Kang Nian Yap, Morag F. Dick, Christopher G. Guglielmo, and Tony D. Williams

Despite widely held assumptions that hematocrit (Hct) is a key determinant of aerobic capacity and exercise performance, this relationship has not often been tested rigorously in birds and results to date are mixed. Migration in birds involves high intensity exercise for long durations at various altitudes. Therefore, it provides a good model system to examine the effect of Hct on flight performance and physiological responses of exercise at high altitude. We treated yellow-rumped warblers (Setophaga coronata) with avian erythropoietin (EPO) and anti-EPO to experimentally manipulate Hct and assessed flight performance at low and high altitudes using a hypobaric wind tunnel. We showed that anti-EPO treated birds had lower Hct than vehicle and EPO treated birds post-treatment. Anti-EPO treated birds also had marginally lower exercise performance at low altitude, committing a higher number of strikes (mistakes) in the first 30 min of flight. However, anti-EPO treated birds performed significantly better at high altitude, attaining a higher altitude in a ramped altitude challenge to 3000 m equivalent altitude, and with longer duration of flight at high altitude. Birds exercising at high altitude condition, decreased Hct, increased glucose mobilization, and decreased antioxidant capacity, regardless of treatment. In summary, we provided experimental evidence that the relationship between Hct and exercise performance is dependent on altitude. Future studies should investigate whether free-living birds adaptively modulate their Hct, potentially through a combination of erythropoiesis and plasma volume regulation (i.e. hemodilution), based on the altitude they fly at during migratory flight.

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