A Lipid Index, the ratio of fat to fat-free dry weight, is proposed as a measure of fat stores in birds. The estimation of the index from field measurements of live birds is illustrated with data on the sandhill crane (Grus canadensis) and greater white-fronted goose (Anser albifrons). Of the various methods of assessing fat stores, lipid extraction is the most accurate but also the most involved. Water extraction is a simpler laboratory method that provides a good index to fat and can be calibrated to serve as an estimator. Body weight itself is often inadequate as a condition index, but scaling by morphological measurements can markedly improve its value.