Here, we present results from two foraging studies with the bumblebee Bombu
s terrestris L. We used the doubly labelled water technique which makes it
possible to estimate energetic costs of free flight in a natural environmen
t. One study was carried out in windy weather in the open, the second in a
large greenhouse under controlled conditions. Individual mass-specific meta
bolic rates from the open-air study varied widely (159.5 to 750.2 W kg(-1);
n = 7) and differed significantly from expected values predicted from labo
ratory data. Results from the greenhouse study were closer to expectations
and the range of individual metabolic rates was much narrower (405.0 to 485
.5 W kg(-1); n = 8). The range of field metabolic rates was three to four t
imes larger than that of laboratory studies (Heinrich, 1975; Ellington et a
l., 1990; Cooper, 1993), which might reflect the fact that only a relativel
y small percentage (25-30%) of bees are 'co-operative' in laboratory flight
cost measurements (Hanauer-Thieser and Nachtigall, 1995; C.P. Ellington an
d T.J. Wolf, pers. observations). However, all bees flew in the field studi
es, even in strong winds. We conclude that estimates of energetic costs of
free flight using laboratory data (time/activity/laboratory = TAL), an appr
oach regularly used in tests of foraging models, might not be appropriate u
nder many natural conditions. Whereas the TAL method provided a good estima
te of mean metabolic rate it is a poor method for studies in which individu
al variations are of interest.