Biology
Energetics of Long-Distance Migration in Bar-tailed Godwits
Quick fact
Bar-tailed godwits perform the longest nonstop flight of any land bird, covering over 11,000 km (nearly 7,000 miles) from Alaska to New Zealand in roughly 8–9 days, fueled entirely by fat reserves that can nearly double their body weight.
Why this is interesting
Every year, a bird weighing less than a pound flies nonstop for eight days across the Pacific Ocean—farther than the distance from London to New York. How does it survive without eating, drinking, or even pausing to rest?
Read the full explanation
Understanding Energetics of Long-Distance Migration in Bar-tailed Godwits
Imagine running a marathon, but you can't carry water or snacks. That's the challenge bar-tailed godwits face on their epic migration. They solve it by turning their bodies into fuel tanks. Before departure, they feed intensely on marine worms and crustaceans, building up thick layers of fat. This fat can make up more than half of their total body mass. Fat is the ideal fuel for long flights because it releases a large amount of energy per gram and produces water as a metabolic byproduct, helping the birds stay hydrated. The godwits also reduce their digestive organs to shrink body weight, and their flight muscles become incredibly efficient at burning this fuel. When they take off, they are flying with a full tank of gas, and they must make that fuel last until they land—thousands of kilometers later.
A deeper explanation
The godwit's migration hinges on the physiology of fuel storage and conversion. Fat is stored in subcutaneous and intra-abdominal depots, yielding about 37 kJ per gram, far more than carbohydrates. During flight, muscles burn a mix of fat and some protein, but fat provides the bulk of energy. As fat is metabolized, it releases metabolic water, helping to offset water loss through respiration—crucial because the birds cannot drink during the flight. However, carrying large fat loads increases the energetic cost of flight: a heavier bird requires more power to stay airborne. To manage this, godwits burn fuel in a way that gradually lightens the load, and they adjust their flight speed to minimize energy use. They also rely on wind patterns, choosing departure times with favorable tailwinds that reduce the ground distance and energy needed. Ultimately, the godwit's success depends on balancing energy intake, storage, and expenditure—an elegant example of how energy budgets dictate the limits of animal endurance.