Magnetic resonance Imaging versus chemical fat extraction in a small passerine, the willow warbler Phylloscopus trochilus: a fat‐score based statistical comparison

Abstract

The amount of fat of a bird is a fundamental metric commonly used by avian biologists in studies of migration, winter energy management and condition. The methods used for estimating fat content range from simply measuring body mass and subtracting the estimated lean mass, visual scoring of subcutaneous fat deposits, to destructive extraction using a Soxhlet apparatus. We used magnetic resonance imaging (MRI) for non‐invasive estimation of fat content in willow warblers Phylloscopus trochilus. According to an indirect comparison, made using a standardized fat scoring system as a common measure, the MRI method gave very similar fat amount as Soxhlet extraction and we discuss advantages of using MRI over alternative methods. It was also shown that the commonly used fat scoring system yielded a nonlinear relationship between fat mass and fat score.