Fat and pectoral muscle in migrating Sedge Warblers Acrocephalus schoenobaenus

Abstract

Increases in fat and pectoral muscle mass are important physiological changes associated with migration, but the extent to which these are linked is uncertain. The relationship between fat and pectoral muscle in first‐year Sedge Warblers Acrocephalus schoenobaenus was investigated using the carcasses of 20 birds that died by flying into the lighthouse on Bardsey Island, North Wales, UK, in autumn 1996, and data for fat and pectoral muscle scores from 3,281 Sedge Warblers ringed while on migration thro ugh the Wetland Trust Reserve at Elms Farm, Sussex, UK, between the end of July and early October in the years 2000, 2001 and 2002. For the Bardsey sample, the mass of tracheal pit fat (claviculocoracoid fat body) correlated with the masses of fat at other body sites, and was a good measure of overall fat levels. Lean dry pectoral muscle mass did not correlate with the mass of tracheal pit fat, suggesting that pectoral muscle mass and fat mass are independent measures of body size. The link between fat and pectoral muscle mass was investigated further using the Elms Farm data. Wing length, time of capture, fat score and pectoral muscle score all made significant contributions to overall body mass. Although fat and pectoral muscle scores were correlated overall, analysis by year and fat score range supported the idea that these can vary independently. Analysis of fat and pectoral muscle scores by different ringers suggested that fat scores were consistently applied in different years; however, pectoral muscle scores may be harder to standardise between ringers. In summary, we suggest that fat and pectoral muscle mass increase independently in preparation for migration, but the factors which determine variation in pectoral muscle scores in relation to fat scores in different years are unknown.