Biology
Energetic constraints on long-distance migration in bar-tailed godwits
Quick fact
A bar-tailed godwit can fly over 11,000 km nonstop across the Pacific, losing about 55% of its body weight in fat fuel, yet it loses little muscle, allowing it to take off again after refueling.
Why this is interesting
Imagine flying nonstop from New York to Sydney—without eating, drinking, or sleeping. That's what a bar-tailed godwit does every year, but how does it survive?
Read the full explanation
Understanding Energetic constraints on long-distance migration in bar-tailed godwits
Bar-tailed godwits are shorebirds that breed in Alaska and winter in New Zealand and Australia. To make their marathon migration, they must first deposit enormous fat stores—almost doubling their body weight. This fat is the primary fuel for the flight. During the journey, they fly at high altitudes where the air is thinner, reducing drag and saving energy. They also time their departure to catch favorable winds, such as the trade winds, which push them along. The flight itself is a delicate balance: they must burn enough fat to stay aloft but conserve enough to reach their destination. Their flight muscles are adapted to be efficient, and they can even shrink some organs to reduce weight, focusing energy on the muscles needed for flapping.
A deeper explanation
The core constraint is the relationship between fuel load, flight cost, and body mass. The energy required to fly increases with body mass, so a heavy bird must burn more fuel per kilometer. However, fat is the most energy-dense fuel available, so godwits maximize fat storage while minimizing non-essential body mass. They achieve this by reducing the size of digestive organs before departure, as they won't need them during the nonstop flight. This trade-off is reflected in their flight speed and altitude: flying faster reduces travel time but increases energy consumption, while flying higher reduces air density and drag but may require more work to stay at altitude. Godwits also use a strategy of 'powered gliding', where they alternate between flapping and gliding to save energy. The physiological limits are set by the need to carry enough fat to cross the ocean while maintaining enough muscle to actually flap. This balance is so tight that even a small misjudgment in weather can be fatal. Understanding these constraints explains why migration routes and timings are so precise, and why climate change, which alters wind patterns and food availability, poses a serious threat to these birds.