Fast food, slow birds – a trade-off between foraging and nest attendance modulates parental mobbing in a long-lived raptor

In birds, mobbing intensity in form of nest defence should depend on its cost-benefit-ratio for the parents, which is explained by two different hypotheses: While the “reproductive value of offspring” hypothesis predicts that parents increase their anti-predator investment with age or condition of the young (i.e. their reproductive value), the “harm to offspring” hypothesis assumes that parents take higher risks if their offspring is in bad shape or at young age when the harm of a lack of parental care will be highest. Feeding experiments were carried out to manipulate the value and vulnerability of the nestlings in selected breeding pairs of the red kite (Milvus milvus), and therefore, to change the costbenefit-ratio of parental mobbing. Research was performed during low- and high-rodent density seasons to consider fluctuations in natural food availability for this large raptor species. A live eagle owl (Bubo bubo) was exposed near red kite nests to trigger mobbing reactions in breeding pairs during catching attempts using a Dho-gaza net. Successful capture of parents was highly correlated to occurrence (φ = 0.82) and number (Cramer´s V = 0.79) of swoops they performed to the predator. Therefore, capturing success and time to capture was used to quantify this high-risk mobbing behaviour. Time to detection of the eagle owl by the breeding pair was considered to not bias the temporal patterns of mobbing. Food supplemented breeding pairs showed weaker mobbing response than non-fed pairs, but the effect was not significant. There was a high between seasonal variation in mobbing behaviour, which was related to fluctuating rodent densities, showing enhanced mobbing intensity when natural food availability was scarce. High temperatures had a negative effect on mobbing reactions and a temporary habituation to the eagle owl upon repeated exposure was observed. Time to detection of the owl was faster at a young offspring age, but age and number of the nestlings had no effect on mobbing. In unfavourable food situations, detection time took longer and was extended with increasing number of nestlings. These results reveal a trade-off between foraging and nest attendance, modulating parental mobbing intensity in red kites. Chasing off predators rapidly provides additional time to forage during low food conditions, and increased nest attendance during high food availability reduces risky mobbing. In this research, food availability changed the cost-benefit ratio of two different anti-predator strategies: Mobbing was in accordance with the harm to offspring hypothesis, while nest attendance corresponded to the reproductive value of the young. Both, active mobbing and passive nest attendance must be considered when drawing conclusions about parental investment in a long-lived raptor.