Barn swallows are small, migratory birds that are found throughout most of North America. They are known for their distinctive forked tails and their habit of building mud nests on the walls and rafters of barns and other structures. Barn swallows have long held a special place in many cultures as symbols of the arrival of spring and summer. However, in recent decades, barn swallow populations have undergone significant declines across much of their range. This has raised concern among ornithologists, conservationists, and others who value these birds. In this article, we will explore what has been happening with barn swallow populations, the potential causes behind their declines, and what can be done to support this beloved bird species.
Historical abundance and range of barn swallows
Barn swallows have an extensive historical range, breeding across North America and wintering from southern North America down through South America. They are found in a wide variety of open and semi-open habitats, including farmlands, meadows, wetlands, and coastal areas. Their populations expanded greatly with the spread of agriculture and the increase in suitable nesting and foraging sites like barns and livestock corrals. By the early 20th century, they were an abundant and familiar sight across rural areas of the United States and Canada. The Breeding Bird Survey estimated there were around 12 million breeding pairs in the contiguous United States alone in the 1960s.
Evidence of population declines
Starting in the 1980s, standardized monitoring programs like the Breeding Bird Survey began detecting downward trends in barn swallow abundance. Data collated from 1966-2015 suggests the overall population in the contiguous United States has declined by over 25% in the past 50 years. The declines appear most severe in northeastern and midwestern states, where losses of over 50% have been recorded in some areas. The situation may be even more serious in Canada, with estimates of a 69% drop between 1970 and 2014.
Some more specific evidence of declines includes:
- In Ontario, barn swallow populations on the shores of the Great Lakes dropped by 90% between 1985 and 2005.
- In Nova Scotia, the number of occupied barns decreased by over 60% between 1989 and 2009.
- Surveys in Wisconsin recorded a 79% decline in roadside nests between 1980 and 2002.
Although still locally common in some areas, the overall population trajectory for barn swallows across North America is clearly downward.
Hypothesized causes of declines
What factors could be behind the disappearance of so many barn swallows? Researchers have proposed several potential causes:
Habitat loss
The conversion of grasslands, wetlands, and agricultural areas into urban and suburban developments has reduced habitat for barn swallows. They rely on open foraging grounds and structures for nesting.
Reduction in nesting sites
Traditional barns and other rural buildings with suitable nesting spots have declined with changes in agriculture and development pressure. The loss of old structures is an important driver of barn swallow declines in many areas.
Climate change
Shifting precipitation patterns and temperatures associated with climate change may be affecting barn swallow prey availability, migration timing, and reproductive success. Extreme weather could also damage nests.
Pesticide use
Barn swallows may be harmed by exposure to pesticides from agricultural operations. These chemicals can reduce insect prey and have toxic effects on birds.
Competition from invasive species
Introduced bird species like house sparrows may usurp nesting sites and compete with barn swallows for food resources.
Decline in cattle farming
The consolidation and intensification of cattle farming has reduced Barn Swallows’ traditional nesting and foraging habitat around barns and livestock corrals.
Evidence for contributors
Research provides some specific support linking several of these factors to barn swallow declines:
Habitat loss
– A study from Ontario found that increased residential density within 1 km of barns was associated with barn swallow declines. (Fig 1)
Residential density | Estimated barn swallow decline |
---|---|
Low | 13% |
Medium | 24% |
High | 41% |
Reduction in nesting sites
– Surveys in Pennsylvania showed 96% of occupied barns had barn swallow nests, compared to only 4% of modern barns.
– Nesting declines were associated with barn renovations that made them less suitable. (Fig 2)
Barn status | % with barn swallow nests |
---|---|
Unmodified traditional barn | 100% |
Renovated traditional barn | 36% |
Modern barn | 4% |
Climate change
– Barn swallow productivity decreased during high temperature and precipitation events in a Texas study.
– Earlier spring arrival linked to warmer temperatures not matched by advancements in peak food availability.
Pesticide use
– Higher reproductive success for barn swallows nesting on organic vs conventional farms in Sweden.
– Declines associated with increased insecticide use in parts of Alberta.
Invasive species
– Modeling suggests house sparrow competition could drive barn swallow declines in Alberta.
– Negative relationship found between house sparrow and barn swallow numbers.
Conservation actions
Given the evidence that habitat loss, lack of nest sites, climate change, pesticides, and invasive species are all contributing to the decline of barn swallows, conservation efforts should focus on addressing these threats. Some recommendations include:
Creating grassland reserves
Protecting remaining grasslands provides habitat for prey species. Programs that pay farmers to take land out of production into conservation grasslands benefit swallows.
Managing agricultural lands sustainably
Encouraging Integrated Pest Management, maintaining hedgerows, and reducing mowing helps provide habitat and insect prey.
Education campaigns about barn renovations
Outreach to help farmers maintain nesting sites for barn swallows during renovations by leaving ledges open or putting up replacement nest cups or shelves.
Monitoring and controlling invasive species
Tracking house sparrow and European starling populations and excluding them from nest sites when possible.
Putting up artificial nest structures
Where appropriate, externally attached artificial nests can provide supplementary nesting habitat. Ongoing maintenance is required.
Further research
Better understanding the impacts of climate change through long-term monitoring will help guide conservation responses. More study on pesticide effects is also valuable.
Outlook for the future
With targeted conservation action, there is hope that barn swallow declines can be reversed. However, the challenges are significant. Habitat loss and degradation will continue driving declines unless substantial effort is made to maintain and restore suitable breeding and foraging habitat across agricultural landscapes. The ongoing pressures of climate change, invasive species, and pesticide use must also be addressed. Stemming the tide of barn swallow losses will likely require coordination across government agencies, conservation groups, agricultural organizations, and landowners. With commitment and creativity, populations of this iconic bird can hopefully be restored. Their acrobatic flights around rural buildings in summer provide joy and connection to nature for many people. Ensuring barn swallows remain a part of the landscape is worth pursuing.
Conclusion
In conclusion, barn swallows have undergone alarming population declines across North America in the past several decades. Habitat loss, reductions in nesting sites, climate change, pesticides, invasive species, and changes in agriculture all likely contribute. Addressing these threats through habitat protection and management, maintenance of nesting sites, controlling invasive species, and improving practices on farms and ranches can support barn swallow recovery. With substantial effort, it may be possible to bring back barn swallows as a common sight each summer. Their cultural significance and important ecological role make finding solutions urgent. More research and monitoring are needed, but there are also many conservation actions that can be implemented now even as we learn more. With creativity and persistence, people concerned about barn swallows can work to provide these special birds with the open skies and safe nesting sites they need to thrive.