Venous blood dynamics after ground-level and high-altitude flights in hermit thrush ( Cathurus Guttatus)

Flight is aerobically demanding, yet some songbirds fly thousands of meters above sea level. Such altitudes should increase metabolic rate and acid-base stress, but this has been rarely tested. We measured venous blood gases, metabolites, and electrolytes in migratory hermit thrushes (Catharus guttatus) flown at ground level, sub-maximum altitude, and maximal high altitude. Birds reached ∼2,300 m during maximal high-altitude flights. Venous PO2 did not differ between rest and flight at high altitude, indicating no detectable oxygen debt, while PCO2 declined with increasing altitude. Venous pH remained stable, but decreases in bicarbonate and base excess with elevated plasma lactate at higher altitudes indicate a mild, compensated metabolic acidosis. These patterns show that high-altitude flight imposes greater metabolic cost than ground-level flight, yet hermit thrush maintain acid-base balance sufficiently to sustain flight even at their maximum tolerated altitude.