Abstract Body composition of birds is often used to assess in part the ''condition'' of individuals in the context of their life history and ecology. We describe contemporary non-destructive (non-lethal) techniques that are available for estimating the body composition of free-living birds. We critically evaluate the strengths and weaknesses of these techniques in the context of bird studies. Although most contemporary techniques are based on theory and first principles, assessing their accuracy and precision requires empirical calibrations. We summarize the results of recent validation studies on songbirds and discuss their implications. Deuterium dilution was the best single technique among those compared for measuring lean and fat dynamics in small songbird species that averaged 9-29 g, although we advocate technique(s) that independently estimate each body component. Interspecific models that estimated lean mass using total body electrical conductivity and structural measure(s), and estimated fat mass using deuterium dilution were as accurate (within 0.3-1.1 g of actual lean mass and 0.2-0.9 g of actual fat mass, respectively, depending on bird species) as intraspecific models for songbirds that averaged 13-29 g in body mass. Thus, separate models for each bird species may not be necessary, and the development and testing of interspecific models for estimating body composition is warranted. Several factors, including body size and physiological state, required accuracy and precision, and the scope of predictions must be carefully considered when any of these non-destructive techniques are used to measure the body composition of birds. Keywords Songbird body composition Á Non-destructive techniques Á Total body electrical conductivity Á Deuterium Á Fat score Á DEXA Á QM
