DIURNAL-VARIATION IN-FLIGHT PERFORMANCE AND HENCE POTENTIAL PREDATION RISK IN SMALL BIRDS

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.