Avian Migration: The Ultimate Red-Eye Flight
Birds that migrate at night enter a state of sleepless mania and gorge on foods by day, behaviors mediated by their biological clocks
Why Fly by Night?
Why do so many species of diurnal birds migrate at night? Humans, another typically day-active species, take red-eye flights because they offer certain advantages. Red-eye tickets are cheaper, the crowds are smaller and the flight schedule allows the maximal amount of time spent at a destination. A recent study by Guy Beauchamp at University of Montreal compared diurnal and nocturnal migration in all North American bird species to identify factors associated with daily migration. The results suggest that daytime travel is beneficial for highly social species that travel in large flocks and rely on visual cues to stick together. Advantages proposed for nocturnal migration include predator avoidance, minimized thermal stress, reduced evaporative water loss and lower energetic costs due to decreased air turbulence. These benefits are thought to be proportionally greater in smaller birds, and indeed, most small avian species migrate at night. In addition, flying at night frees up daylight hours for foraging.
Energy, Metabolism and Clocks
Migration is analogous to an extreme endurance sport, but even the most impressive human athletic endeavors pale in comparison to bird migration. The Badwater Ultramarathon, one of the most extreme endurance races, covering 135 miles from Death Valley to Mt. Whitney, is nominal in light of the migration of the bar-tailed godwit (Limosa lapponica), which makes a nonstop, eight-day journey of 6,800 miles. To be fair, more energy is expended moving a unit of mass by running than by flying the same distance. Nevertheless, birds are hardly loafing. Aerobically speaking, flight is high-intensity exercise requiring 70 to 90 percent of their maximal aerobic capacity. Unlike human endurance athletes, birds have no access to external sources of water, electrolytes or food during exercise. Humans need external fuel sources during long-term, high-intensity exercise because mammals preferentially burn carbohydrates to provide energy, and these reserves are rapidly used up. The body switches to a lipid fuel source after carbohydrate stores are depleted, but the fat-burning process is inefficient, limiting our ability to exercise continuously even if we have excess fat to burn. In contrast, migrating birds preferentially use fat for energy, and each bird bulks up before its long flight.
The accumulation and internal storage of fuel is necessary for long-distance avian migration. Songbirds double their body mass to prepare for migration, mostly due to increased subcutaneous fat stores. Premigratory fattening is controlled by a circannual timer in many species. Photoperiod and food availability also serve as cues to stimulate fattening. In some species, a change in metabolic efficiency prompts fat accumulation, even without increased food intake. For the most part, however, seasonal changes in appetite and satiety lead to increased food intake, accounting for a significant amount of the body mass in many species. The hypothalamic region of the brain controls appetite and satiety, and seasonal increases in neurotransmitters in the hypothalamus (for example, one called neuropeptide Y) are associated with seasonal hyperphagia in birds.
In mammals, a major signal for satiety, which basically indicates, “Stop eating, you’re full,” is a hormone called leptin. This hormone may be involved in seasonal changes in eating, fat storage and lipid utilization in migratory birds. Strangely, the gene that encodes leptin is absent from the avian genome. However, research from Christopher Guglielmo at University of Western Ontario shows that birds are responsive to leptin: They possess a functional receptor for the hormone, and injecting leptin has dramatic effects on their metabolism. Do birds make use of a signal other than leptin that serves to indicate the levels of fat stores? The answer is probably yes. One candidate is adipon