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House Finch Eye Disease
The epidemic is still spreading rapidly In 1994 bird watchers from West Virginia to Massachusetts started seeing House Finches with infected, runny eyes. Soon, a group of volunteers, including FeederWatchers, Lab members, and others, rose to the challenge of tracking the spread of a contagious bacterial disease among House Finches in eastern North America. In the first 13 months of gathering data, we have received more than 12,000 data forms and made two important observations. First, the disease is affecting an increasing number of House Finches and spreading to new locations. Second, changes in the incidence of the disease correspond to times when the birds disperse (for example, after the breeding season). Incidentally, preliminary studies at three different wildlife labs suggest that other songbird species don’t catch the disease from finches, a finding that agrees with our feeder observations. When our survey began, the incidence of House Finch conjunctivitis was already high (19 percent) in the mid-Atlantic region—from Maryland, Pennsylvania, and New York to Massachusetts (Figure 1). It increased to 31 percent in March 1995, gradually decreased throughout the breeding season, then rapidly increased again to 41 percent in November 1995.
The disease was initially much less common in other regions. In the southeast region (from Virginia and Tennessee south to Missouri, Alabama, and Georgia) only 9 percent of observers reported diseased birds in November 1994. The incidence of disease fluctuated in the next few months, then increased to 23 percent in July 1995 and 30 percent in November 1995. In the northeast region (New Hampshire and Vermont north into Canada) the incidence of conjunctivitis was only 7 percent in November 1994 and stayed low for a year. The apparent increase in October and November of 1995 was not statistically significant. The disease was almost absent in the midwest region (from Ontario west through Ohio and West Virginia to Missouri, Iowa, and Minnesota) when we started the survey; less than 2 percent of observers reported diseased House Finches. It slowly crept up to 9 percent in May 1995, decreased slightly in July, then increased rapidly, with diseased birds seen by 18 percent of observers in November 1995. Maps (Figure 2) show how the disease has spread geographically. These maps represent the probability of a sick bird being reported at any given location. (A few cities are indicated as reference points.) We calculated how likely it was that an observer at any given location would report diseased House Finches during a particular month—the darker the color on the map, the higher the probability of seeing a diseased bird. Values range from a one-in-eight (or less) chance of seeing a House Finch with conjunctivitis symptoms in the palest gray regions to a more than one-in-two chance in the black regions.
In November 1994 the probability of seeing a diseased bird was highest in the mid-Atlantic region. In June 1995 (the height of the breeding season) the epidemic was still focused in this region, although it had become firmly established in the Southeast. By November 1995 it expanded dramatically, especially toward the west. The incidence of disease, indicated by the amount of black, also increased. To all the volunteers who help track the disease outbreak—thank you. We’ll keep you posted as the House Finch Disease Survey continues this summer. André A. Dhondt is director of the Bird Population Studies program. |
