The anthropause as a behavioral reset: persistent reduction in territorial aggression in an urban songbird

Cities are evolutionarily novel environments that can impose unique stressors on wildlife. The period of movement restrictions during the COVID-19 pandemic, dubbed the “anthropause,” temporarily decoupled human activity from urban landscape stressors, creating a natural experiment otherwise impossible to replicate. We investigated whether the post-anthropause increase of human activity affected territorial behavior in urban dark-eyed juncos (Junco hyemalis), as we previously observed that territorial aggression decreased during the anthropause. In this longitudinal study, we measured post-anthropause (2022–2024) intraspecific territorial aggression in resident dark-eyed juncos in Los Angeles, USA, and compared responses to measurements from before (2019) and during (2021) the anthropause. We found population-wide territorial aggression has remained significantly lower than the pre-anthropause baseline for three years post-anthropause. Furthermore, because individual birds were uniquely color-banded, we found differences between cohorts that would have otherwise been masked: younger individuals who began breeding during the anthropause (2020–2021) increased their aggression post-anthropause (2022) while older individuals with pre-anthropause experience did not. We suggest that the anthropause acted as a behavioral reset, allowing the population to transition away from a potentially maladaptive, high-aggression phenotype. Additionally, we show that behavioral plasticity appears to be key in facilitating rapid responses to temporary environmental shifts, and that flexibility in territorial aggression may be informed by prior life experiences.