navigate across thousands of miles?

Birds use a combination of the sun's position, star patterns, Earth's magnetic field, and learned landmarks. Magnetic sensing appears to involve specialized proteins in the eye that allow birds to perceive field orientation visually. Young birds often rely more heavily on innate compass direction, while experienced adults integrate multiple cues.

Nature Editorial Team

Staff Writer

Nature Editorial Team is the collective byline for our editorial team and contributor network. Articles published under this byline or an editorial pen name are researched, written, and reviewed according to our editorial standards for clarity, consistency, and independence before publication.