UCLA

Urban areas often impose strong, novel selection pressures on wildlife. Phenotypic plasticity is an important mechanism helping enable organisms to colonize and establish populations in these novel environments. Phenotypic plasticity can be difficult to study in urban wildlife because many urban environmental variables are challenging to isolate and manipulate experimentally. The COVID-19 lockdowns created a natural experiment in which urban wildlife populations normally exposed to high levels of disturbance were released from stressors associated with humans. We took advantage of this to measure the territorial aggression responses of resident dark-eyed juncos (Junco hyemalis) on an urban college campus in in Los Angeles, USA. We assessed whether individual birds expressed aggression differently when relieved from frequent encounters with humans. We quantified their aggression using simulated territorial intrusions and compared measurements from the pre-pandemic year 2019 to those from the pandemic year 2021. We found that the population overall displayed significantly reduced aggression responses in 2021. Furthermore, individuals measured in both 2019 and 2021 showed significantly reduced aggression responses during 2021, demonstrating that individual birds maintain phenotypic plasticity in this trait. Our results show that human disturbance likely has a significant effect on the aggressive behavior of urban birds.