Diurnal mass gain strategies and perceived predation risk in the great tit Parus major

1. Individual animals optimize their body mass to enhance fitness and mass is expected to be optimized over both short and long time-scales. On the short time-scale small birds increase mass as they build up energy reserves over each day to avoid starving at night. Theory predicts that starvation risk should be minimized by gaining mass at the start of the day thus insuring against unpredictable food supply later. However, if predation risk is mass-dependent due to reduced escape flight performance, birds should delay mass gain until later in the day to minimize predation risk. 2. Regulation of mass change over longer time-scales has been well documented and a number of studies have been able to show that over weeks, months and years mass is regulated consistent with the starvation-predation risk trade-off being mass-dependent. However, it is crucial to mass-dependent predation theory that it functions over shorter time-scales as well and that birds are able to regulate their diurnal mass gain strategies in response to predation risk. This has not yet been investigated and recent studies of flight performance have been unable to show that small-scale mass change (< 10%) over short time-scales (e.g. diurnal mass change) affects escape flight performance as predicted. 3. We used a unique design of automated identification and weighing system that for the first time allowed the diurnal mass gain of individual great tits Parus major to be monitored remotely in the wild. While automatically tracking diurnal body mass changes of individuals we manipulated perceived predation risk by using model sparrowhawks. We were thus able to investigate experimentally the effect of increased perceived predation risk on diurnal mass gain patterns. 4. We show, for the first time, that birds are capable of manipulating their diurnal body mass gain strategy in response to heightened predation risk and that they do so by delaying mass gain until later in the day, as predicted by mass-dependent predation risk theory. Furthermore, the results are consistent with changing flight performance rather than changing exposure time to predators being the driving force for mass-dependent predation risk.