Biology
The Physiological Costs of Long-Distance Migration in Shorebirds
Quick fact
During a single non-stop migration flight, a shorebird may lose up to 50% of its body mass, much of it from fat stores, and may experience measurable shortening of its telomeres—a cellular sign of aging.
Why this is interesting
Bar-tailed godwits fly non-stop for over 11,000 kilometers, but what does such an epic journey do to their bodies?
Read the full explanation
Understanding The Physiological Costs of Long-Distance Migration in Shorebirds
Imagine you are about to run a marathon—just one, not several, and without any food or rest until you cross the finish line. Shorebirds like the bar-tailed godwit face a similar challenge, but on a scale that would be the equivalent of running a marathon every day for a week, non-stop. To fuel this, they prepare by gorging on food, building up large fat reserves. Before a long flight, a bird's body may be over half fat. As they fly, they burn these fats for energy, but they also break down protein from their muscles and organs—not just as fuel, but to provide water, since fat metabolism produces water as a byproduct. By the time they reach their destination, they are emaciated. Their flight muscles have withered, their digestive organs have shrunk, and their immune system is suppressed. This is the immediate physiological cost: the bird arrives exhausted, many pounds lighter, and vulnerable to disease. But there are also hidden, longer-term costs, such as accelerated cellular aging. Throughout their journey, intense exercise produces reactive oxygen species—damaging molecules that attack cells and DNA. To cope, birds have evolved antioxidant defenses, but these are not perfect. Over many migrations, damage accumulates, telomeres shorten, and the bird ages faster. So, while migration is a marvel of endurance, it is not without a price: each journey chips away at the bird's health and longevity.
A deeper explanation
The physiological costs of migration can be understood at several levels. First, the energetic cost: flight is among the most energetically expensive activities in the animal kingdom. During long flights, metabolic rate can be up to 10 times the resting rate. To sustain this, shorebirds rely on stored fat, which yields more energy per gram than protein or carbohydrate. But fat is not unlimited. They also catabolize protein, particularly from flight muscles and digestive organs. This muscle breakdown provides amino acids for gluconeogenesis and for maintaining water balance, but it comes at a cost: after arrival, the bird must rebuild these tissues, a process that requires time and food. Second, the immune system is down-regulated during migration. The energetic demands of flight and the stress hormone corticosterone suppress immune function, making birds more susceptible to infections. This is a trade-off: energy is diverted from immune defense to flight, and the risk of disease increases. Third, at the cellular level, intense exercise increases oxidative stress. The high metabolic rate produces free radicals that can damage lipids, proteins, and DNA. Shorebirds have evolved strong antioxidant defenses, but these can be overwhelmed, leading to oxidative damage. One measurable consequence is telomere shortening: telomeres are protective caps at the ends of chromosomes that shorten with each cell division and under oxidative stress. Studies have shown that migratory shorebirds that fly longer distances have shorter telomeres, suggesting that migration accelerates aging at a cellular level. These costs have evolutionary implications: they help explain why shorebirds may not migrate every year, why they choose certain stopover sites, and why they time their departures to avoid the worst weather. Understanding these costs is also crucial for conservation: habitat loss at stopover sites can increase the physiological burden on birds, reducing their chances of survival and successful breeding.