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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\u2005m\u2005s\u22121 in a wind tunnel. Both Pmet and Pmech showed the expected U-shaped relationship with airspeed. Whole-animal \u03b7 showed a shallow peak at 7% at 11\u2005m\u2005s\u22121. Partial efficiency (\u03b7p), estimated from horizontal and 4\u00a0deg climbing flight, was 27% (95% confidence interval CI: \u22122 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 \u03b7 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 \u03b7 peak. Although \u03b7 has been studied in few species, our results suggest that \u03b7 is both species and speed dependent, probably shaped by species-specific ecological demands.<\/jats:p>","DOI":"10.1242\/jeb.252390","type":"journal-article","created":{"date-parts":[[2026,9,17]],"date-time":"2026-09-17T10:05:50Z","timestamp":1789639550000},"source":"Crossref","is-referenced-by-count":1,"title":["Flight energetics of barn swallows across speeds: high aerodynamic efficiency, but low energy conversion efficiency"],"prefix":"10.1242","volume":"229","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-4503-164X","authenticated-orcid":false,"given":"Pablo","family":"Mac\u00edas-Torres","sequence":"first","affiliation":[{"name":"Lund University Department of Biology , , 223 62 Lund ,","place":["Sweden"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-4272-7681","authenticated-orcid":false,"given":"Sonja I.","family":"Friman","sequence":"additional","affiliation":[{"name":"Lund University Department of Biology , , 223 62 Lund ,","place":["Sweden"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-1851-3635","authenticated-orcid":false,"given":"L. 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