Great tits, Parus major, increase vigilance time and reduce feeding effort during peaks of aircraft noise

Antipredator vigilance may take a significant amount of time, leading to a trade-off between foraging and vigilance. Studies show that this trade-off is modulated by the perception of predation risk faced by individuals. Anthropogenic noise can disturb acoustic communication by masking both direct predator detection and the perception of conspecific alarm calls. We would expect a decrease in acoustic awareness to be compensated for by an increase in visual awareness. Thus, we tested whether increases in noise produced by commercial aircraft reduced foraging effort and increased vigilance time in great tits. We videotaped birds feeding on peanut feeders in the vicinity of Barajas airport (Madrid, Spain), and measured behavioural sequences before, during and after aircraft sound events. Our results show that, when aircraft noise peaked, the proportion of time devoted to vigilance was maximal, while that devoted to feeding was minimal. As a result, the ratio between vigilance and foraging was at its maximum when aircraft noise was loudest, being almost double that during baseline levels. The duration of vigilance episodes was strongly correlated with noise levels. Also, the duration of feeding bouts was at its lowest during the peak of noise, and increased after this period. We suggest that these behavioural modifications help foragers visually detect possible predators in those situations in which high levels of noise hamper sound perception. Our study suggests that behavioural plasticity could contribute to the resilience of avian populations exposed to anthropogenic disturbance.