Anti-predation behaviour during bird migration; the benefit of studying multiple behavioural dimensions

Predation and predation risk have recently been shown to have profound effects on bird migration, but we still know relatively little about how birds respond to predation risk en route and how this is translated into fundamental aspects of optimal migration. Here, we make the case that to understand the fitness consequences of foraging and anti-predation behaviour en route we cannot rely on single behaviour relationships but must take many aspects of behaviour into account, because of predation risk compensation. We show this in a case study of fat and vigilant birds feeding close to cover, which emphasises the importance and potential of predation risk compensation. Another reason for taking many aspects of behaviour into account is that different behaviours need not contribute equally to individual fitness. Birds faced with an increased predation risk during migration can compensate for increased predation risk in different ways. This implies that the adaptive value of a behavioural trait can still be ambiguous even if a survival cost can be correlated with particular behaviour where all other things are equal (e.g. in an experiment). That is because in natural systems there may frequently be many other ways for animals to compensate, because all other things are never equal, so that the particular behaviour can actually be of little consequence to individual fitness. In conclusion, when studying foraging decisions and anti-predation behaviour during stopover potential compensatory mechanisms should be incorporated. This knowledge is also critical for improving future models of optimal migration.