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. J. WILDL. MANAGE. 49(3):569-575 Body weight and nutrient reserves, which are often used to characterize condition, have been related to both survival (Lack 1966:276277) and breeding performance (Jones and Ward 1976) of birds (but see King and Murphy 1984). Assessing the condition of birds is therefore important in the study and management of bird populations (Bennett and Bolen 1978). Birds are capable of storing several nutrients for mobilization during critical periods of their life cycle. Although fat, protein, and Ca have each been identified as potentially limiting for breeding females (Ankney and Maclnnes 1978), we believe that fat is the most frequent limiting nutrient during the year because of its numerous functions, including lipid source for egg synthesis (Raveling 1979), energy source during migration (Odum et al. 1964, Blem 1980) and food deprivation (Hanson 1962), and as insulation (Evans and Smith 1975). Protein and Ca requirements are relatively small except during egg production, when a larger turnover of these nutrients occurs (Robbins 1981). Our purpose is to recommend a Lipid Index that represents fat stores of birds of various sizes and to indicate how the index can be estimated from measurements taken in the field on live birds. We also evaluate the performance of several published condition indices. The methods are illustrated with data on the sandhill crane and greater white-fronted goose. 1 Present address: Patuxent Wildlife Research Center, Room 509, 820 South Street, Vicksburg, MS 39180. 2 Present address: School of Forest Resources, Nutting Hall, University of Maine, Orono, ME 04469. We thank R. Atkins, C. M. Boise, C. R. Frith, B. A. Hanson, C. Jorgenson, T. C. Tacha, and P. A. Vohs for their assistance in obtaining specimens from various study areas. R. R. Campbell, J. R. King, J. Longmuir, and D. W. Sparling provided valuable comments on an earlier draft of the report. STUDY AREAS AND METHODS Study Areas All sandhill crane specimens were collected in 1978-79. From late February to mid-April, 119 were taken in the Platte River Valley of Nebraska. Additional samples included 28 taken in late April to early May near Last Mountain Lake in Saskatchewan, 20 taken during May or early June at Clarence Rhode National Wildlife Range in Alaska, 14 taken in late August or mid-October in central North Dakota, and 15 taken in mid-February near Muleshoe National Wildlife Refuge in Texas. Most greater white-fronted geese (49) were collected from late February to early April in 1979-80 near the Platte River or in the Rainwater Basin of Nebraska. Six others were taken in late April or early May of 1979 in the Last Mountain Lake area. Field and Laboratory Methods Specimens were weighed and measured at field laboratories. Measurements included (flattened) Wing, (diagonal) Tarsus, and Culmen (post nares). External features and gonads were examined to ascertain the sex and age (young of the year and older; Lewis 1979) of each bird. Subspecies were identified according to Johnson and Stewart (1973).