According to Jennifer McCabe, a postdoctoral researcher at the University of Wisconsin in Madison, whose study focused on birds in the Chicago area, many hawk species had declined
significantly by the middle of the 20th century because of hunting
and pesticide use. Populations of most hawks, including the Cooper’s and
sharp-shinned, have rebounded since then – largely due to legal protections and
the banning of particularly harmful pesticides – enabling the birds to colonize
areas that they had previously ignored.
|Cooper's Hawk (stock)|
In a research paper published in November in the Proceedings of the Royal Society B, McCabe found that the two hawk species, which look similar and are collectively called Accipiters for their genus name, occupied about 26 percent of the area in and around Chicago in the 1990s. Two decades later they were found in nearly 67 percent of the area.
Birders in Rhode Island have also reported anecdotal evidence of an increase in Accipiter numbers in recent decades, especially Cooper’s hawks. Rachel Farrell, a member of the Rhode Island Avian Records Committee, has noted several Cooper’s hawks nesting in Providence in recent years, and she calls their presence at feeders in winter “commonplace, unremarkable, and therefore not generally reported [any more] from suburban areas.”
“In the beginning years of our study, sites were occupied around the fringe of the city, and through time they moved into the inner city,” said McCabe of her study site in Chicago. “The main driver for this colonization is prey abundance. They seem to be cuing in on feeders that have a lot of birds. That’s the driver that keeps the hawks there – prey abundance at feeders.”
Her findings were initially counterintuitive, because Accipiters nest in forested habitats. Their narrow wings and long tail enable them to maneuver quickly through densely forested landscapes and chase down small birds, a behavior the larger soaring hawks like the common red-tailed hawk cannot do. The soaring hawks typically feed on slower-moving rodents.
“We did our study in winter, so the birds weren’t concerned about finding the perfect tree for nesting,” McCabe said. “They were more concerned about survival.”
The relative absence of tree cover in urban areas and the abundance of pavement and other impervious surfaces did not seem to discourage the hawks from colonizing cities, she said. In fact, the more tree cover a site had, the less likely it was to attract Accipiters in winter. The key factor was prey availability. As long as there were bird feeders attracting an abundance of small songbirds to the area, the hawks moved in.
The data for the study comes from Project Feederwatch, a citizen science project in which participants periodically count the birds and bird species at their feeders. Sponsored by the Cornell Lab of Ornithology and Bird Studies Canada, the program began in 1987 and now includes more than 20,000 volunteers from across North America.
Since bird feeding is among the most popular pastimes in the United States, with some surveys finding that more than 40 percent of households participate, it is likely that the Accipiters that have colonized urban and suburban areas will not go hungry.
The effect the hawks are having on the population of common feeder birds like sparrows, chickadees, titmice and nuthatches has not been measured, but it is unlikely they will be impacted in the long term. They may even receive a boost, since other studies have found that urban Accipiters primarily target invasive city birds like pigeons, starlings and house sparrows, potentially easing competitive pressures on native species.
A study of the recolonization of Britain by sparrowhawks, which also feed on birds, provides additional insights. When sparrowhawks were extirpated from Britain, it became less necessary for their primary prey – house sparrows – to be vigilant for the predators.
“Over 30 years, they lost this anti-predator behavior,” McCabe said, “and when the hawks came back, they ended up decimating the house sparrow population.”
Whether North American feeder birds’ vigilance for predators declined following the eradication of hawk populations half a century ago is uncertain. But even if they did, it’s not likely to last long.
“If the birds lost their anti-predator behavior, they’ll regain it pretty quickly now that the hawks are back,” McCabe said. “People’s backyards won’t be picked clean by hawks.”
This article first appeared on EcoRI.org on February 2, 2019.