The speed and agility with which small birds can take flight are crucial in determining whether or not they will survive an attack by a predator. Theoretical models of bird flight predict that these aspects of performance will be influenced by variations in body mass; recent influential models of optimal foraging routines and energy storage are based on the resulting assumption that predation risk is mass-dependent. Small birds show slight but very consistent changes in body mass during the course of a day, being lightest at dawn and heaviest at dusk. Here we show for the first time that this natural diurnal variation in mass has a highly significant and disproportionate impact on flight performance: individual zebra finches were over 30% slower at dusk than at dawn, despite being less than 7% heavier. These results are in close agreement with predictions from flight mechanical theory, but the consequent mass-dependent diurnal variation in predation risk is predicted to be far greater than previously thought.