September 30, 2013 at 6:45 pm #427MikeKeymaster
Long-term effects on birds uncertain
Sunday March 28, 2004
By Mark Schleifstein
A collection of crows is correctly called a murder. But scientists
across the nation are worrying that the moniker has begun to fit too
The American crow, that large black bird heard cawing in New Orleans
neighborhoods, is again turning up dead in back yards and on streets
and sidewalks, the victim of West Nile virus.
Recent research shows that in some areas of the country, West Nile
epidemics have destroyed hundreds, maybe even thousands, of the
But scientists say it is unclear whether the disease will have long-
term effects on crow populations, and the effects on other birds also
One study published in the April edition of Emerging Infectious
Diseases, a magazine produced by the Centers for Disease Control and
Prevention, documents the effects of the virus on 156 crows.
Taking the bait
Studying crows has its challenges, said graduate researcher Sarah
Yaremych of the University of Illinois.
“Our first objective was to get our hands on a lot of crows from the
area, and the best way to do that is by trapping,” she said.
It is a laborious process. Large wooden cages with chicken-wire sides
are placed in areas where crows normally congregate, with doughnuts,
Cheetos or bread used as bait.
“They hop down into the trap and eat the bait, but their wingspan is
too big for them to get out,” she said.
After they were tagged with tiny transmitters, the researchers
tracked them from Yaremych’s Jeep Cherokee.
“We found they’d move around so much in a daily pattern, and were
able to identify what they feed on and where they sleep.
“You could tell after a while that the crows were used to our
presence,” she said. “But then, through the course of us being out so
much with them, we began finding crows that had died.”
Within a year, 64 percent of the crows were dead. Tests confirmed
that the crows were dying of West Nile virus, and Yaremych said
researchers suspect the northern house mosquito was the source of the
High mortality rate
Similar results were registered by Carolee Caffrey, a zoologist with
the National Audubon Society’s Audubon Science Center in Ivyland, Pa.
Caffrey has studied crows for 20 years, and has determined that the
American crow is normally a remarkably healthy bird.
Studies in California have found that the annual survival for crows
was 97 percent for males and 95 percent for females, she said, “but
that was before West Nile.”
Working at Oklahoma State University, Caffrey and two graduate
students were tracking about 120 crows.
When West Nile first hit Oklahoma in mid-September 2002, it killed a
third of the crows in only six weeks. The next year, the virus killed
64 percent of the remaining crows; she and her graduate students were
“These were birds we marked as nestlings,” she said. “We watched them
grow up, so it was heartbreaking. My graduate students and I cried.”
A similar study in 2000 in Ithica, N.Y., by a different researcher
resulted in a 40 percent mortality rate for crows, she said.
‘So many variables’
The virus and the way it spreads is complicated, she said, as shown
by what happened in ensuing years in Ithica. In 2001 and 2002, very
few crows died, as West Nile seemed to abate. But in 2003, another
major kill occurred, she said.
Determining whether there are long-term effects could take years, she
said. That’s because so many variables are involved.
For instance, although the virus is passed from bird to bird — and
from bird to human — by some species of mosquitoes, not all species
pass the virus along as effectively as others. And not all species
live in all locations.
Another obstacle in tracking long-term effects is a drop-off in the
reporting of dead birds.
“Part of the slowdown in the reported numbers of dead birds is that
all the facilities doing dead bird collecting and testing in the
first years of the disease were public health departments whose
budgets were strapped,” Caffrey said. “Their mission is public
health, not wildlife monitoring.”
Applying the experience of crows with West Nile to other bird species
also is difficult, Caffrey said.
Nick Komar, an arbovirus specialist with the Centers for Disease
Control and Prevention, conducted experiments on 25 species of birds
“The range of responses was huge,” Caffrey said. “For some species,
the virus was not even replicated by their bodies. Others do, and
their immune system increases, the virus goes down and is beaten back
to undetectable levels.”
“But in other species, including crows and house finches, the virus
is deadly,” Caffrey said.
Researchers have noticed anecdotally a reduction in common birds such
as cardinals and mockingbirds, said David Muth, a biologist at Jean
Lafitte National Historical Park and Preserve. “There’s been a lot of
talk about incredibly low populations of what people perceive as
common garden birds.
“And people who raise birds in and around New Orleans also have had
mortalities,” he said. “Autopsies showed the cause was West Nile
Muth conducts an annual bird count, which did indicate a decrease in
sightings of live American crows in 2002: The volunteers saw an
average of 4.76 crows per hour across Louisiana, compared with the
previous 10-year average of 6.2 birds per hour.
Volunteer efforts vary
Muth said the count can be misleading because it depends on the
varying levels of effort by volunteers.
Officials at the Audubon Zoo are a bit more certain about the effects
of West Nile on birds there, said Dan Maloney, general curator and
vice president for the Audubon Nature Institute.
The zoo is vaccinating animals and birds susceptible to the virus, he
“Last year, we started not only making sure adult flamingos were
protected, but we also started vaccinating the chicks,” he said.
“That seemed to improve the survivability of the chicks,” Maloney
said. “We went from losing five of six flamingo chicks in the 2002
season, to last year having 11 of 14 chicks survive.”
“That one-season difference was really encouraging for us.”
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