Do parents defend their offspring whenever necessary, and do self-sacrificing parents really exist? Studies recognized that parent defence is dynamic, mainly depending on the threat predators pose. In this context, parental risk management should consider the threat to themselves and to their offspring. Consequently, the observed defence should be a composite of both risk components. Surprisingly, no study so far has determined the influence of these two threat components on parental decision rules. In a field experiment, we investigated parental risk taking in relation to the threat posed to themselves and their offspring. To disentangle the two threat components, we examined defence behaviours of parent blue tits Cyanistes caeruleus towards three different predators and during different nestling developmental stages. Nest defence strategies in terms of alarm call intensity and nearest predator approach differed between the three predators. Defence intensity was only partly explained by threat level. Most importantly, parental risk management varied in relation to their own, but not offspring risk. Parent defence investment was independent of nestling risk when parents followed a high-risk strategy. However, parents considered nestling as well as parental risk when following a low-risk strategy. Our findings could have general implications for the economy of risk management and decision-making strategies in living beings, including humans.
Keywords: altruistic behaviour; blue tits; dynamic defence behaviour; predator defence; threat.
© 2014 The Author(s) Published by the Royal Society. All rights reserved.