Urban birds become less fearful following COVID-19 reopenings

Abstract

Following the COVID-19 pandemic, many people around the world stayed home, drastically altering human activity in cities. This exceptional moment provided researchers the opportunity to test how urban animals respond to human disturbance, in some cases testing fundamental questions on the mechanistic impact of urban behaviours on animal behaviour. However, at the end of this 'anthropause', human activity returned to cities. How might each of these strong shifts affect wildlife in the short and long term? We focused on fear response, a trait essential to tolerating urban life. We measured flight initiation distance-at both individual and population levels-for an urban bird before, during and after the anthropause to examine if birds experienced longer-term changes after a year and a half of lowered human presence. Dark-eyed juncos did not change fear levels during the anthropause, but they became drastically less fearful afterwards. These surprising and counterintuitive findings, made possible by following the behaviour of individuals over time, has led to a novel understanding that fear response can be driven by plasticity, yet not habituation-like processes. The pandemic-caused changes in human activity have shown that there is great complexity in how humans modify a behavioural trait fundamental to urban tolerance in animals.

Keywords: anthropause; behavioural plasticity; behavioural response; flight initiation distance; urban ecology.

Figure 1.

Population-level fearfulness remained unchanged during lockdowns but decreased following reopenings. Population-level flight initiation distance (FID) before (n = 71), during (n₂₀₂₀ = 135; n₂₀₂₁ = 131) and after (n = 67) the anthropause. The dark-eyed junco population at University of California Los Angeles (UCLA) did not shift their FID across the anthropause (GLMM contrast: p > 0.05). FID significantly dropped in the 2022 post-anthropause environment in comparison to both years in the anthropause and the pre-pandemic baseline (GLMM contrasts: p < 0.05 for pre-pandemic, 2020 anthropause, and 2021 anthropause compared to 2022 post-anthropause). FID data are log₁₀ transformed for visual aid, but not in the formal statistical analysis. Data points represent mean log₁₀ FID ± standard error for each time period assessed. Groups with the same letter are not statistically significantly different from each other. Groups with different letters are statistically significantly different.

Figure 2.

Individuals became less fearful after reopenings in comparison to before the pandemic closures. Individual shifts in flight initiation distance (FID) before, during and after anthropause. (a) FID measurements only for individuals that were tested repeatably for at least one time point in each period: before, during and after the campus closures (n = 11). Trends reflect a similar decrease in FID from before the pandemic to the end of the anthropause. Lines are fitted linearly to demonstrate the change from before to after the anthropause. Each line represents one individual. Dashed red vertical lines denote the beginning and end of the anthropause, respectively. A GLMM, only including birds tested across all time periods and accounting for potential habituation to the investigator (by including trial number), and treating the anthropause as a single category, produced similar results. Here, differences in pre-pandemic fearfulness compared with post-anthropause fearfulness were significant (p = 0.05) and differences in anthropause compared to post-anthropause fearfulness were significant (p = 0.02). (b) Mean FID values per individual (grey) in pre-pandemic, anthropause, and following reopening ‘post-anthropause' time periods. The thick black line represents shifts across all individuals sampled repeatedly between the pre-pandemic and anthropause (n = 33) or the anthropause and post-anthropause time periods (n = 24). These demonstrate pairwise shifts in mean fear response for each individual to account for individuals that might not have been tested in one of the time periods. A GLMM accounting for potential habituation (by including trial number), closest cover, time by month, and considering the anthropause as a single category, revealed significant pairwise differences between pre-pandemic and post-anthropause environments (p = 0.02) and between the anthropause and post-anthropause environments (p < 0.001). Though FIDs were not log₁₀ transformed in our analyses (given their gamma distribution), they are log₁₀ transformed here for visual ease.