City hawks flock to feeders to find prey

Bustling cities like Chicago are not usually known for their wildlife, but Cooper’s (Accipiter cooperii) and sharp-shinned hawks (Accipiter striatus) have been establishing populations in the city. In Chicago, researchers say, they’re drawn to backyard bird feeders — or at least, they’re drawn to prey on the birds that feed at them.

“People have seen hawks in urban areas a lot for the last 20 to 30 years,” said Jennifer McCabe, a postdoctoral research associate at the University of Wisconsin-Madison and lead author of the recent study published in Proceedings of the Royal Society B. “People haven’t looked at what’s allowing them to colonize or stay there.”

In the study, a research team took advantage of a citizen science dataset, Cornell Lab of Ornithology’s Project FeederWatch, to determine the extent to which hawks were flocking to feeders and their reasons for doing so, including food availability. The citizen scientists who were involved followed a protocol, recording birds that showed up at their feeders and taking note of how long they were watching.

McCabe and her colleagues originally thought the hawks would be driven by tree cover in urban and suburban areas. In past research in Europe, afforestation in suburban and urban areas was correlated with hawk colonization. “We figured that would happen here, but it actually didn’t,” McCabe said. Remotely sensed data ruled out this reason for hawks showing up in Chicago and even showed a negative relationship between tree canopy cover.

Instead, based on wintertime FeederWatch data, the team determined that the hawks were mainly driven by food sources. While the hawks need trees for nesting, they weren’t nesting during the winter. “They were more worried about their own survival, and food is more important than nest site,” she said. “They can persist and live in areas where they don’t have as many trees, and we found food is the big driver for where they move and stay.” McCabe thinks that this was the case because in the winter, the birds are more concerned about survival than nesting.

The team also found that it didn’t matter what species were at feeders but simply the abundance of prey that was there. “They were cueing in on the areas of high bird abundance,” she said. “This was not only allowing them to move in, but it correlated with having them stay.”

The team also didn’t look at what’s happening to hawks’ prey sources, McCabe said. One of the next research steps will be to look at prey composition and if that changes with the presence of hawks, she said.

McCabe said citizen science data are important for this as well as other projects for collecting long-term data covering large spatial scales that researchers wouldn’t be able to collect on their own.