"Endurance in the Skies: How Migratory Birds Rest During Long-Distance Flights"

Frigatebirds in flight in the Galápagos Islands. Photo: Frans Lanting/National Geographic Creative Image Source audubon.org

*Heri Tarmizi

Birds employ a range of remarkable strategies to rest during their long migratory journeys, including unihemispheric sleep, short stopovers, and energy-efficient flight modes like soaring and gliding.

In migratory journeys, birds demonstrate incredible endurance, often traveling thousands of kilometers across diverse terrains and weather conditions. During these long migrations, birds must find ways to rest without halting their progress. This essay delves into how birds rest during migration, drawing on peer-reviewed studies and scientific publications, offering an in-depth exploration of flight physiology, behavioral adaptations, and ecological factors that influence avian resting strategies during long flights.

1. Introduction: The Migratory Challenge

Migratory birds undertake some of the longest and most challenging journeys in the animal kingdom. Species like the Arctic Tern (Sterna paradisaea) fly over 70,000 kilometers annually, while others, such as the Bar-tailed Godwit (Limosa lapponica), perform non-stop flights exceeding 11,000 kilometers across open oceans. These feats are astonishing given the physical demands placed on birds, raising critical questions about how they rest and recover during such long flights.

2. The Physiology of Long-Distance Flight

Migratory birds have evolved remarkable physiological adaptations that enable them to sustain extended flights. According to a study by Piersma and Gill (1998), long-distance migrants undergo significant pre-flight preparations. They accumulate fat reserves, sometimes doubling their body mass, which serves as the primary energy source during migration. Additionally, migratory birds experience morphological changes, such as muscle hypertrophy, which increases the endurance of flight muscles. Despite these adaptations, the energy cost of migration remains high, and birds must employ efficient strategies to conserve energy and rest in mid-flight.

2.1 Sleep Deprivation and Energy Expenditure

Research indicates that birds experience sleep deprivation during migration. In contrast to most terrestrial animals that require extended periods of sleep, migratory birds can drastically reduce their sleep duration. A study by Rattenborg et al. (2004) found that Swainson’s Thrush (Catharus ustulatus) and other songbirds engage in unihemispheric sleep, where one hemisphere of the brain remains awake while the other sleeps. This adaptation allows birds to rest one part of their brain while continuing to fly and maintain alertness, particularly for predator detection and navigation.

2.2 Energy Conservation Through Flight Modes

Migratory birds employ various flight modes to conserve energy and minimize fatigue. Soaring and gliding are common strategies used by large birds, such as raptors and storks. Research by Hedenström (2010) showed that soaring birds exploit thermal currents to gain altitude without expending energy, which allows them to rest during long migrations. Albatrosses, for example, are known to use dynamic soaring, a technique that leverages wind gradients to maintain flight with minimal energy output. This energy-efficient flight mode is a form of passive resting, as it reduces the need for constant wing flapping.

3. Behavioral Adaptations for Rest During Migration

Migratory birds also exhibit unique behavioral adaptations that enable them to rest during long flights. These adaptations vary depending on the species and the environmental conditions encountered during migration.

3.1 Unihemispheric Sleep

As mentioned earlier, unihemispheric sleep is a crucial adaptation for migratory birds. In this state, one half of the brain sleeps while the other remains active. This ability allows birds to rest without stopping their flight, enabling them to maintain navigation and predator vigilance. Rattenborg et al. (2016) conducted studies on the Great Frigatebird (Fregata minor), a species known for its long, continuous flights over oceans. Their findings revealed that frigatebirds sleep in brief episodes of unihemispheric sleep while flying, typically for only a few seconds at a time. The ability to engage in this form of sleep allows these birds to cover vast distances without the need for prolonged rest stops.

3.2 Short Rest Stops

While some migratory birds can rest in flight, others rely on brief stopovers to rest and refuel. These stopovers, often referred to as staging sites, are critical for the survival of migratory species. According to a study by Newton (2008), stopover sites are carefully selected based on resource availability, safety from predators, and proximity to migratory routes. For example, wetlands and coastal areas often serve as key stopover points for migratory shorebirds like the Red Knot (Calidris canutus), where they can rest and forage before continuing their journey.

The timing and duration of rest stops vary between species. Some birds, such as the Northern Wheatear (Oenanthe oenanthe), engage in “hop” migration, where they alternate between short flights and frequent rest stops. In contrast, species like the Blackpoll Warbler (Setophaga striata) undertake “jump” migrations, characterized by long, non-stop flights followed by extended periods of rest at staging sites. A study by Bairlein (2002) found that these strategies are influenced by environmental conditions, food availability, and the distance to the next suitable resting site.

4. Environmental and Ecological Influences on Resting Strategies

Environmental factors play a crucial role in determining how and when migratory birds rest during their journeys. The availability of suitable stopover sites, weather patterns, and predation risks all influence the resting strategies of migratory birds.

