We investigated daily changes in body mass, fat reserves and crop contents, and diurnal organization of behaviour, in the zebra finch, Taeniopygia guttata, in relation to experimental manipulations of food availability. Diurnal mass change and the organization of foraging behaviour during the day were in general agreement with recent theoretical predictions. Foraging intensity, and hence rate of mass gain, was most rapid immediately after dawn and before dusk. The experimental birds did not alter either mean body mass or their diurnal mass trajectory after a period of 2 weeks when food was made unavailable for 2 h a day at unpredictable times. Instead, they changed their allocation of time to different activities during the day, decreasing the mean amount of time spent locomoting and increasing the mean amount of time spent inactive over the day. Thus, contrary to a number of recent studies on different species, zebra finches appear to respond to unpredictable interruptions in food supply by reducing energetically expensive activities rather than adjusting their levels of energy reserves. Copyright 1998 The Association for the Study of Animal Behaviour Copyright 1998 The Association for the Study of Animal Behaviour.