Seasonal phenotypic flexibility of flight muscle size in small birds: a comparison of ultrasonography and tissue mass measurements

Abstract

Changes in flight muscle size are important mediators of phenotypic flexibility in birds, so the ability to track such changes over time in individual birds is a valuable tool for investigating phenotypic flexibility. Ultrasonography has been used to track changes in flight muscle size in shorebirds, but has not been previously used to track such changes in small birds, despite variation in flight muscle size being an important contributor to phenotypic flexibility in these birds. One prominent avian example of phenotypic flexibility is the seasonal phenotypes of small birds in response to climatic variation. The winter phenotype in these birds is characterized by increases in organismal metabolic rates and pectoralis muscle mass. We measured seasonal flight muscle size in House Sparrows (Passer domesticus, 25–30 g) using both ultrasonography and wet muscle mass and tested the correlation between ultrasonographic measures of breast muscle thickness and muscle mass. We further tested whether ultrasonographic measures of muscle thickness were sufficiently precise to detect seasonal variation in flight muscle mass. Muscle mass was significantly and positively associated with ultrasonographic measurements of breast muscle thickness for short-axis (SA), long-axis (LA), and combined SA and LA measurements. Breast muscle mass was significantly greater in winter than in summer (17.5 %) and muscle thickness also increased significantly in winter for both SA (9.1 %) and LA (7.5 %) measures. Thus, these data confirm that winter elevations of flight muscle mass consistently contribute to the winter phenotype in House Sparrows and that ultrasonography is effective in detecting seasonal changes in muscle mass in small birds.