Current Biology
Volume 31, Issue 15, 9 August 2021, Pages 3433-3439.e3
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Extreme altitude changes between night and day during marathon flights of great snipes

https://doi.org/10.1016/j.cub.2021.05.047Get rights and content
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Highlights

  • Great snipes follow a diel altitude cycle, flying much higher at day than at night

  • Most birds reached above 6,000 m and one bird reached a record height of 8,700 m

  • Daytime ascents may relate to orientation, predator avoidance, or need for cooling

  • Repeated flight altitude changes may be a common phenomenon among migrating birds

Summary

Several factors affect the flight altitude of migratory birds, such as topography, ambient temperature, wind conditions, air humidity, predation avoidance, landmark orientation, and avoiding over-heating from direct sunlight.1, 2, 3, 4, 5, 6 Recent tracking of migratory birds over long distances has shown that migrants change flight altitude more commonly and dramatically than previously thought.4, 5, 6, 7, 8 The reasons behind these altitude changes are not well understood. In their seasonal migrations between Sweden and sub-Saharan Africa, great snipes Gallinago media make non-stop flights of 4,000–7,000 km, lasting 60–90 h.9,10 Activity and air pressure data from multisensor dataloggers showed that great snipes repeatedly changed altitudes around dawn and dusk, between average cruising heights about 2,000 m (above sea level) at night and around 4,000 m during daytime. Frequency and autocorrelation analyses corroborated a conspicuous diel cycle in flight altitude. Most birds regularly flew at 6,000 m and one bird reached 8,700 m, possibly the highest altitude ever recorded for an identified migrating bird. The diel altitude changes took place independently of climate zone, topography, and habitat overflown. Ambient temperature, wind condition, and humidity have no important diel variation at the high altitudes chosen by great snipes. Instead, improved view for orientation by landmarks, predator avoidance, and not least, seeking cold altitudes at day to counteract heating from direct sunlight are the most plausible explanations for the diel altitude cycle. Together with similar recent findings for a small songbird,6 the great snipes’ altitudinal performance sheds new light on the complexity and challenges of migratory flights.

Keywords

Gallinago media
wader
shorebird
migration
flight altitude
diel altitude cycle
circadian pattern
multisensor dataloggers

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