4.1 Weather and Wind Conditions

Weather and wind patterns significantly impact the ability of migratory birds to rest. Favorable winds can reduce the energy expenditure of birds in flight, allowing them to fly longer distances without needing to rest. A study by Shamoun-Baranes et al. (2017) demonstrated that birds use weather forecasts and adjust their flight altitudes to take advantage of favorable tailwinds. Conversely, adverse weather conditions, such as storms and strong headwinds, increase energy demands and may force birds to seek shelter and rest.

Some birds, such as swifts, are known to ascend to higher altitudes to avoid bad weather. During these ascents, swifts enter a state of torpor, a form of energy-saving rest where their metabolic rate decreases. This allows them to conserve energy until the weather improves and they can resume their journey.

4.2  Predation Risk

Predation risk is another factor that influences resting behavior during migration. Birds must balance the need to rest with the risk of predation, especially in areas where stopover sites may harbor predators. According to Ydenberg et al. (2002), birds modify their stopover duration based on predation risk. For example, migratory shorebirds are known to reduce their foraging time and increase vigilance when resting in areas with a high density of predators.

In response to predation threats, some species opt for nocturnal migration, resting during the day when predation risk is higher. Night-flying birds, such as the Common Cuckoo (Cuculus canorus), avoid predation by migrating under the cover of darkness, using celestial cues for navigation. Nocturnal migration allows these birds to rest during the day in safer, concealed locations.

5. Case Studies: Resting Strategies in Specific Species

5.1 Bar-tailed Godwit (Limosa lapponica)

One of the most extraordinary examples of non-stop migration comes from the Bar-tailed Godwit. This species is known for its epic flight from Alaska to New Zealand, a journey of over 11,000 kilometers without stopping. A study by Gill et al. (2009) revealed that during this non-stop flight, the birds rely on a combination of fat reserves and efficient wing mechanics to sustain flight without the need for rest. Despite the lack of rest, the birds manage to reach their destination in good condition, demonstrating the extreme endurance of migratory species.

5.2 Swainson’s Thrush (Catharus ustulatus)

Swainson’s Thrush, a small songbird, migrates from North America to Central and South America each year. Unlike larger birds, Swainson’s Thrush cannot soar or glide to rest during migration. Instead, these birds employ unihemispheric sleep while flying at night. A study by Fuchs et al. (2009) found that Swainson’s Thrush can engage in short bursts of sleep during flight, with each episode lasting only a few seconds. This allows the bird to rest without compromising its migration speed or route.

5.3 Common Swift (Apus apus)

The Common Swift is another species that has evolved remarkable adaptations for resting during migration. A study by Hedenström et al. (2016) revealed that swifts spend the majority of their migration in flight, sometimes flying continuously for months without landing. Swifts enter a state of torpor during flight, reducing their metabolic rate to conserve energy. This allows them to rest while flying, an adaptation that enables them to cover long distances without the need for stopovers.

6. Conservation Implications of Resting Strategies

Understanding how migratory birds rest during their journeys has important conservation implications. Migratory routes and stopover sites are increasingly threatened by habitat loss, climate change, and human activity. A study by Runge et al. (2014) emphasised the importance of preserving stopover habitats for migratory birds. Loss of these critical resting sites can lead to increased mortality rates, as birds may be unable to find sufficient resources to recover during migration.

Conservation efforts must prioritise the protection of key stopover sites, particularly wetlands, coastal areas, and forested regions that serve as important resting points for migratory species. Collaborative international efforts are essential, as migratory birds often cross multiple countries during their journeys. Conservation organisations, such as BirdLife International, have called for the establishment of transboundary protected areas to ensure the survival of migratory birds.

7. Conclusion

Birds employ a range of remarkable strategies to rest during their long migratory journeys, including unihemispheric sleep, short stopovers, and energy-efficient flight modes like soaring and gliding. These adaptations enable birds to conserve energy, avoid predators, and complete their migrations successfully. However, the increasing threat of habitat loss and climate change underscores the urgent need for conservation efforts to protect migratory routes and resting sites. By understanding the physiological and behavioural mechanisms that allow birds to rest during migration, we can develop effective conservation strategies that support the survival of these extraordinary travellers.

References

- Piersma, T., & Gill, R. E. (1998). Guts Don’t Fly: Small Digestive Organs in Obese Bar-tailed Godwits. Auk, 115(1), 196–203.
- Rattenborg, N. C., Mandt, B. H., & Obermeyer, W. H. (2004). Unihemispheric sleep and dynamic visual processing in the freely flying frigatebird. Nature, 441(7091), 585-588.
- Hedenström, A. (2010). Migration by soaring or flapping flight in birds: The relative importance of energy cost and muscle work. Philosophical Transactions of the Royal Society B, 365(1550), 315-332.
- Newton, I. (2008). The Migration Ecology of Birds. Academic Press.
- Gill, R. E., Tibbitts, T. L., Douglas, D. C., Handel, C. M., Mulcahy, D. M., Gottschalck, J. C., & McCafferty, B. J. (2009). Extreme endurance flights by land birds crossing the Pacific Ocean: ecological correlates and physiological consequences of trans-oceanic migrations. Proceedings of the Royal Society B: Biological Sciences, 276(1654), 447–457.