A severe limitation in studies of avian ecological energetics is the l
ack of an accurate, noninvasive technique for determining whole body f
at storage in living birds. We explored a technique of assaying total
body potassium as a predictor of lean mass (LM) and then derived fat m
ass (FM) by subtracting LM from total body mass. Body potassium (K), p
resent in lean tissue but not in fat, was estimated noninvasively from
naturally-occurring radioactive K-40, which Occurs as a fixed ratio t
o total body K. We assayed 29 pigeon (Columba livia) carcasses for K-4
0 and then measured LM from body composition analyses in which fat mas
s was extracted using petroleum ether. The K-40 results were regressed
against LM using five different combinations of independent variables
. Regression equations were tested by comparing predicted LM (and FM p
redictions by subtraction from body mass) to measured LM values obtain
ed from a separate group of pigeons. Whole body assay of K-40 was not
a useful predictor of LM in pigeons (r(2) = 0.51; mean absolute error
was 14 +/- 7%). Absolute errors increased with FM predictions (96 +/-
50%). Adding body mass as an independent variable increased the r(2) t
o 0.97, but body mass alone explained 96% of the variability in LM.