Flight energetics of barn swallows across speeds: high aerodynamic efficiency, but low energy conversion efficiency.

Bird flight conversion efficiency (η) determines the amount of metabolic power (Pmet) transformed into mechanical power (Pmech) to remain aloft, and it is central to understanding animal flight energetics, yet it remains difficult to quantify. Using 13C-labelled sodium bicarbonate (NaBi) and particle image velocimetry (PIV), we measured Pmet and Pmech, respectively, in un-instrumented barn swallows (Hirundo rustica) flying across 7.8 to 13.6 m s-1 in a wind tunnel. Both Pmet and Pmech showed the expected U-shaped relationship with airspeed. Whole-animal η showed a shallow peak at 7% at 11 m s-1. Partial efficiency (ηp), estimated from horizontal and 4 deg climbing flight, was 27% (95% confidence interval CI: -2 to 56.1). Compared with thrush nightingales previously studied under identical conditions, barn swallows exhibited higher weight-specific flight Pmet but lower lift-specific Pmech, reflecting a higher basal metabolic rate but greater aerodynamic efficiency in barn swallows. Barn swallows showed lower peak and flatter η curve across speeds compared with that of the nightingales, suggesting a trade-off between flight energetics and flight speed: higher efficiency peak over a narrow speed range (thrush nightingales) versus lower, more consistent efficiency across a broader speed range (barn swallows). Whereas nightingales fly mainly during migration, barn swallows spend much of the day airborne and use a wider range of flight speeds, potentially favouring versatility over a narrow η peak. Although η has been studied in few species, our results suggest that η is both species and speed dependent, probably shaped by species-specific ecological demands.