TOTAL-BODY WATER AND BODY-COMPOSITION IN PHALAROPES AND OTHER BIRDS

Contrary to some reviews, total body water (TBW) may fall below the expected value of about 65% in birds if measured as a percentage of total body mass. However, water constitutes a relatively stable proportion (65%-70%) of body composition when measured as a percentage of lean body mass (LBM). We demonstrate this by using data from two migratory phalaropes (Phalaropus tricolor and Phalaropus lobatus), which exhibit inverse relationships between TBW and body mass and between TBW and fat content. As the phalaropes fatten before migration, lipids increase but not at the expense of water, which also increases with gains in lean tissue. The gain in lean dry mass with body size is also correlated with migration. Because absolute water increases with body mass, TBW is a poor measure of hydration; however, because of the inverse relationship between TBW and fat, TBW can predict body fat accurately if a calibration curve is first generated. We provide equations that predict fat content in both species of phalaropes. We suggest that water normally be reported not as TBW (a percentage of total body mass) but preferably as "lean" body water (LBW, a percentage of lean body mass). In the absence of quantitative estimates of fat, we suggest that water be communicated in absolute amount, preferably with some estimate of condition (fat).