When invasive species clash: Competition between the House Sparrow and House Finch
Among the feeder birds of North America, few are as recognizable and widespread as the House Finch and House Sparrow. The House Sparrow, originally from Europe, is one of the most well-established invasive species in the United States. The House Finch, however, is invasive in its own right. Originally native to only the western United States and Mexico, it has spread rapidly through the east since a small number of caged birds were released in New York in 1940.
Although House Sparrows have successfully spread across North America, these populations have not always been stable. In an attempt to explain recent declines in populations of House Sparrows in the US, some scientists suggested that House Finches could be providing significant competition as they rapidly expanded their range from west to east. In 1994, the rapid decline in the House Finch population following the emergence of mycoplasmal conjuctivitis (more commonly known as House Finch Eye Disease) provided a unique opportunity to conduct a natural experiment: did populations of House Sparrows increase in areas where the House Finch population had decreased?
By using data from the Christmas Bird Count, Project FeederWatch, and the Breeding Bird Survey, Cornell Lab researchers led by Dr. Caren Cooper were able to show that House Sparrow numbers vary inversely with those of House Finches in the northeast. That is to say, as House Finches increase, House Sparrows decrease, and as House Finches decrease, House Sparrows increase. These results suggest that competition between the two species does exist.
Source: Cooper, C. B., W. M. Hochachka, and A. A. Dhondt. Contrasting natural experiments confirm competition between House Finches and House Sparrows. 2007. Ecology 88(4): 864-870.