Metabolic reduction after long duration flight is not related to fat-free mass loss or flight duration in a migratory passerine

Migratory birds catabolize large quantities of protein during long flights, resulting in dramatic mass reductions of organs and muscles. One of the many hypotheses to explain this phenomenon is that decreased lean mass could reduce resting metabolism, saving energy after flight during refueling. However, the relationship between lean body mass and resting metabolic rate remains unclear. Furthermore, the coupling of lean mass to resting metabolic rate and to peak metabolic rate before and after long duration flight have not previously been explored. We flew migratory yellow-rumped warblers (Setophaga coronata) in a wind tunnel under one of two humidity regimes to manipulate the rate of lean mass loss in flight, decoupling flight duration from total lean mass loss. Before and after long duration flights, we measured resting and peak metabolism, and also measured fat mass and lean body mass using quantitative magnetic resonance. Flight duration ranged from 28 to 600 min, and birds flying under dehydrating conditions lost more fat-free mass. After flight there was a 14% reduction in resting metabolism but no change in peak metabolism. Interestingly, the reduction in resting metabolism was unrelated to flight duration or to change in fat-free body mass, indicating protein metabolism in flight is unlikely to have evolved as an energy saving measure to aid stopover refueling, but metabolic reduction itself is likely beneficial to migratory birds arriving in novel habitats.