Contrasting natural experiments confirm competition between House Finches and House Sparrows This study examines evidence that suggests the decline of House Sparrows in the United States correlates with the introduction and rapid increase from west to east of House Finch populations. Without ruling out other potential causes, scientists blamed interspecific competition from House Finches for the decline of House Sparrows. Then, in 1994, the House Finch population experienced a rapid decline following the emergence of a new disease (mycoplasmal conjunctivitis), providing a natural opportunity to test whether this hypothesis was true. If interspecific competition from House Finches is the culprit that drives House Sparrow populations, then a decline in House Finches should lead to an increase in House Sparrow population. Using three independent volunteer programs that monitor bird species’ abundance and distribution, including the Christmas Bird Count, Project FeederWatch, and Breeding Bird Survey, the authors of this study examined northeastern data to test the notion. In the first analysis, House Sparrow and House Finch numbers varied inversely during a time when House Finch populations were increasing, as well as when their numbers were decreasing. In the second analysis, the authors found that the rates of geometric change in House Sparrow abundance were negatively correlated with House Finch and sparrow abundances at individual sites, regardless of the time period. These analyses suggest it would be very unlikely for competition or false correlations to cause changes in House Sparrow populations, given that finch abundance and declines are the result of two very different phenomena. As a result, the authors conclude that interspecific competition does exist between House Sparrows and House Finches. This article was written by Caren B. Cooper, Wesley M. Hochachka, and Andre A. Dhondt of the Cornell Lab of Ornithology. It appeared in the April 2007 issue of Ecology, a publication by the Ecological Society of America and a leading international journal in its field that reports and interprets the results of original scientific research in basic and applied ecology.