Barn swallows (Hirundo rustica) are a familiar sight in rural areas, where they nest in barns and other structures and dart gracefully through the air catching insects. However, there are concerns that barn swallow populations may be declining in parts of their range.
Summary
Evidence suggests barn swallow populations have declined in some areas, but the global population remains large. Causes of declines likely include loss of nesting habitat, climate change, and reduced insect prey availability. More research is needed to fully understand population trends and causes of any declines.
What are barn swallows?
Barn swallows are small, sparrow-sized birds recognized by their long, deeply forked tails and iridescent blue-black upperparts and orange underparts. They are found across most of North America, Europe, Asia and Africa. Barn swallows migrate long distances between their breeding and wintering grounds. They are aerial insectivores that catch flying insects on the wing.
Breeding habitat and behavior
Barn swallows nest inside or on the outside of structures such as barns, sheds, bridges and culverts. They build cup-shaped mud nests and lay clutches of 4-7 eggs. Barn swallows are very social, often nesting in colonies of up to several dozen pairs. Both parents help build the nest and care for the young. Barn swallows are dependent on suitable structures for nest sites and mud for building nests.
Wintering grounds
Barn swallows winter in Central and South America. They form enormous communal roosts numbering in the millions of individuals in wetlands and reedbeds. The availability of suitable wetland stopover and wintering habitat may influence barn swallow populations.
Evidence of declines
Barn swallow populations decreased significantly across much of North America from 1966-2015 according to the North American Breeding Bird Survey, with a loss of 49% of the population overall. Parts of Europe, including the UK, have also seen declines of 30-60% in recent decades. However, global populations remain high at an estimated 190 million individuals.
North American Breeding Bird Survey data
The North American Breeding Bird Survey monitors bird population trends by collecting data along thousands of survey routes across North America each year. Barn swallow trends from 1966-2015 show:
| Region | Population Change 1966-2015 |
| Eastern Breeding Bird Survey | -2.1% per year |
| Western Breeding Bird Survey | -3.5% per year |
| Over all North America | -49% total loss |
These declines were statistically significant, indicating they were unlikely to be due to chance. However, a subset of routes showed increases, suggesting regional variability in trends.
UK population trends
According to the British Trust for Ornithology, barn swallow populations in the UK underwent moderate declines from the mid-1960s through the late 1980s. From 1994-2016, populations were relatively stable, though still 20% lower compared to the pre-decline level:
| Time Period | UK Barn Swallow Population Change |
| 1966-1989 | -30% |
| 1994-2016 | Stable at around 20% below 1966 level |
Causes of declines
The causes behind barn swallow declines in parts of their range are likely multi-factorial and require more research. However, several probable contributing factors include:
Loss of nesting habitat
The conversion of old barns and other rural outbuildings to modern standards reduces nesting sites. One UK study found 65% fewer suitable nesting sites in 2011 compared to 1971. Nest sites may limit barn swallow populations in some areas.
Reduced insect prey
Barn swallows depend on flying insect prey, which some studies suggest has declined due to factors like pesticide use and habitat loss. One study found European barn swallows have faced a 59-87% decline in insect food availability which could negatively impact reproduction.
Climate change
Changing precipitation patterns may limit mud availability for nest building in some regions. Hotter, drier summers may reduce insect prey populations. Delayed migration timing due to climate change may cause mismatches with peak food availability.
Conservation status
Due to their large global population size, barn swallows are listed as a species of Least Concern on the IUCN Red List of Threatened Species. However, they are listed on the State of North America’s Birds Watch List, which highlights species potentially needing conservation action. Several European countries have implemented barn swallow conservation programs.
Research needs
More research is needed to clarify barn swallow population trends and the relative importance of factors driving declines in different regions. Specific areas of study could include:
- Continued long-term monitoring across all regions
- Studies on limiting factors like nest site or food availability
- Research into how climate change may affect breeding, migration, and wintering
- Investigating impacts of agricultural intensification and insect abundance
Conclusion
Barn swallow populations have declined significantly in parts of North America and Europe, likely driven by a combination of habitat loss, climate factors, and reduced insect prey populations. However, global populations remain large. Targeted conservation programs and further research can help ensure stable future populations of this iconic aerial insectivore.
