Flight muscle shape reliably predicts flight muscle mass of migratory songbirds: a new tool for field ornithologists

The pectoral muscle is the biggest organ within a passerine bird. It provides flight locomotion and is known to act as a protein source during periods with increased protein demands or decreased protein availability. The mass of the flight muscle is dynamic and changes during juvenile growth, reproduction, seasonal acclimatization, fasting and migration. Thus, a tool that accurately and non-invasively quantifies this phenotypic flexibility in flight muscle mass is of interest to ornithologists. We provide a calibration and validation of a "muscle meter" device designed to accurately measure the shape of the flight muscle. For two species of different size, the European starling (Sturnus vulgaris) and the garden warbler (Sylvia borin), we compared the accuracy and precision of different linear regression models for predicting flight muscle mass. The multifactorial linear regression model with the most support for both species included “muscle meter score” (mmscore), tarsus length and body mass (m b), although a simpler model with mmscore and m b had as much support for predicting flight muscle mass of European starlings. A validation exercise revealed that flight muscle mass of these two species could be estimated with a relative error of about 3%. The muscle meter is a simple device, easy and quick to handle, that can reliably and non-invasively estimate flight muscle mass of captive and wild birds when used in conjunction with standard measurements of tarsus length and m b.