Thousands of hawks, eagles, kestrels, falcons and owls have died in recent
months in a swath of the United States from Minnesota to the Gulf of Mexico and
Nebraska east through Ohio into Pennsylvania.
West Nile virus was - and still is - a prime suspect in the raptor deaths.
But tests on some of the dead birds show they were not infected, and avian
experts are investigating other factors that could be acting alone or in
combination with the virus to bring the big birds down.
Southwestern Pennsylvania appears to be on the edge of the massive and
mysterious raptor die-off that has taken a tremendous toll on birds in the broad
Midwest but, for reasons that are still unclear, have resulted in few raptor
deaths and affected fewer species from the Appalachian Mountains to the East
Coast.
Large numbers of crows and blue jays and a few red-tailed hawks, great horned
owls and kestrels have died of West Nile in the East, but it's nothing like the
toll in the Midwest.
Patricia Bright, director of the pesticides and birds program for the
American Bird Conservancy in Plains, Va., said her organization guessed that the
number of dead raptors was in the thousands.
"But there are no hard numbers," she added. "And where they do
have them they're usually dramatically understated because sick birds sequester
themselves and dead birds are hard to find.
"The real concern is what effect this will have on long-term raptor
populations. The ones with high reproductive rates will be OK, but the concern
comes when birds like the California Condor, which are already existing in low
numbers, get it."
Birds serve as hosts for West Nile virus, which is spread by mosquitoes to
other birds as well as to humans, horses, squirrels and dogs. The virus can
cause encephalitis, a potentially fatal inflammation of the brain and spinal
cord, but only a small percentage of humans who contract the disease suffer any
ill effects. Many birds also contract West Nile, but only a small percentage
die.
The virus cannot be spread from person to person or from birds to humans.
While many of the dead birds have tested positive for West Nile virus, others
have not. Even birds that display West Nile symptoms have tested negative for
the virus.
Beth Shoaf, a wildlife rehabilitator, said the neurological symptoms
displayed by the birds - head tremors, difficulty swallowing, inability to
recognize food - are also suggestive of exposure to pesticides or some other
environmental toxin.
Some longtime observers of raptors are blaming the Hippoboscid fly - actually
a group of bird parasites also known as louse flies or flat flies. The flies
proliferated because of last year's mild winter and infested birds of prey in
unusually large numbers this summer. The flies feed on blood in the quill of
emerging feathers on large birds.
No one knows why raptors are dying in greater numbers in the Midwest than
they are in the East, but the question has hatched many theories.
One is that the West Nile virus could be mutating as it moves west,
developing a strain that is more deadly to bird populations. Another is that
bird populations in the Midwest are genetically different than those in the East
and for some reason more susceptible to West Nile virus.
A third theory holds that the birds, already weakened by West Nile, are being
killed off by other viruses, bacteria or pesticides. Or it could be that
different species of mosquitoes in the Midwest may target raptors over other
birds.
Yet another theory holds that raptors in the East have long been exposed to
Eastern equine encephalitis, a virus related to West Nile, and have developed an
immunity to it.