http://www.eurekalert.org/pub_releases/2007-10/uonc-edc101607.php
Ecologists discover city is 'uber-forest' for big owls
Public release date: 16-Oct-2007
It may be news to its bankers, but Charlotte, the biggest city in North
Carolina and a major center of the American financial industry, is
actually an old growth forest.
At least that’s the way the barred owls see it. .
Charlotte is famous for having two kinds of green. It is home to two of
the nation’s largest banks and its downtown residential neighborhoods
and near-suburbs are also known for their lush yards and green streets,
lined with large trees. Less well-known is the fact that the city is
almost as well populated with large owls – particularly barred owls –
as it is with bankers. Harry Potter would feel very much at home.
In fact, the barred owl population in Charlotte is so strong that the
city was chosen to be the site for the most extensive barred owl
research study that has ever been attempted, with fieldwork going on in
the manicured front lawns and gardened back yards of urban and suburban
neighborhoods.
Urban wildlife numbers have been increasing in recent decades, notably
in populations of squirrels, Canada geese, raccoons and deer, but the
appearance of significant urban populations of barred owls, the third
largest owl species in the US, is a surprise to many biologists.
“If you read about barred owls in the textbooks, it says they need
large stands of old-growth forest to survive,” notes University of
North Carolina at Charlotte ecologist and ornithologist Rob
Bierregaard, who has directed the six-year-old research study. “Either
the barred owls in Charlotte haven’t read that book or the book is
wrong, because they are really here and apparently doing quite well.”
“We have concluded is that there may be a third possibility: that old
suburban neighborhoods in fact are an old growth forest, at least as
far as the barred owls are concerned.”
Bierregaard’s study has now found and monitored more than 200 nesting
attempts by 78 different pairs in both suburban Charlotte and the
surrounding countryside, but the project began when he first considered
doing a study of barn owls, which are common in farm country, as a
thesis project for a graduate student. A team of volunteers was
necessary to support the effort and, unfortunately, they all lived in
the city -- a long drive from the proposed rural study sites.
In order to accommodate the volunteers, the researchers pragmatically
changed the target species and put up nest boxes in the wooded suburban
neighborhood where the volunteers lived. Barred owls, they discovered,
were common there.
“Barred owls need old growth forest because they need trees big enough
to have holes to nest in,” Bierregaard noted. “They also need a pretty
open understory, because their hunting technique is to sit on a branch
and wait for something to move. If you have a young forest with a
really thick undergrowth, they are not going to be able to see enough
to hunt.
“When you look at suburban Charlotte, what do we have? We’ve got giant
old willow oak trees with plenty of holes in them and we’ve got mowed
lawns and azalea bushes, which is a very open understory, so they can
see a long way. The habitat is an ‘uber’ old growth forest for owls
because the understory is so open and there are plenty of birdfeeders
to attract prey.”
The research study, which began in 2001 and has been sponsored by the
Carolina Raptor Center, has been large-scale and in-depth, with
researchers monitoring about 40 nesting sites each year and tracking
many sets of young as they mature through attached radio transmitters.
Using radio telemetry, the team has mapped out a dozen or so owl
territories in south Charlotte, each of which is about 200 acres in
size. Locating the birds and their nests, normally a very difficult
task in wild forests, has been greatly simplified thanks to the
reporting of ordinary Charlotteans, who apparently love their city’s
owls.
“Probably half the nests we’ve found because someone either called in
to the Raptor Center and reported young on the ground or we’re
wandering around a neighborhood with a tape recorder playing owl calls
and someone will ask ‘what are you doing?’ I’ll explain and they’ll
say ‘there’s a pair three blocks over that way.’ Since they are so
vocal, you can’t be around a barred owl nest and not know it,”
Bierregaard said.
Though amused to see the biologists afield in their yards, the
Charlotte community seems to have eagerly embraced the project. “Pretty
much everybody knows us when they see us and the antennas,” he said.
“There aren’t many neighborhoods where we haven’t been.”
Public enthusiasm and interest aside, some very serious science is
going on in people’s backyards. One of the most important ecological
questions that the study is close to answering is the question of
whether or not the barred owls are really as successful in Charlotte as
they appear to be.
The answer to the question of whether or not the city’s many owls have
been able to be at home is not as obvious as it would seem on the
surface. For example, the cooper’s hawk, another raptor known to be
common in cities like Charlotte, has been shown not to be able to breed
successfully in the urban environment by a recent scientific study.
