Adaptations of the Gray Catbird Dumetella carolinensis to Long Distance Migration: Energy Stores and Substrate Concentrations in Plasma

Abstract

Abstract The major body components (water, lean dry, and fat) were measured in the carcasses of Gray Catbirds from which the flight muscles had been removed. Birds were collected from May through October near Ann Arbor, Michigan and during September and October near Gainesville, Florida. Additionally, the glycogen content of muscle and liver and the concentrations of glucose and triglycerides in plasma were determined in catbirds sampled during fall migration in Florida. Catbirds attained maximum body masses of ∼50 g in Florida, largely due to the addition of fat. Relatively lean birds (∼3-4% body fat) in spring through fall weighed approximately 35 g. Thirteen percent of the birds sampled were estimated to have had sufficient reserves of fat to cross the Gulf of Mexico, although a larger proportion of the population probably makes this crossing. The lean dry mass of the carcass (without the flight muscles) is related significantly to structural body size and time of day, but is not related to molt, sex, or carcass fat content. Plasma glucose and triglycerides in free-living fall migrants do not vary diurnally. Liver glycogen, however, is four times higher in the evening than in the morning (77 and 19 mg/g, respectively), and muscle glycogen is five times higher in the evening (20 and 4 mg/g, respectively). Evening concentrations of glycogen are among the highest values reported for birds and do not confirm the reduction in glycogen reported for some other migrants.