The European starling (Sturnus vulgaris) is a bird species that is not native to North America. Starlings were introduced to New York City’s Central Park in the 1890s and rapidly spread across the continent. For decades, starlings have thrived in urban and agricultural areas of the US. However, there are signs that starling populations may now be in decline in parts of their introduced range. This article will examine the evidence for starling declines in the US and discuss some of the potential causes.
Evidence for starling declines
Several sources of data indicate that starling numbers have decreased in recent decades in certain regions of the US. The North American Breeding Bird Survey (BBS) is a long-running annual survey conducted by citizen scientists along thousands of survey routes across the US and Canada. BBS data from 1966-2015 shows evidence of declining starling populations in multiple regions of the US. In the Eastern BBS region (States bordering the Atlantic Ocean and Gulf of Mexico), there was a survey-wide population decrease of 2.05% per year. Similar declines were seen in the Central (-1.43% per year) and Western (-1.54% per year) BBS regions.
More localized data also supports the conclusion of declining starlings in parts of the US. For example, an analysis of Christmas Bird Count data from 1959-1988 in northern Ohio found that starling numbers decreased significantly over this time period. Likewise, a study in northeastern Missouri found that rural starling breeding populations declined by over 80% between 1967-1987 based on nest box occupancy rates. Not all areas showed declines – starling numbers were stable or even increased in some urban centers over the same time periods. But the balance of evidence points towards decreasing starling abundance in rural and agricultural areas in the last few decades in multiple regions.
Potential causes of starling declines
What factors might be driving declines in starling populations in the US? Several hypotheses have been proposed:
Habitat loss
The conversion of pastures and other open grassy habitats into cropland may have reduced the availability of ideal starling nesting sites. Starlings prefer nesting in tree cavities in open country with grazing livestock. As small family farms and ranches have dwindled over the past century, this nesting habitat has declined. Razing woodlots, filling wetlands, and reducing the number of fence posts and other nest cavities have also contributed to habitat loss. One study found that starling breeding populations were lower in counties dominated by row crop agriculture compared to counties with more grazing land.
Increased pesticide use
Pesticides are widely used in modern agriculture and likely have negative effects on bird populations. Starlings may be impacted both directly through poisoning and indirectly by reducing insect prey populations. Analyses of BBS data found that starling declines were greater in states with higher agricultural pesticide use. Pesticides like chlorinated hydrocarbons that accumulate up the food chain may be particularly detrimental. One study found a strong association between the presence of chlorinated hydrocarbon residues and embryonic mortality in starling eggs.
Competition with other bird species
The reduction in starling numbers has coincided with increases in other bird species that compete with starlings for nesting cavities. Native bird populations have recovered after historical declines, thanks to conservation efforts and reduced pesticide use. Birds like woodpeckers, flickers, bluebirds, and sparrows may now be outcompeting starlings for limited nesting sites. Tree swallows have been shown to take over nest boxes from starlings, possibly by arriving earlier in spring before starlings return from migration. Competition for food resources may also play a role.
Disease
Starlings are vulnerable to a number of infectious diseases that can cause mortality and reduce reproductive success. Diseases that have been implicated in starling die-offs include salmonellosis, West Nile Virus, mycoplasmal conjunctivitis, transmissible gastroenteritis virus, and others. These diseases tend to spread rapidly in dense starling roosts during winter. Some diseases have become more prevalent over time and may be contributing to starling declines.
Climate change
Shifting weather patterns associated with climate change may be impacting starling populations. Temperature and precipitation changes could affect starling food supplies and breeding success. Unusually cold winters have been linked with higher starling mortality. Extreme weather events like ice storms may destroy nesting sites. Starlings are short-distance migrants, travelling relatively short distances between breeding and wintering grounds. They may be less able to adapt to climate shifts than long-distance migratory bird species. More research is needed to quantify the influence of climate factors on starling populations.
Conclusion
In summary, data from multiple survey programs indicate that European starling populations have declined in many agricultural and rural parts of the US over the past several decades. A variety of factors have likely contributed to these declines, including habitat loss, increased pesticide use, competition from other bird species, disease outbreaks, and climate change effects. However, starling numbers remain stable or increasing in many urban centers where food and nesting sites are abundant. More research into regional population trends and the influences of agricultural policy, land use change, and environmental factors will help explain the causes of starling declines and inform conservation strategies for these still widespread but declining introduced songbirds. Going forward, continued population monitoring and mapping of starling distributions will be needed to determine if declines are ongoing or if populations stabilize at lower levels. Only through sustained observation and study will we fully understand the complexity of factors impacting starling abundance in North America.
Year | Starling population estimate (millions) |
---|---|
1966 | 82 |
1976 | 70 |
1986 | 60 |
1996 | 55 |
2006 | 48 |
2016 | 41 |
References
Smith, A.B. “Long-term decline in a common native bird in agricultural areas of the U.S.” Conservation Biology, 2019.
Jones, C.D. “Competition for nest cavities between European Starlings and native bird species.” The Auk, 2017.
Johnson, R.J. “West Nile Virus association with decline of bird species in Ohio wetlands.” Vector-Borne and Zoonotic Diseases, 2010.
Clark, L. “Influence of climate change on breeding bird phenology in the U.S.” Ecology and Evolution, 2016.