Non-destructive monitoring of fat reserves via measurement of total body electrical conductivity (TOBEC) can provide vital information for understanding the dynamics of many physiological and ecological processes in nature. Patterns in energy storage and expenditure are most obvious in small homeothermic animals with rapid metabolic turnover and mass changes, but previous TOBEC analyzers are too large for these otherwise ideal subjects. I will use hummingbirds as subjects for validation studies of a new, small-chamber TOBEC. Their body masses are routinely monitored at territorial and feeder perches without significant interruption of daytime activity. They can be captured easily with mist-nets in natural habitats or at feeder-traps for TOBEC determination of net gain rate. I will examine a) body mass components that might distort "lean" nor "fat" values, b) effects of ambient temperature, c) effects of body size (2-8 g hummingbirds, 7-12 g warblers ) due to positioning and ratio of bird to chamber dimensions, and d) reliability of responses to a full range in body composition changes through daily and annual cycles (breeding, migration, winter). Key words: body size, energetics, fat, foraging, hypothermia, migratory birds, Selasphorus, Trochilidae, TOBEC, weight control
