Solar heating may explain extreme diel flight altitude changes in migrating birds

Area of Science:

• Ornithology
• Animal Physiology
• Ecology

Background:

• Migratory birds exhibit diel cycles in flight altitudes, flying higher during the day than at night.
• A prevailing hypothesis suggests birds climb to cooler altitudes to mitigate solar heat gain during daytime migratory flights.

Purpose of the Study:

• To investigate the quantitative effect of solar radiation on the heat balance of flying migratory birds.
• To test the hypothesis that solar radiation influences diel cycles in flight altitude.

Main Methods:

• Temperature data loggers (MDLs) were attached to great reed warblers and great snipes, shielded from direct solar radiation.
• Logger temperatures were compared between day and night at similar flight altitudes to infer solar heat gain.

Main Results:

• Logger temperatures were significantly higher during the day than at night for both species, even at comparable altitudes.
• Great reed warblers showed a 5.9°C-8.8°C increase, while great snipes showed a 4.8°C-5.4°C increase during the day.

Conclusions:

• The findings provide strong evidence that solar radiation affects the heat balance of flying migratory birds.
• Solar radiation is a key factor contributing to the observed diel cycles in flight altitude in great reed warblers and great snipes.