By TODD McLEISH/ecoRI News contributor
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| Watching my bird feeders, well, like a hawk. Photo by Will Collette |
Now, an analysis of 21 years of
data collected by Cornell University has confirmed those observations by noting
that Cooper’s and sharp-shinned hawks, which prey primarily on songbirds, have
been colonizing urban and suburban areas during winter because of the
availability of prey at bird feeders.
According
to Jennifer McCabe, a postdoctoral researcher at the University of
Wisconsin-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 because of legal
protections and the banning of particularly harmful pesticides — enabling the
birds to colonize areas that they had previously ignored.
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.”
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 such as the common red-tailed hawk can’t 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 didn’t 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.
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’s
likely that the accipiters that have colonized urban and suburban areas will
not go hungry.
The
impact the hawks are having on the population of common feeder birds such as
sparrows, chickadees, titmice, and nuthatches hasn’t been measured, but it’s
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 birds such as pigeons, starlings, and house sparrows, potentially easing competitive pressures on native species.
They may even receive a boost, since other studies have found that urban accipiters primarily target invasive birds such as 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.”
Rhode
Island resident and author Todd McLeish runs a wildlife blog.