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House Finch Disease
New data shed light on basic questions Understanding the role of disease in wild-bird populations is critical for their long-term management and conservation. Since last winter, participants in the redesigned House Finch Disease Survey (HFDS) have had a unique opportunity to increase our knowledge and understanding of the conjunctivitis (eye disease) found in House Finch populations across the country. This particular conjunctivitis is caused by the parasitic bacteria Mycoplasma gallisepticum, or MG. Although the HFDS has successfully tracked the spread of the disease since November 1994, ornithologists and wildlife health researchers had limited quantitative information regarding the disease in various areas during different seasons. We could not answer many simple questions, such as what proportion of birds at feeders have clinical conjunctivitis, how many were male and how many were female, and how severe were their infections? By applying Project FeederWatch counting techniques and harnessing the energy of dedicated volunteer observers, we are beginning to answer these questions. What follows are the preliminary findings from last winter, including comparisons of data received by mail and via the Internet. We believe this information is an invaluable resource for researchers and wildlife managers, because the effects of infectious diseases on songbird populations are often underestimated. Between November 1997 and March 1998, 355 participants provided 1,166 observations to the HFDS from 32 eastern states and 2 Canadian provinces. Eighty percent of the total observations contained data on House Finches (312 from the mailed survey and 622 via the Internet). Twenty-five percent of the participants who counted House Finches noted at least one diseased bird. In all, participants counted 407 diseased House Finches among 8,313 individuals observed during the five-month period. Diseased finches were generally not numerous at a given site; 50 percent of the observations that reported diseased birds described only one finch with conjunctivitis. Though slightly more females with conjunctivitis were counted at sites with diseased finches, the difference between the mean number of diseased males and females was statistically insignificant. Susceptibility to MG infections in domestic poultry is not linked to the sex of a bird. A recent publication from researchers at Auburn University, however, suggests House Finch populations may be shifting from a male-dominated sex ratio to a female-dominated ratio, supposedly because of greater male mortality during the epidemic stage of the disease three years ago. At present, HFDS data show an apparently equal distribution of conjunctivitis between the sexes in all regions with adequate sample sizes. Regionally, conjunctivitis is less prevalent in northeastern and Great Plains House Finches (2.4 percent in Maine and Vermont and 3.1 percent in Kansas, North Dakota, Nebraska, South Dakota, Texas). With low densities of House Finches in these areas, we would expect less transmission of infection between birds, thus resulting in fewer cases of disease being observed by participants. This survey needs additional participants and observations in these areas, however, to increase our confidence in these estimates.
As shown in Figure 1, the disease remains prevalent but shows remarkable dynamics over time, both within and between the mid-Atlantic (Connecticut, Delaware, Maine, Maryland, New Jersey, New York, Pennsylvania), Southeast (Alabama, Georgia, Louisiana, Missouri, North Carolina, South Carolina, Virginia, West Virginia), and Midwest (Iowa, Illinois, Indiana, Kentucky, Minnesota, Michigan, Missouri, Ohio, Tennessee, Wisconsin, and Ontario) regions. In the mid-Atlantic region, House Finches show the greatest prevalence (or percentage) of conjunctivitis. Interestingly, the disease appears to be maintained at a low level within the population through the end of the year, then it increases rapidly during the coldest winter months. Yet similar trends are apparent in the Southeast and Midwest. It seems likely that increased flocking behavior, which creates locally high bird densities favorable for spreading disease, is responsible for these midwinter increases. But the cold stress experienced by the more abundant mid-Atlantic wintering population may be responsible for the high prevalence of disease noted in this region, because this stress may create a larger pool of susceptible hosts. Many new participants from various western states also provided the HFDS with observations. Fifty-one new participants provided 159 observations from nine western states and one Canadian province. Observations of House Finches with conjunctivitis in Colorado and New Mexico are extremely important, because these may represent the first reports of the MG-associated disease in western House Finches. The spread of this disease from the introduced eastern population to native western House Finches is expected at any time. (MG has been confirmed as far west as Waco, Texas, and Lincoln, Nebraska.) A confounding factor in this region, however, is the presence of avian poxvirus. This disease is common in western House Finches and may cause a conjunctivitis that is easily confused with MG-associated disease (this is the likely scenario for reports received to date from California and Washington State). Additional reports are needed to target these populations so that conclusive diagnostic sampling can be done and the cause of these eye diseases can be determined. On the whole, the mailed survey and Internet submissions were similar across the continent (as we had hoped). The geographic coverage, the percentage of participants counting House Finches, the mean numbers of House Finches counted per period, and diseased birds per count were remarkably consistent in data obtained from both reporting methods. The participation rate by Internet users, however, was lower than that of the mailed survey participants (3.7 observations per participant for the mailed survey versus 3.1 observations for Internet users), and the mailed survey was available a month later. Such lowered reporting opens up two potential biases in our survey that can have serious consequences for the validity of our estimates: 1) selective reporting of diseased House Finches, and 2) lack of additional negative data because some participants don't bother to report if they don't see any diseased birds. Both of these biases may inflate our estimates. Negative data are extremely important in charting the distribution of this disease and in gaining the proper perspective on how common this disease is in an extremely large, mobile population. In hopes of gaining many new participants this year, we encourage Internet users who participated in a limited fashion last year to consider rejoining our team for the months ahead, especially into the breeding season. If you would like more information on the House Finch Disease Survey, please contact us at (800) 843-BIRD (2473) or visit us on the Internet at http://birds.cornell.edu/hofi/. Thank you in advance for contributing to the health of wild birds. |
