Using plasma metabolite profiling to assess refueling performance and diet of migrating songbirds

Background/Question/Methods

Migrating songbirds require frequent stops at migration stopover sites to rest and replenish energy stores before continuing migration.  Plasma metabolite profiling offers a useful means to evaluate refueling performance and physiological condition of migrating birds because certain metabolites indicate fat deposition (plasma triglyceride) and dietary protein intake (uric acid).  Thus, measurement of these metabolites in migrating songbirds can provide valuable information about food availability and habitat quality at migration stopover sites.  I compared refueling rates and nutritional state of birds captured during spring and fall migrations of 2009 at the Braddock Bay Bird Observatory, an important stopover site for passerines on the south shore of Lake Ontario.  I focused on seven species that are commonly captured at this site and display different degrees of seasonal frugivory during fall migration. 

Results/Conclusions

There were no interspecific differences in plasma metabolite concentrations among spring migrants.  During the fall, Swainson’s Thrushes had higher plasma triglyceride concentrations than White-throated Sparrows and lower plasma uric acid concentrations than White-throated Sparrows and Red-eyed Vireos.  Seasonal differences in plasma metabolite concentrations were detected in Swainson’s Thrushes and Red-eyed Vireos; Swainson’s Thrushes had higher plasma triglyceride and lower plasma uric acid concentrations during fall versus spring, whereas Red-eyed Vireos had higher plasma triglyceride in the spring.  My results suggest that there are seasonal differences in dietary protein intake for certain species, most likely representing a dietary switch to frugivory in the fall.  In addition, many birds are able to refuel at a higher rate during fall stopover, possibly due to their consumption of abundant fruit resources at this site.  Results of this study emphasize the importance of considering seasonal differences in food availability when managing stopover sites in order to provide high-quality habitat across seasons for a range of songbird species.