City life doesn’t stress burrowing owls out

Over the past several decades, as human settlements sprawl and natural habitats shrink, wildlife species have been moving to cities. Now, scientists are going beyond observing and documenting these new arrivals to figure out what makes urban wildlife tick at a physiological level.

You might think, for example, that humans, traffic, and pollution in cities would cause increased stress for wild creatures. But that’s not the case for burrowing owls living in Bahía Blanca, Argentina, according to a new study.

A team of Spanish and Argentine researchers studied burrowing owls (Athene cunicularia) nesting in the city, which has 300,000 human inhabitants, as well as in rural grasslands, pastures, and grain fields nearby. The robin-sized owls have colonized Bahía Blanca over the past few decades and are now even more abundant there than in surrounding rural areas.

The researchers walked up to 121 owls resting near the entrance to their burrows and recorded how close they could get before the owls flew away, a measure known as the flight initiation distance. According to their paper published 8 September in Scientific Reports, urban owls allow humans to approach closer than rural owls do, a finding that is both intuitive and confirms earlier studies.

They also captured and banded 183 adult birds to track their survival over time, finding that urban owls have twice the survival rate of their rural cousins. This pattern is often seen among birds, due to the scarcity of predators in the city.

Finally, the researchers plucked a tail feather from each banded bird and tested the level of corticosterone, a stress hormone. Past studies of urban birds have measured stress hormones in blood, which only reflect an animal’s current physiological state. The researchers say feather corticosterone is a better measure of how stressful urban life is for the birds because it captures the amount of stress experienced over a period of several weeks while the feather was growing.

We humans may long to escape the pressure and fast pace of urban life, but it turns out that living in the city is not a sustained, consistent source of stress for burrowing owls. The researchers found that urban and rural owls have the same level of corticosterone in their feathers.

They also found no link between corticosterone and flight initiation distance, meaning that letting people get close to them doesn’t result in increased stress for urban owls.

Instead, the researchers believe that individual birds that intrinsically have less fear of humans to begin with are the ones that tend to colonize cities.

That hypothesis also fits with previous studies showing that a particular bird’s flight initiation distance is consistent over time in both urban and rural habitats. In other words, it’s not that urban birds delay flying off because they’ve gotten used to the presence of humans; flight initiation distance is a matter of temperament or personality.

It’s not yet clear whether the relationships between stress, habitat, and behavior seen in burrowing owls apply more broadly to other urban species. But in the meantime, the study hints at some interesting questions about the nature of humans and of owls.

On one hand, the finding that city life isn’t stressful for burrowing owls is welcome news. Maybe to the owls, humans are just another species of urban wildlife that happens to share their environment and poses little direct threat.

On the other hand, thinking about our indirect effects is a bit discomfiting. If urban habitat selects for individual animals with less fear of humans, then as our cities expand we may be subtly shaping the characteristics of certain species, taming them without really intending to. — Sarah DeWeerdt | September 22, 2015

Source: Rebolo-Ifran N. et al. “Links between fear of humans, stress and survival support a non-random distribution of birds among urban and rural habitats.” Scientific Reports DOI: 10.1038/srep13273

Header image: A burrowing owl in an urban environment. Credit: Natalia Rebolo.