Testing the utility of condition indices in nestling swallows: a quantitative magnetic resonance approach

Abstract

Nutritional condition is a key factor influencing the survival of migratory birds, and condition indices derived from morphometric measurements are commonly used to assess nutrient and energy stores. The performance and utility of condition indices used to examine energetics of growing juvenile birds remain uncertain. Recently, quantitative magnetic resonance (QMR) has been used to provide a non-invasive and non-lethal way to obtain measures of body composition. We calculated three widely used body condition indices using both wing length and tarsus length for nestling Barn Swallows Hirundo rustica erythrogaster at two ages (Day 8 and 15) and compared them to relative fat and lean mass calculated using QMR measurements. Additionally, we used QMR to develop multiple regression formulas to predict nestling fat mass, lean mass, and fat-free lean body mass (LBM) from easily obtainable body metrics. In Day 8 birds, variation in both relative fat mass and relative lean mass were adequately explained by total body mass alone, but this relationship was strongest for lean mass. Almost every condition index evaluated failed to explain variation in relative fat or lean mass in Day 15 birds. While our best multiple regression formulas for predicting fat mass and LBM had low predictive power, our formulas for predicting lean mass were precise (R2 > 90%) in both ages, providing a method to estimate the lean mass of nestlings from more readily obtainable metrics. Estimates of fat mass calculated from LBM and total body mass showed poor precision (R2 ≤ 30%).