Testing an emerging paradigm in migration ecology shows surprising differences in efficiency between flight modes.

Area of Science:

• Ornithology
• Bioenergetics
• Animal Migration

Background:

• Flying animals balance speed and energy for optimal fitness.
• Large raptors in North America use thermal gliding and slope soaring for migration.
• Slope soaring is often assumed to be faster than thermal gliding.

Purpose of the Study:

• To test the hypothesis that slope soaring is faster than thermal gliding in migrating golden eagles.
• To understand the trade-offs between time and energy expenditure in avian migration.

Main Methods:

• High-frequency GPS-GSM telemetry devices were used to track golden eagles during their northbound migration.
• Flight speeds and migratory paths were analyzed for both slope soaring and thermal gliding behaviors.

Main Results:

• Contrary to expectations, golden eagles flew slower when slope soaring.
• Slope soaring diverted eagles from their migratory path, potentially increasing energetic costs.
• Fast gliding speeds between thermals compensated for slower progress during thermal soaring.
• Eagles opted for energetically expensive slope soaring when thermals were unavailable to minimize migration time.

Conclusions:

• Slope soaring is not necessarily faster than thermal gliding for migrating raptors.
• Avian risk of collision with wind turbines may be linked to evolutionary trade-offs in migratory strategies.
• Understanding these trade-offs is crucial for conservation, especially in areas with wind energy development.