Effects of exposure to predator models on fledging behaviour in blue tits

Abstract

Leaving the nest is a key transition in the life of altricial birds, whereby fledging decisions should depend on multiple factors, including the risk of predation. High postfledging predation risk may favour fledging at a more advanced stage of development, if more developed fledglings are better at escaping predation, or together with others. While comparative studies have highlighted the role of predation risk for between-species variation in the timing of fledging, drivers of within-species variation in fledging behaviour remain largely unknown. We presented owl models near blue tit, Cyanistes caeruleus, nests during the first half of the day throughout the fledging period to simulate an increased risk of postfledging predation. Using an automated monitoring system, we then recorded the precise fledging times of 595 nestlings from 105 nests (52 predator-treated, 53 control nests). Contrary to our predictions, the predator presentations did not affect the age at which nestlings fledged, the time of day of fledging or other aspects of fledging behaviour. The tendency to fledge together with siblings was affected, but the effect was in the opposite direction to that expected, with nestlings exposed to the predator treatment being more likely to fledge alone. Parents visited predator-treated nests less often, but this effect diminished over the course of the morning. We suggest several explanations for why the behavioural responses to the predation risk manipulation were generally limited.