Elevational differences in estimated fattening rates suggest that high-elevation sites are high-quality habitats for fall migrants

Abstract

ABSTRACT. Many migrant songbird species use high-elevation habitats for stopovers in fall throughout North America, but whether these are good migration habitats as indicated by high fueling rates or other measures has not been previously quantified. At high-quality stopover sites, birds can refuel while maintaining their optimal or preferred migration schedules. We used plasma metabolite analysis to estimate fueling rates of four songbird species during the fall migration period over 3 years at two high-elevation (1,200 m above sea level) and two low-elevation (<25 m above sea level) sites in southwestern British Columbia. For three species with more frugivorous diets during fall—the Fox Sparrow (Passerella iliaca), Golden-crowned Sparrow (Zonotrichia atricapilla), and Hermit Thrush (Catharus guttatus)—estimated fattening rates (defined as residual plasma triglyceride levels) were 37–65% higher at high-elevation sites than at low-elevation sites. By contrast, the largely insectivorous and smaller-bodied ...