Many people put up bird feeders in hopes of attracting avian wildlife. It turns out those backyard birds are attracting even bigger birds.
As birds come to cities for the feeders, woodland hawks are flocking to the "urban buffet" they create, according to a study conducted by researchers at the University of Wisconsin-Madison. The hunting is so good, in fact, that many hawks are now city-bred.
"For hawks, the secret is out: There is a hyperabundance of prey" in the city, Benjamin Zuckerberg, a University of Wisconsin–Madison professor of wildlife ecology and a senior author of the study, said in a statement.
A new concrete perch
In the past, hawks struggled to survive as habitat loss, hunting and the pesticide DDT reduced their populations. Eventually regulations were put into place, including stronger protections for migratory birds, and hawks staged somewhat of a comeback. Habitat loss, however, wasn't easily undone, and as the woodland hawks' population rebounded, they had to find new hunting grounds. Luckily, cities and bird-loving humans provided some assistance.
"Bird feeders are like buffets," Zuckerberg said, "It is an easy meal."
Researchers looked at 20 years of data collected by participants in the Cornell Lab of Ornithology's Project FeederWatch. This citizen science project covered backyard birding information in Chicago from 1996 to 2016. What they found was a steady increase in the hawk population in the city's center, flying away from rural areas.
"Project FeederWatch is the perfect program for this kind of research because you can use that information not only to document hawks, but also their prey," Zuckerberg said.
The researchers published their findings in the Proceedings of the Royal Society B.
Hawks find plenty of good perching spots in the city. (Photo: p611938/Flickr)
Two things surprised the researchers as they studied the data. The first was that the birds seemingly adapted to life in the big city quickly. Woodland hawks, like the Cooper's hawk (Accipiter cooperii) and the sharp-shinned hawk (Accipiter striatus), are considered "perch-and-scan" predators. They sit still on a branch, hiding in tree cover, and then swoop onto their prey once it comes within striking distance. Branches, it turns out, weren't a deal breaker for these hawks; food was.
"I was surprised that tree canopy cover was not important in colonization by these woodland hawks," Jennifer McCabe, a postdoctoral fellow at Wisconsin-Madison who led the study, said. "However, they aren't nesting in the winter, meaning they are more concerned about their own survival and not raising young. So, it makes sense that food availability would be so important."
The second surprise was related to food availability. The hawks didn't seem to care how large or small the prey was. They just wanted a bird snack.
"Prey biomass wasn't an important driver of colonization or persistence," McCabe explained. "Much of the literature states, at least for Cooper's hawks, that they prefer larger-bodied prey like doves and pigeons. Perhaps these hawks are cueing in on the sheer number of birds and not particular species."
The biggest takeaway is that urban areas are now an important wildlife habitat, a place where nature has adapted to urban life.
"Don't discount urban areas as habitat," Zuckerberg said. "The more we know about which species and what landscape factors allow those species to colonize and persist in urban areas, the better we can manage wildlife in an ever-developing world."