Protein metabolism during flight in migratory birds

Migratory birds perform some of the most extreme endurance feats in the animal kingdom, often flying non-stop for hours or days. While fat is the primary fuel supporting these flights, birds also show dramatic reductions in lean body mass due to the catabolism of protein. The adaptive significance of this protein catabolism has been an unresolved issue for decades and this Review examines several non-exclusive hypotheses explaining protein use in flight: (1) as a last-resort fuel, (2) to produce endogenous water, (3) to replenish metabolic intermediates, (4) to reduce mass and metabolic rate, and (5) as a consequence of protein turnover. Evidence from wind tunnel studies, body composition analyses using quantitative magnetic resonance (QMR), and metabolic assessments suggests that while a baseline level of protein catabolism may support metabolic processes during prolonged fat oxidation, environmental stressors - particularly dehydration - consistently increase protein breakdown. This supports the 'protein-for-water' hypothesis as a key driver of flexible fuel use during migration, but evidence also suggests a background rate of protein breakdown occurs as a result of turnover. Potential mechanisms involving glucocorticoid hormones and plasma osmolality are discussed as mediators of this response. Understanding the ecological and physiological consequences of protein catabolism is increasingly important as migratory birds face changing environmental conditions. Shifts in flight fuel use could impact arrival condition, stopover ecology and, ultimately, migration success. Further research into the mechanistic underpinnings of protein metabolism in flight will help clarify how migratory species might adapt - or be challenged - by ongoing climate change.