Why does diurnal mass change not appear to affect the flight performance of alarmed birds?

To survive, animals need to trade-off gaining energy with the avoidance of predation. In small birds, this trade-off is thought to be mass dependent because increased mass is expected to reduce maximum flight performance during escape from predators. For large mass changes, flight performance during escape follows the inverse relation predicted by Newtonian physics but apparently not for small mass changes. I investigated three possible explanations for this apparent breach of Newtonian physics. In a study of free-living great tits, Parus major, I found no significant change in flight performance in relation to diurnal mass change. First, the novel design of the experiment rules out various potential confounding factors common to previous experiments as an explanation. Second, I used the work–energy theorem from physics to show that birds did the same amount of work in morning and evening escape flights. This indicates that the birds were maximizing their flight performance and rules out physiological compensation to increased mass as an explanation. Third, by calculating the evening flight performances predicted by Newtonian physics for each individual in my experiment, I show that the level of diurnal mass change present in both this and previous studies could not be expected to produce a statistically significant change in flight performance. The results suggest that an effect of diurnal mass gain on alarmed flight speed would be undetectable with the sample sizes used so far.