Urban Ecology

The Good Life for Birds

by James Hathaway

Some birds prefer the view from a high tree branch. Others like to sit on top of a cactus or along a power line. A team of Arizona State University scientists found that birds also prefer to live the high life in other ways, at least in the Phoenix metropolitan area.

For two years, ecologists Ann Kinzig and Paige Warren studied the birds living in small Phoenix parks. What they found was a bit surprising. They discovered that more birds and more types of birds actually live in the city than in the surrounding Sonoran Desert.

The real surprise came when Kinzig and Warren took a look at exactly where birds live in the city. In Phoenix, bird populations seem to prefer high-income neighborhoods. The scientists found that more species of birds live in wealthy neighborhoods than in middle and lower income areas.

The study was done as part of the Central Arizona-Phoenix Long Term Ecological Research Project (CAP-LTER). Supported by the National Science Foundation, CAP-LTER is an ongoing study of how humans interact with ecological systems here in the Sonoran Desert of central Arizona.

The ASU scientists studied 15 small community parks in Phoenix. The parks were located in a variety of residential areas. Some were found in lower income areas. Others were in very wealthy neighborhoods. Kinzig and Warren measured both the number and types of birds and trees living in the parks.

They chose to look at parks rather than residential yards for a reason. The parks provided comparable environments to study. Each site had similar landscape. Each had grass, athletic fields, playgrounds, and scattered trees.

Together, the scientists counted birds living in about 2,000 trees. These results also surprised them. They found that an average of 30 different species live in the trees in upper income parks. The average was spread across one year. That compares to the 23 species of birds they found living year round in middle income parks. In the lowest income area parks they found an average of only 18 bird species.

The findings are strange, but interesting.

“We can’t explain bird diversity in the parks by the size of the parks, or the types or sizes of trees in the parks,” says Kinzig. “That is what we might expect. Instead, the characteristics of the neighborhood, including the income of the residents, seem to play a significant role in influencing the number of species that live in the park.”

Trees and other vegetation are important to birds. They rely on trees for food and shelter. So, if you have lots of trees in a park, you should see lots more birds, right? Sounds logical. But the scientists actually found the opposite to be true.

Kinzig and Warren found more birds and more types of birds in upper income neighborhood parks. They found fewer birds and fewer types of birds in parks near middle and lower income neighborhoods. However, the amount and types of trees was actually highest in lower income neighborhood parks.

The study eliminated park landscaping as a factor. But the scientists have not yet found a good explanation for why birds like wealthy neighborhoods best.

“Something that happens near the park boundaries is influencing the diversity of birds inside the parks,” Kinzig says. “We can’t explain it with the park itself. The answer might be related to what people are planting in their yards. It could be how often people feed the birds. Maybe the rich people have more bird feeders.”

Other factors might be at work. The scientists say it could be something as small as the number of bird-eating cats and other predators that live in the neighborhoods.

“Or it could be zoning,” Kinzig says. What does the city plant along the median strips on near sidewalks? How much industrial activity is allowed near the park? There are lots of questions that need answers.

“We don’t know the exact answer,” she adds, “but we know it’s related to the differences in people’s lifestyle.”

The scientists want to look at other ecological factors that might be important.

“We still want to look at reproductive success in the parks. There may be something that’s really influencing reproduction and that has an influence on the bird community,” she says.

Kinzig and Warren plan to look at food sources during the breeding season. What do people plant in their yards that birds eat when they breed? Birds love to eat bugs. Maybe some neighborhoods are better than others for insects.

Maybe the answer is related to available water. Dog dishes are important water sources for birds. Maybe rich neighborhoods have more dog dishes and bird baths.

Kinzig and Warren and other ecologists have plenty of work to do. For the first time in human history, more people are living in urban environments. Parks now provide more and more people with their daily access to nature.

“We need to fundamentally learn how these neighborhoods differ from each other,” Kinzig adds. “We want to understand what kind of nature people will have in their yards, parks, and green spaces. We want to know what affects that nature.”