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

Abstract

Urban areas often impose strong, novel selection pressures on wildlife. Phenotypic plasticity is an important mechanism helping organisms establish populations in novel environments. Phenotypic 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 aggression differently when relieved from frequent encounters with humans. We measured the territorial aggression responses of resident dark-eyed juncos (Junco hyemalis) on an urban college campus in Los Angeles, USA. We found that the population overall displayed significantly reduced aggression in pandemic year 2021 compared to the typical year 2019. Furthermore, individuals measured in both 2019 and 2021 showed significantly reduced aggression 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.

Urban birds become less fearful following COVID-19 reopenings

“…Entering into COVID-19 lockdowns and reopenings, contrasting shifts in human activity—one increasing human activity and one decreasing human activity—led to contrasting fitness consequences reflecting this shift in the landscape of fear: great tits ( Parus major ) in an area with lower human activity had higher reproductive output than that with higher human activity 31 . However, there was no evidence for increased fitness by means of increased nestling condition and nest success when comparing 2021 and pre-pandemic 2019 breeding seasons in this population 24 . Additionally, there were no changes in aggressive interactions in the population following reopenings in 2022 and pre-pandemic 2019 breeding seasons 24 , suggesting that fear response is not a by-product of shifting behavioral strategies due to a different socioecological context or due to indirect effects on predator density during the anthropause.…”

“…However, there was no evidence for increased fitness by means of increased nestling condition and nest success when comparing 2021 and pre-pandemic 2019 breeding seasons in this population 24 . Additionally, there were no changes in aggressive interactions in the population following reopenings in 2022 and pre-pandemic 2019 breeding seasons 24 , suggesting that fear response is not a by-product of shifting behavioral strategies due to a different socioecological context or due to indirect effects on predator density during the anthropause. Urban song rapidly shifted during San Francisco’s lockdowns in a related species (white-crowned sparrows) potentially because there was a clear communicative signal being interrupted by urban stressors 2 .…”

“…UCLA remained remote until Fall 2021, when classes resumed on campus. Campus closures caused human activity to be approximately 7x lower than “normal” in 2021 24 , and therefore even lower during 2020 at the height of Los Angeles lockdowns and UCLA campus restrictions. We assessed the fear levels of individually identifiable birds using a flight initiation distance assay 25,26 repeatedly during the 2018-2022 breeding seasons (January-June/July).…”

Urban birds become less fearful following COVID-19 reopenings

“…Entering into COVID-19 lockdowns and reopenings, contrasting shifts in human activity—one increasing human activity and one decreasing human activity—led to contrasting fitness consequences reflecting this shift in the landscape of fear: great tits ( Parus major ) in an area with lower human activity had higher reproductive output than that with higher human activity [45]. However, there was no evidence for increased fitness by means of increased nestling condition and nest success when comparing 2021 and pre-pandemic 2019 breeding seasons in this dark-eyed junco population [25]. Additionally, there were no changes in aggressive interactions in the population following reopenings in 2022 and pre-pandemic 2019 breeding seasons [25], suggesting that fear response is not a by-product of shifting behavioural strategies due to a different socioecological context or due to indirect effects on predator density during the anthropause.…”

“…Each survey lasted two minutes and all individuals, vehicles, and dogs crossing the observer's eyeline were counted. Campus closures caused human activity to be approximately 7× lower than ‘normal' in 2021 [25], and therefore even lower during 2020 at the height of Los Angeles lockdowns and UCLA campus restrictions.…”