Measuring the individual body conditions of living birds is essential for understanding tradeoffs in life-history studies, especially in long-lived species. The aim of this study was to further develop a method for measuring body condition (lipid content), without killing birds. In theory, the lipid content mass in the body should have a close inverse relationship with water mass. Using this relationship, body condition can be evaluated without killing because the amount of water in the body can be estimated in live birds. However, the inverse relationship has only been shown to exist in a few studies of bird species. In this study, significant inverse relationships between water and lipid contents of three seabird species, Leach's Storm-Petrel Oceanodroma leucorhoa, Rhinoceros Auklet Cerorhinca monocerata and Black-tailed Gull Lams crassirostris, were shown. These facts highlight that body condition (lipid content) , can be estimated from the water content of the body.