COVID lockdowns accidentally created an experiment scientists could never have deliberately run: What happens when the roads and buildings stay, but the people disappear?

In the spring of 2020, something happened that scientists could never have planned as an experiment. People suddenly vanished from streets, offices, highways, campuses and public spaces, but the cities themselves stayed where they were. The roads were still there. Buildings were still standing. Parks, campuses and highways had not moved. What changed was the amount of human activity happening around them.

That strange period, later called the "anthropause", gave researchers a rare chance to separate the effects of human presence from the effects of the environment humans had already built. And studies from the US, India and elsewhere have since shown that wildlife, birds and even the ground itself responded when people stopped moving around so much.

The city stayed the same, but wildlife noticed the difference

A new study from researchers at the University of California, Santa Barbara, the Smithsonian's National Zoo and Conservation Biology Institute and Yale University took that idea further.


Published in Science, the research compared GPS data from 37 animal species with anonymised cellphone location data across the US. The team examined 4,581 individual mammals and birds and found that animals were responding not just to roads, cities and other changes to their habitat, but also to how many people were actually around.

Overall, 57% of the species were affected by both human presence and landscape modification.


"Humans have complicated effects on wildlife -- from our physical presence to how we reshape habitats -- but we can't understand our full impact without information on both," said co-lead author Ruth Oliver, an assistant professor at UCSB's Bren School of Environmental Science & Management.

The finding matters because a road does not disappear when traffic stops using it. A city does not become a forest when its streets become empty. The lockdown effectively allowed researchers to watch what happened when one part of that human environment suddenly changed.

India saw birds moving into cities

A similar pattern appeared during India's lockdown.

Research by Raahil Madhok and Sumeet Gulati, published in Biological Conservation, looked at bird observations from India's 20 most populous cities. After accounting for birdwatching effort, researchers found a 16% increase in the number of species observed during the lockdown.

The change did not happen immediately. Species diversity increased around one to two weeks after human activity dropped, which the researchers said pointed towards a gradual recovery rather than simply people spotting more birds because they had more free time.

Reduced noise and air pollution were among the possible reasons. Some rare and at-risk species were also among those recorded during the period.

The silence could be measured underground too

The lockdown changed something humans normally do not notice: the amount of vibration produced by everyday activity.

A 2021 study published in Scientific Reports examined seismic recordings from 12 stations in Gujarat. Researchers found that COVID restrictions caused seismic noise to fall by 1–19 decibels, with daytime ground-motion displacement dropping by 27–79% in populated areas.

The strongest effects appeared around cities, industries and highways, showing just how much ordinary human movement contributes to the background "noise" of the planet.

A separate global study published in Science in 2020 found that seismic noise linked to human activity fell by as much as 50% in some places during the lockdown period.

But animals did not all respond in the same way

This is where the lockdown experiment gets more interesting.

Research on dark-eyed juncos at the UCLA campus found that an over 80% reduction in human activity during the pandemic modestly increased nesting success, but the birds did not change their preference for novel nesting sites. The researchers concluded that the built environment or urban predators may have been more important than human disturbance in determining where the birds nested.

Other research has found that urban juncos could respond physically to the sudden change in their surroundings. A 2025 PNAS study reported temporary changes in the birds' beak shape during the pandemic, with the changes reversing after human activity returned.

So there was no single "lockdown effect" on wildlife. Some species moved into quieter spaces, some changed their behaviour, and others barely changed their use of the urban environment.

The biggest lesson may be about people themselves

That is what makes the latest research important. Scientists have long measured the impact of humans through things such as roads, buildings, farming and deforestation. The lockdown showed that this may leave out another major factor: the physical presence of people.

"The cell phone data we used was made available to researchers during the pandemic to help reveal the impacts of COVID-19 shutdowns," said co-lead author Scott Yanco, a research ecologist at the Smithsonian's National Zoo. "Typically, private companies hold onto these, which made this a rare opportunity for us to quantify how human presence impacts wildlife, and to demonstrate that there is more to consider than just land modification to create robust conservation plans."

The "anthropause" therefore became much more than a strange moment when empty streets filled with unusual wildlife sightings. It became a real-world experiment that separated the city from the people using it, even if only for a short time.

And the results suggest that for some animals, simply giving them more space from humans can matter almost as much as changing the physical landscape itself.