Super Size Me: An Experimental Test of the Factors Affecting Lipid Content and the Ability of Residual Body Mass to Predict Lipid Stores in Nestling European Starlings

1. Lipids are a major source of energy storage in vertebrates. A cross-fostering brood manipulation was conducted to assess the role of rearing environment vs common origin on lipid stores. 2. Rearing conditions, especially nest temperatures, explained more variation in lipid levels than did nest of origin. As nest temperatures increased, lipid levels in nestlings also increased. Larger individuals, reflected in both wet and lean dry mass, tended to store more lipids. 3. There was no trade-off between structural growth and lipid stores. However, body mass growth was linked with lipids. In addition, nestlings raised early in the season tended to maintain greater lipid levels, suggesting a role of individual quality. 4. Because the assessment of lipid stores is difficult, it is common to use residual body mass (RBM) as a surrogate. Multiple methods of calculating RBM were compared with actual lipid mass in nestling European Starlings (Sturnus vulgaris). RBM was correlated with lipid levels (R2up to 0·66); models calculated using ordinary least squares regression predicted lipid stores better than reduced major axis regression. 5. These results indicate that RBM can be a non-invasive predictor of lipid levels in nestling birds and that studies examining variation in physiological stores should consider development conditions and individual quality.