Urban parks and other green spaces are a welcome refuge from our busy lives, especially now. And research suggests that being outside in nature no matter how wild provides important benefits to our mental and physical well-being.
Green spaces also provide important places for birds to take refuge, but not all green spaces are the same. In a study out this week in Landscape and Urban Planning, Frank La Sorte, a research associate at the Cornell Lab of Ornithology, and colleagues wanted to find out what aspects of green spaces support the greatest number of bird species throughout the year.
Green spaces differ in all sorts of ways; some have a lot of trees while others have fewer, some are long and skinny, while others take up just one city block. The size and shape of a green space can affect what species are present due to what scientists call edge effects. Smaller parks have more area exposed to non-green space borders or edges that allow predators such as feral cats or invasive plant species to sneak in, which can make that green space less ideal for some species. Green spaces that are farther apart also might make it more challenging for birds to navigate the dangers of city life, such as buildings, cars, and predators. If green spaces are closer together one might expect there to be more birds.
To determine what features of a green space support more birds, La Sorte and his team used land cover data for 1481 urban green spaces in New York City (NYC). They calculated the size and shape of each park, tallied the amount of canopy cover, and calculated the distance to the nearest park. They paired land cover data from 102 green spaces with bird observations from eBird, the world’s largest biodiversity community science database, to better understand which types of green spaces held more species of birds at different times of the year. “Because observations of birds reported in eBird are year round, we could look at patterns of bird occurrence throughout the entire year, unlike previous studies that only looked at one season,” says La Sorte. eBird data also allowed the team to be certain that all of the species present at each site were included in the analysis, what La Sorte calls completeness.
To their surprise the shape of the green space didn’t change how many bird species were present during the year nor did the distance between green spaces. What mattered most was the size of the green space. Larger green spaces supported a greater number of species year-round. Green spaces with more tree cover also supported more songbird species that migrate at night in the spring.
So what does this mean for urban planners in NYC? “If you want to support birds in urban green spaces,” says La Sorte, “you should make them larger and plant more trees.”
“What’s unique about our study is that we were able to make quantitative predictions that can help city planners know what will happen if they change the size of an urban park,” says La Sorte. According to the study, if planners in NYC increase the size of an urban green space by 50%, the number of bird species should also increase within each season by 8%; that’s a lot more birds.
Visit the top 5 green spaces in NYC with the greatest number of bird species throughout the year, but don’t forget to visit the lesser known parks too.
Although La Sorte and his team found that large urban green spaces support a greater number of bird species, they warn that other factors such as increased risk of collision with buildings, and a greater number of predators such as cats can mean that urban areas might be risky for birds. But La Sorte adds, “birds are attracted to urban areas during migration by artificial light, so if they are going to survive, they need urban green spaces to refuel.” Even in small urban green spaces, La Sorte says, “you do find birds, suggesting that birds are being forced to stop there to refuel,” so it’s good to have places in urban areas for birds to take refuge.
La Sorte hopes the study will compel ornithologists to conduct more detailed studies of urban green spaces and how they can help migrants. And that’s where eBirders can help. “It’s important to go birding along the full urban to suburban gradient, including from areas that may have very little greenery,” says Chris Wood, eBird project director. “Even if you are just stopped for a coffee and only see House Sparrows and a Feral Pigeon, that is still very important to know, because it helps us understand the areas that are being avoided,” says Wood. “I think in the next few years we will see some very interesting work to explore the interaction between climate change and direct changes to the landscape,” says Wood.
La Sorte, F. A., M. F. J. Aronson, C. A. Lepczyk, and K. G. Horton. (2020). Area is the primary correlate of annual and seasonal patterns of avian species richness in urban green spaces. Landscape and Urban Planning 203:103892.