SPRING 2007/VOLUME 21, NUMBER 2
Why Did House Sparrow Numbers Rise, then Fall?
Citizen-science data point to competition with House Finches
House Finches are common backyard birds in the eastern United States today—but just 70 years ago, the species was found only in the western United States and Mexico. In the 1940s, some House Finches from California were released on Long Island, New York. Within 50 years, they had spread across the eastern United States and southern Canada, their numbers swelling into the millions. Were House Finches so successful because they outcompeted other birds for food and other resources? Or did they find a niche that was unoccupied by other birds species?
Nonnative, invasive bird species are often assumed to compete with and displace native birds. However, documenting the long-term, large-scale effects of competition among birds is very difficult, and evidence for the impact of competition on bird communities is rare. In a study published in the April 2007 issue of Ecology, we used data collected by citizen-science participants to show some of the first clear evidence that an invasive bird species can cause declines of another species as the invasive species expands its range and becomes more abundant. Ironically, the invasive species in question is the House Finch, native to North America but of western origin, which outcompeted the well-established European exotic, the House Sparrow.
It may seem simple to document the effect of competition on birds, for example by observing the outcome of fights between birds of different species. However, aggression among individual birds does not necessarily mean that one species is displacing another at the population level. For example, European Starlings aggressively usurp nesting cavities of native North American birds, but a study by researcher Walter Koenig found that starlings have not caused detectable population declines in the majority of cavity-nesting species (Conservation Biology 17: 1134–1140).
To document population-level competition among wild birds, researchers often look for evidence that one species is increasing in number while another species is decreasing. However, this pattern could be coincidental, caused by many other factors besides competition. In experiments with laboratory organisms, you could demonstrate competition by putting two species together, then separating them. If competition is occurring, the stronger competitor would outnumber the weaker competitor when they are together, but the effect would reverse when the competitor is removed.
Such a controlled experiment is impossible with most wild birds. However, years of observations from citizen-science participants show that long-term fluctuations in numbers of House Finches and House Sparrows are linked.
A tale of two species
In the mid-1800s, long before House Finches had been introduced to the eastern United States, humans brought House Sparrows from Europe to New York and other locations. The adaptable birds multiplied and spread across the United States and much of Canada. After House Finches were introduced to the East in the 1940s, several researchers noted that House Sparrow numbers began to decline. Was this merely a coincidence or were the finches outcompeting the sparrows?
A new disease affecting House Finches provided a clue. In 1994, an eye disease called mycoplasmal conjunctivitis began to plague House Finches. The disease impaired the birds' vision, leading to many finches' deaths. Data from citizen-science participants showed that some House Finch populations declined by half during a three-year period. Did House Sparrow numbers rebound as the House Finches declined?
They did, according to data from Project FeederWatch and the Christmas Bird Count. The pattern is consistent with the hypothesis that House Finches can outcompete House Sparrows. Sparrow numbers were declining in the years when finch numbers were increasing, before the finches had been affected by conjunctivitis. As the disease spread and House Finch numbers declined, House Sparrow numbers increased.
In a finer-grained analysis, we also found a pattern in the rate of change in the abundance of the two species at individual sites, regardless of the time period: when House Finch numbers increased, House Sparrows decreased, and vice-versa. Together, our analyses suggest that the pattern is more than a coincidence; it is highly likely that House Finches caused the observed changes in House Sparrow abundance.
The role of competition in structuring bird communities has important conservation implications. According to the "driver hypothesis," exotic species can displace native species, steering the composition of species to a less diverse end point. In contrast, the "passenger hypothesis" suggests that exotic species may expand into areas where other birds are not already using resources, perhaps because of habitat degradation or disease. In this case, the exotic species would be "passengers" filling otherwise empty seats.
Our results support the idea that competition can be a driver of community structure in birds, leading to regional declines of a bird species. Furthermore, in this study, neither species was native to the area, suggesting that the outcome of competition was determined by the birds' preference for similar resources, rather than traits inherent to their status as introduced species.
This study was possible only because of the wealth of long-term data provided by citizen-science participants and the natural circumstances that enabled us to detect a relationship between the abundance of House Finches and House Sparrows. Unfortunately, given the difficulty of obtaining data with which to test for evidence of competition, we may never know the proportion of exotic species that adversely affect established species.
Caren Cooper is a research associate in the Lab's Bird Population Studies and Citizen Science program. Wesley Hochachka is assistant director and André Dhondt is director of Bird Population Studies.
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