An evaluation of condition indices and predictive models for noninvasive estimates of lipid mass of migrating Common Yellowthroats, Ovenbirds, and Swainson's Thrushes

Noninvasive methods of measuring lipid mass in birds are widely used, but not frequently evaluated. I evaluated the ability of three noninvasive indicators of fat content (fat scores, body mass, body mass/wing chord) and regression models to predict lipid mass in two migratory songbirds previously unexamined in this context-Common Yellowthroat (Geothlypis trichas) and Ovenbird (Seiurus aurocapillus). I also examined the accuracy of these methods for Swainson's Thrushes (Catharus ustulatus) for comparison to a previous study. Fat score, body mass, and body mass/wing chord were highly correlated with chemically extracted lipid mass in each species. In all three species, birds with no visible subcutaneous fat possessed considerable quantities of fat, ranging from 9.8 to 19.7% of total dry body mass. Forward-selected regression models explained 69-87% of lipid mass variation, with prediction errors of 14.6-27.5%. An existing predictive model for the Swainson's Thrush overestimated lipid mass by an average of 92%. Fat score, body mass, and the regression models generated here are reliable predictors of lipid mass in two of the three migrating species examined. The accuracy of the methods, in addition to their low cost and simplicity, justifies their continued use in field studies of birds.