Carrying large fuel loads during sustained bird flight is cheaper than expected

Area of Science:

• Avian biology
• Physiological ecology
• Biomechanics

Background:

• Migratory birds must balance fuel use during flight with fuel accumulation during stopovers.
• Carrying fuel increases metabolic power demands, impacting flight efficiency and migration strategies.
• The precise impact of fuel load on metabolic power input during flight remains empirically undetermined.

Purpose of the Study:

• To empirically determine the effect of fuel load on the total metabolic power input of a long-distance migratory bird.
• To investigate how fuel load influences the relationship between metabolic power input and mechanical power output during flight.

Main Methods:

• Utilized the doubly labeled water technique to measure total metabolic power input.
• Observed a long-distance migrant, the red knot (Calidris canutus), during 6–10 hour wind tunnel flights.
• Varied fuel loads to assess their impact on metabolic and mechanical power.

Main Results:

• Total metabolic power input increased with increasing fuel load.
• This increase was proportionally less than predicted based on mechanical power output.
• Observed an increase in the efficiency of converting metabolic to mechanical power with higher fuel loads.

Conclusions:

• Flight muscle efficiency likely increases with fuel load in migratory birds.
• Findings necessitate adjustments to current models of bird flight and migration.
• This phenomenon may explain how some shorebirds achieve exceptionally long nonstop flights despite high energy costs.