The fat reserves of small birds are built up daily as insurance against starvation. They are believed to reflect a trade–off between the risks of starvation and predation such that in situations of high predation risk birds are expected either to reduce their fat reserves in response to mass–dependent predation risk or to increase them in response to foraging interruptions. We assessed the effect on fat reserves of experimentally altering the perceived (but not the actual) risk of predation of wild great tits at a winter feeding site. The perceived predation risk was alternated between ‘safe’ and ‘risky’. Increasing the perceived risk of predation involved ‘swooping’ a model sparrowhawk over the feeder at four unpredictable times each day, using a remote mechanism. We produce evidence that the experiment was successful in altering the perceived risk of predation. As predicted from the hypothesis of mass–dependent predation risk, great tits (Parus major) carried significantly reduced fat reserves during the ‘risky’ treatment. Furthermore, dominant individuals were able to reduce their reserves more than subordinates. As birds returned to feeders within seconds after a predator ‘attack’, the reduction in fat reserves cannot be attributed to an interruption in feeding.