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Cornell Ornithology Scientists Track Impact of Bird Disease, for First Time, Summer 2000

Salmonella strikes redpolls, Spring 2000

  Cornell Ornithology Scientists Track Impact of Bird Disease, for First Time, June 2000.

 Ithaca, NY — Thanks to a unique, continentwide network of bird-feeding enthusiasts, researchers at the Cornell Lab of Ornithology have been able to substantiate a long-standing theory that a naturally occurring disease can regulate a wildlife population. The findings, from a study of mycoplasmal conjunctivitis in House Finches (Carpodacus mexicanus) and published recently in the Proceedings of the National Academy of Sciences, could give scientists insight into the dynamics of other host/disease systems as well, including an epidemic currently affecting fish in the Chesapeake Bay, possibly even AIDS.

"Our data showed that high-density populations of House Finches that became infected with mycoplasmal conjunctivitis experienced a dramatic drop in numbers within two to three years after the epidemic began and that they stabilized at about 40 percent of their previous abundance," says André Dhondt, director of Bird Population Studies at the Cornell Lab of Ornithology and coauthor of the study. "At the same time, emerging House Finch colonies increased to approximately those same levels, despite the presence of the disease."

Theoretically, says Dhondt, it makes sense that when a disease emerges in a dense population, it moves quickly through that population and wipes out vast numbers because healthy individuals have a higher chance of coming in contact with infected animals. Conversely, healthy animals in lower-density populations are less likely to cross paths with infected individuals, allowing those populations to continue to grow until the disease reaches a density threshold.

This theory, however, has never before been proven because it is extremely difficult to follow the course of a naturally occurring wildlife disease. Scientists need information on host abundance both before, during, and after the epidemic sweeps through the population. They also need to be able to begin collecting data at the onset of the disease. To do that, symptoms need to be readily detectable and a large network of observers need to be in place. Previously, large-scale studies have focused on diseases that were deliberately introduced as a means to control non-native species.

Cornell Lab of Ornithology researchers were in a unique position. Since 1988, thousands of backyard bird-feeding enthusiasts across North America have been collecting information about their winter feeder birds as part of the Lab’s Project FeederWatch. During the winter of 1993—94, FeederWatchers in the Washington, D.C. area, were among the first to observe and report House Finches with swollen, crusty eyes. In response, Lab scientists developed a new protocol to help FeederWatchers collect information about this mysterious new disease. From this evolved the House Finch Disease Survey. "Fortunately, we were in position to act quickly and therefore had an unprecedented opportunity to monitor a wildlife disease from its origin and follow it as it spread," says Laura Kammermeier, project leader for Project FeederWatch. By 1997, participants were reporting diseased House Finches throughout most of the species’ eastern range, extending as far as Texas and the Dakotas.

Dhondt and his colleague Wesley Hochachka combined House Finch Disease Survey reports with National Audubon Society’s 100-year-strong Christmas Bird Count database to determine the numbers of affected birds. They estimate that House Finch numbers have fallen in the East from some 300 million birds to 180 million. The disease has now stabilized; Dhondt believes that in New England, for example, only about 1 in 20 birds are infected.

Historically, House Finches were a western species until they were introduced in 1940 on Long Island, where they were sold as caged birds. Referred to by bird sellers as "Hollywood finches," the males are rosy red and brown, the females primarily streaked brown. When a new law prohibited the sale of American songbirds, the finches were released. The species made optimum use of bird feeders and rapidly established populations throughout eastern North America.

Mycoplasmal conjunctivitis is caused by a previously unknown strain of mycoplasmal gallisepticum, a bacterium that typically leads to respiratory infections in domestic poultry. The new strain, which also affects American Goldfinches (Carduelis tristis) and a few other wild bird species, causes the eyes of an infected bird to become swollen and crusty. This may lead to blindness and death from starvation, as the birds become unable to forage for food.

The disease is spread when healthy birds come into contact with an infected bird or an object touched by a diseased bird. For example, tube feeders require the bird to stick its head into a hole in order to extract seeds. When the eyes of an infected bird come in contact with the opening, the next bird to feed at that perch may pick up the disease. For this reason, feeders should be cleaned with a solution of 1part bleach to 10 parts water every two weeks or so. Nevertheless, the disease probably spreads most rapidly where the birds sleep together in large, crowded roosts, quite often among populations that do not make regular use of bird feeders.

Dhondt assures that mycoplasmal conjunctivitis cannot be transmitted to humans.

Bird-feeding enthusiasts of all skill levels and backgrounds are encouraged to participate in the House Finch Disease Survey and Project FeederWatch. A small fee for these projects helps defray costs of materials. To sign up, call (800) 843-2473. Visit the Lab on the Web or e-mail them at <cornellbirds@cornell.edu>. You can also write to them at Cornell Lab of Ornithology, 159 Sapsucker Woods Road, Ithaca, NY 14850.