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Negative Data Have Please cite this Page as:
During the last couple of years many people have been telling us that they see fewer House Finches at their feeders. They ask if we know whether the birds’ numbers are decreasing and, if so, whether the Mycoplasma gallisepticum infection sweeping through their population could be responsible. The instructions accompanying the House Finch Disease Survey data forms ask participants to return them whether they see any House Finches or not. Out of 21,924 data forms returned between November 1994 and October 1996, 3,240 (sent in by 774 different observers), or 14.8 percent, report the absence of House Finches at their feeders in that month, which is negative data. Eighteen participants in New Brunswick returned 222 data forms, although House Finches are reported on only six forms; and 12 participants in Nova Scotia returned 111 data forms, 54 of which reported seeing House Finches, only one of which had conjunctivitis. So, clearly the message that negative data are valuable is coming across loud and clear. But can we really use this negative information? We definitely can. Let us take a look at what we can learn from the data forms reporting no House Finches. For that we must first take a step backward. The western population of House Finches—from which the ancestors of the eastern population were introduced—is nonmigratory. Although western birds do not always stay year-round on their breeding grounds, the western House Finches do not make regular autumn movements. Using data on band recoveries from the Bird Banding Lab, Belthoff and Gauthraux published an interesting paper in 1993 reporting that migratory behavior was gradually developing in the eastern House Finch population. A certain proportion of the birds was moving south each autumn and returning north each spring. This clearly suggests that, in some parts of the House Finch’s new range, migration was advantageous, and that we were witnessing an evolutionary change through natural selection. Furthermore, the proportion of birds migrating and the distances they moved seemed to be increasing. Then came the Mycoplasma infection. Once the disease arrived in the Southeast, it spread rapidly. In the northern states, however, the disease arrived, disappeared, arrived again, and disappeared again. In Wisconsin, for example, participants reported seeing House Finches with conjunctivitis in November and December 1994, but not in January 1995; again in February 1995, but not in March; again in April, but not in May and June; and again in July, August, and September, but not in October and November. Since March 1996, however, diseased House Finches have been reported each month. Similarly, in Maine the disease was reported from December 1994 through August 1995, but not after that. It will probably reappear. House Finches with conjunctiv-itis become lethargic. They are often easy to approach and pick up, and they are often blind in one or both eyes. They hang around feeders, barely surviving. Some infected birds do recover, however: Jean Bickal, a bird bander and Lab member from New Jersey, recaptured infected birds up to two months after their initial capture. In parts of North America where winters are cold and many birds rely on feeders to survive the difficult times, House Finches (and also their predators) come to feeders. If given a choice, predators take the easy prey—for example, slow-moving, ailing House Finches. Our FeederWatch data (see Living Bird, Autumn 1996) show that winter House Finch numbers decreased about two years after the Mycoplasma infection arrived in a state or province. What do we learn from our negative data? In Figure 1 we analyze as data the reports of the participants who have NOT seen any House Finches in a given month. The graph compares the percentage of negative data in some months of the second survey year (1996) with the percentage in the same period during the first survey year (1995). If a higher proportion of participants reported not seeing House Finches in 1996 than in 1995, then the ratio would be larger than 1.0. In the Northeast, for example, 48 percent of the participants did not observe House Finches in February 1995 and 86 percent did not see any in February 1996. The ratio, therefore, is 1.79. The left side of the graph compares midwinter months (January and February). In three of the regions (the Northeast, the Mid-Atlantic States, and Ohio and Ontario), the ratio is much larger than 1.0; in the two other regions (The Midwest and the Southeast) there was no such change. Thus in the Northeast, the Mid-Atlantic States, and in Ohio and Ontario, the proportion of participants who did not see any House Finches in winter increased strongly, but not in the Midwest and Southeast. The right side of the graph shows the results for two breeding season months. Here we see that no ratio is very different from 1.0. None of the regions showed changes in the proportion of participants not seeing any House Finches. So what does this all mean? Winter House Finch numbers are rapidly decreasing in parts of the birds’ range where winters are very cold, where a high proportion of birds gets conjunctivitis. Perhaps infected birds cannot survive the extreme conditions. We know that in partially migratory species migration is to a large extent genetically controlled. Birds that leave the breeding grounds to winter farther south have the genetic make-up to do this, whereas the birds that stay around in winter do not. If many of the resident birds are killed through a combination of mycoplasmal conjunctivitis and winter cold, with some predators thrown in for good measure, and the migrating individuals are more likely to survive, then there is a strong selective force that favors migratory individuals. The House Finch populations living in cold climates will rapidly evolve and probably become mostly migratory. Just how rapidly? If the negative data from our Northeastern participants are any indication, very rapidly. So by looking at negative data we might be able to see evolution taking place right in front of our eyes.
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