A sport-physiological perspective on bird migration: evidence for flight-induced muscle damage.

Exercise-induced muscle damage is a well-described consequence of strenuous exercise, but its potential importance in the evolution of animal activity patterns is unknown. We used plasma creatine kinase (CK) activity as an indicator of muscle damage to investigate whether the high intensity, long-duration flights of two migratory shorebird species cause muscle damage that must be repaired during stopover. In two years of study, plasma CK activity was significantly higher in migrating western sandpipers (a non-synchronous, short-hop migrant), than in non-migrants. Similarly, in the bar-tailed godwit (a synchronous, long-jump migrant), plasma CK activity was highest immediately after arrival from a 4000-5000km flight from West Africa to The Netherlands, and declined before departure for the arctic breeding areas. Late-arriving godwits had higher plasma CK activity than birds that had been at the stopover site longer. Juvenile western sandpipers making their first southward migration had higher plasma CK activity than adults. These results indicate that muscle damage occurs during migration, and that it is exacerbated in young, relatively untrained birds. However, the magnitude of the increases in plasma CK activity associated with migratory flight were relatively small, suggesting that the level of muscle damage is moderate. Migrants may avoid damage behaviourally, or have efficient biochemical and physiological defences against muscle injury.