Causes and consequences of mass loss upon predator encounter: feeding interruption, stress or fit‐for‐flight?

1. Birds have been shown to lose mass upon predator encounters. This mass loss has generally been assumed to be caused by the feeding interruption the birds experience upon encountering the predator. However, birds may lose this mass because of predator stress and because they prepare themselves for flight (fit‐for‐flight). In this experiment the aim was to distinguish between effects of feeding interruptions and stress or fit‐for‐flight on the mass loss of Yellowhammers ( Emberiza citrinella L.) upon predator exposure. 2. When exposed to a 45‐min feeding interruption, the birds lost only a quarter of the mass they lost when they were moved to another room and exposed to a stuffed Sparrowhawk ( Accipiter nisus ) for 1 min at that beginning of the feeding interruption. This indicates that mass loss upon predator exposure is not just due to the feeding interruption birds experience upon encountering a predator, but is probably, to a large extent, due to both predator stress and fit‐for‐flight. 3. When the stuffed Sparrowhawk was replaced with a dummy (an opaque plastic bottle), mass loss upon exposure was similar to the loss in the Sparrowhawk treatment. This indicates that moving the birds to another room, which occurred in both these treatments, may to a large extent be the cause of the mass loss. 4. During the same day, the birds regained 92% of their losses. However, regaining those losses was partly postponed to the end of the day, which indicates that the birds faced a trade‐off between starvation and predation risk, and were able to respond to that trade‐off by altering their diurnal trajectory of mass increase. By postponing foraging to the end of the day, the birds decreased the mass‐dependent costs of predation risk.