Abstract
Parental investment theory proposes two non-mutually exclusive
hypotheses to explain variation in anti-predator behaviour in relation
to the age of offspring: the “reproductive value of offspring”
hypothesis and the “harm to offspring” hypothesis. The relative
importance of the two factors underlying the hypotheses, reproductive
value and harm, may change depending on environmental conditions such as
food availability. To test the relative importance of the two hypotheses
under different food conditions, we conducted a supplementary feeding
experiment in red kite (Milvus milvus) breeding pairs and used a live
eagle owl (Bubo bubo) as decoy nest predator to trigger anti-predator
behaviour. We used time-to-capture in mist nets mounted next to the
decoy predator as proxy for mobbing intensity. Under natural food
conditions we found a nearly constant mobbing intensity throughout the
entire nestling period. However, under food-enhanced conditions mobbing
intensity was reduced in parents with young nestlings and increased in
parents with old nestlings. These results suggest greater importance of
the “reproductive value of offspring” hypothesis in situations of
favourable food availability. Moreover, mobbing intensity depended on
brood size and weather conditions. The results suggest that parental
anti-predator investment increases with the reproductive value of the
brood under favourable breeding conditions, but that this pattern is
adjusted to the current context, including the vulnerability of the
brood and environmental conditions.