The bald eagle reported hunting near the University Hospital
last week would have been unusual several years ago.
But now, city ornithologist Dea Armstrong to whom the bird
sightings are reported — says she’s not so surprised.
Birds of prey like the bald eagle — once rare enough to be
counted on the U.S. endangered species list — have rebounded
in the state and nationwide.
University students, faculty and staff can now expect to see
birds of prey even near campus, she said.
Armstrong said the recovery of these birds is due largely to the
1972 U.S. ban on the pesticide DDT, Armstrong said.
Once used for crop pest control, DDT causes birds of prey to lay
eggs whose shells are too thin, said ecology and evolutionary
biology Prof. David Mindell, co-curator of birds at the
University’s Museum of Zoology. Parent birds may
inadvertently crush these thin-shelled eggs while moving about the
Predatory birds are exposed to pesticides like DDT because they
eat smaller birds and fish, which in turn have eaten insects or
seeds contaminated with the pesticide.
“The pesticides get concentrated in birds of prey because
they’re at the top of the food chain,” Mindell
Birds of prey recover slowly from environmental shocks compared
to other birds, due to several biological factors, he added.
First, birds of prey breed slowly compared to songbirds.
Peregrine falcons, for instance, breed only once a year, while
smaller songbirds can lay two or more clutches of eggs per summer,
he said. Secondly, young birds of certain species are slow to reach
“For species like the bald eagle, they don’t breed
until they’re five or six (years old),” Mindell said.
Birds of prey also need larger territories than small birds and
tend to have lower population densities, he said, which further
slows their recovery.
Armstrong said, since the ban on DDT, “almost all kinds of
large birds at the top of the food chain are increasing in
numbers.” She added that in the Ann Arbor area, the
populations most positively affected by the ban are eagles,
Cooper’s hawks, peregrine falcons and osprey.
The Cooper’s hawk is now common even in the middle of
town, she said. “You’ll see Cooper’s hawks on
campus, on the Diag. It’s just a great place to see (them)
Hawk populations in Michigan took several years to recover after
the ban, said Michael Kielb, author of “The Birds of
Washtenaw County, Michigan.”
“The population of Cooper’s hawks at migration sites
has been rising since about the 1980s,” nearly 10 years after
the ban was put in place, he said. “(Now), we’re close
to reaching the carrying capacity of the town.”
Ann Arbor’s ability to support birds like the
Cooper’s hawk depends on the amount of available nesting
territory, Kielb said. The town’s current population of
Cooper’s hawks is about one nesting pair per square mile,
which he called a high density.
“With the park lands in Ann Arbor, I think they should be
pretty secure for awhile.”
Both Mindell and Kielb cited habitat destruction as a major
challenge still facing birds of prey.
“As woodlands get cut away, hawk numbers will go
down,” Kielb said. “They don’t like to get bugged
too much by people.”
In the spring, two to three pairs of Cooper’s hawks
usually nest in the Nichols Arboretum, he said, and one pair
regularly nests on North Campus near the University School of
The hawks usually hunt away from their nesting territory, and
can be found on the Diag year-round chasing sparrows, starlings and
“When you see something bop through campus and bang a bird
out of the air, that’s probably a Cooper’s,” he
said. Statewide, 403 nesting pairs of bald eagles were found in
2003, up from 83 in 1980, according to informal bird surveys.
Peregrine falcons remain more rare in the state at only a dozen
known breeding pairs, but can be found nesting as nearby as
The increase of birds of prey is good environmental news for
Michigan, Mindell said. “Birds of prey are good indicators of
environmental health,” he said. “If there are toxins in
the environment, often they will wind up in predators.”