Behavioural plasticity and the anthropause: an urban bird becomes less aggressive

Urban areas often impose strong, novel selection pressures on wildlife. Phenotypic plasticity, including behavioural plasticity, is an important mechanism helping organisms establish populations in novel environments. Behavioural plasticity can be difficult to study in urban wildlife because many urban environmental variables are challenging to isolate and manipulate experimentally. We took advantage of the COVID-19 lockdowns to assess whether urban birds expressed territorial aggression differently when relieved from frequent encounters with humans. We used simulated territorial intrusions to measure the behavioural responses of resident dark-eyed juncos, Junco hyemalis, on an urban college campus in Los Angeles, U.S.A. We found that the population overall displayed significantly reduced movement and singing behaviour associated with territorial aggression in a pandemic year (2021) compared to a typical year (2019). Furthermore, individuals measured in both 2019 and 2021 had significantly reduced responses in 2021, demonstrating that individual birds maintained behavioural plasticity in these traits. Our results show that human disturbance likely has a significant effect on the expression of behaviours associated with territorial aggression in urban birds.