“Coopers hawks are drawn by the food,” Bierregaard notes. “Everyone’s
got birdfeeders up and a birdfeeder is just a two-step hawk feeding
platform. The local Coopers hawks have these kamikaze raids where they
will fly though a neighborhood at full speed and they will come around
a corner where they know there’s a birdfeeder and just see what flies
up in front of them. It is like they are trap-lining the local
birdfeeders.”
The visiting hawks are not as successful, however, when it comes to
nesting. Cities abound with pigeons and doves, which are good prey for
hawks but often carry a microbial parasite that is fatal to the hawks’
young. The urban environment thus creates an ecological condition known
as a “sink” – the area looks friendly to the birds but is really
causing a net loss to the overall hawk population. Birds are attracted
into the area but are not able to replace themselves in the next
generation.
The situation of barred owls in Charlotte is very different,
Bierregaard and his students believe. Though their findings are not
fully complete, the researchers have so far found that the urban barred
owls are able to reproduce effectively – perhaps significantly more
effectively than in wild forests – because their rate of reproduction
exceeds their rate of mortality. In the city, owl death tends to happen
either from disease or from cars, the owls’ most serious predator.
The researchers have attached miniaturized radio transmitters to young
owls. The ecologists then track the movements of the owls as they
mature and note where they finally settle among the network of
well-mapped territories and nesting sites, establishing a kind of
on-going community history of the owl population in south Charlotte.
“If you see an owl in south Charlotte, chances are we know it by name,”
said Bierregaard. “There’s a location in Lata Park, for example, that
has apparently had barred owls almost forever. But just since we’ve
been studying that pair, it has been replaced by a completely new pair.
Three years ago, the male died – he was replaced. The next year, the
female died – the male raised the young as a single dad – and then the
next year the female was replaced. If we didn’t have radios and know
those birds, nobody would have known that they were new birds. It’s
been amazing how quickly they are replaced.”
Bierregaard notes that Charlotte wasn’t always such prime owl habitat.
A hundred years ago when the city was much smaller, most of the current
residential area was farmland – open country with few trees that would
be suitable for barred owl nests. As the land was sold for residential
neighborhoods, trees were planted which eventually grew to old growth
forest size and, apparently, the owls moved in.
Now, as newer suburban continue to mature, the owl habitat is steadily
expanding. “As the farms have been abandoned, the new neighborhoods
that replaced them have planted trees,” he said. “If you wait long
enough, the barred owls are going to expand their territory, as the
trees start to grow up in the newer suburban neighborhoods.”
Other mysteries of urban owl life are beginning to emerge, including
the question of what it is that city birds most like to eat – squirrel
or cardinal, crawfish or koi? In the last couple of seasons the
researchers have installed video recorders in some of the nest boxes,
and Cori Cauble, one of Bierregaard’s graduate students, has been
researching a thesis on the owl’s food habits and how they compare to
owls in the wild.
Before the video cameras, the researchers had noted the prominence of
bird feathers in the nest boxes, but were unwilling to draw any
conclusions because they noted that feathers were more likely to be
left and preserved from kills than other kinds of remains. The videos
of owl home life answered the question.
“We scaled back our estimation on how important birds are in their diet
until the first day we had a video camera in a box: they brought in
eight prey items and four were birds. That result has held – for two
years we have had cameras in four or five different nests. They have
diverse diets, depending on territory,” he noted. “We have one nest we
call the ‘sushi box’ because they bring in so many fresh fish, but even
there the owls bring in a lot of birds.”
All-in-all, the researchers think a picture is emerging of barred owls
that are nearly as happy in cities as people are, though like the
humans, they hate and fear the traffic, and living space is at a
premium.
“The biggest source of mortality in an urban environment is flying into
cars,” Bierregaard noted. “We’ve had a couple die of diseases, but for
most of the birds that we have had tagged, where we know how they died,
they flew into a car. But it seems that mortality even from that isn’t
that high.
“It certainly seems that they are cranking out enough young to more
than make up the difference. We are getting to the point now where
young that we radio-tagged back in 2002 are having young. It’s neat to
watch how the young birds that we tagged wander around and find a spot
where there is a vacancy, where a bird has died. There are enough birds
floating around that when a bird dies, that spot is filled really
quickly.”
In the world of urban owls, it would appear, there is no downturn in
the real estate market.
###
To see further information about the south Charlotte barred owl study,
see the web pages at:
http://www.bioweb.uncc.edu/bierregaard/barred_owls.htm
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