Phenotypic flexibility of skeletal muscles during long-distance migration of garden warblers: muscle changes are differentially related to body mass.

Mass changes of skeletal muscles occur in a variety of species during the migratory period. Phenotypic flexibility of flight muscle mass is considered to represent adaptations of the flight muscle to changing power requirements associated with changes in body mass. We analyzed the relationship between muscle masses and body mass for garden warblers (Sylvia borin) sampled during spring migration in Tanzania, Ethiopia, and Egypt, and during autumn migration in Turkey. Flight muscle mass was positively related to body mass of warblers at only one of the four sites, in Egypt where warblers had just arrived after a long migratory flight. Analysis of covariance (ANCOVA) revealed a significant interaction term between sampling site and body mass (P < .001), indicating that flight muscle and body mass differed from site to site. We therefore question the idea that changing power requirements associated with changes in body mass cause mass changes of the flight muscle. We further suggest that different migration strategies across different landscapes shape the relationship between flight muscle and body mass. Flights across major ecological barriers may cause substantial catabolism of flight muscle protein until the limit necessary for flight, while migration across "common landscape," which enables a bird to land and to feed and/or drink, may occur without the need to catabolize flight muscle protein. However, a differential relationship between flight muscle mass and body mass described here for a long-distance migrant seems as well to occur in a short-distance migrant and is therefore unlikely to be part of an adaptive syndrome typical for long-distance migration.