Volume 31 Issue 1, Winter 2026
by Rich Wailes, Volunteer Data Coordinator
The 2025 nesting season brought significant shifts in Loudoun County’s cavity-nesting bird populations, as prolonged June heatwaves and hyperlocal environmental conditions reshaped breeding outcomes across the county. The Loudoun Wildlife Bluebird Monitoring Program’s 2025 nest-box results report was compiled from 62 of the county’s 63 monitored trails and 742 of our total of 753 nest boxes. Volunteer monitors documented 3,159 total fledged birds this year — a complex mix of gains, losses, and changing species dynamics.
The headline finding: Eastern Bluebird fledging declined sharply, while Tree Swallows surged, hinting at how these two species may be responding to rising temperatures.
Bluebird fledging fell by 314 birds compared with last year. At the same time, monitors recorded 85 fewer Bluebird nest-building attempts, signaling that the species may be reducing breeding activity during harsh conditions.
The report identifies prolonged heat as a key factor. From June 22-26, temperatures exceeded 100°F in parts of Loudoun County — dangerously near the threshold known to threaten Bluebird survival. Temperatures inside nest boxes can be much higher than outside the boxes. Scientific studies and prior experience show that Eastern Bluebird eggs and nestlings cannot survive temperatures above 107°F, and extended exposure can even cause temporary infertility in male Bluebirds.
That combination appears to have struck hard in 2025. “Heat adversely affects Bluebirds,” the report states plainly, underscoring how temperature extremes may increasingly shape the species’ long-term viability in the region.
In contrast, Tree Swallows flourished. The species produced 1,329 fledglings, an increase of 230 over last year, with 70 more nest-building attempts recorded.
The data suggests not only resilience but possible adaptation to changing climate conditions. Research cited in the report notes that Tree Swallows employ both behavioral and physiological strategies to withstand heat. These include panting, spreading wings to shed heat through convection, and increasing cellular heat-shock proteins and antioxidants to protect against temperature stress.
This may explain why Tree Swallows did not merely maintain previous levels but expanded their presence during the very same heatwave that suppressed Eastern Bluebird activity. The report’s summary for Hanson Park illustrates this contrast clearly: fewer Bluebirds this year, but more Tree Swallows, with the latter potentially better adapting to higher temperatures.
Beyond the headline species, the county’s nest boxes also supported 432 House Wren, 53 Chickadee, and 64 other native cavity-nester fledglings. Total fledging success rates — measured as
birds fledged per eggs laid — remained similar to last year for Eastern Bluebirds, Tree Swallows, and House Wrens, despite the environmental challenges.
The stability of smaller cavity-nesting species suggests that the year’s ecological stresses did not uniformly affect all bird populations, reinforcing the idea that heat tolerance varies significantly across species.
Data compiled by our volunteer nest-box monitors shows that Eastern Bluebird fledging numbers were down in 2025, likely as a result of prolonged summer heat in Loudoun County. Photo by Larry Tipton.
Tree Swallows like these may be better at adapting to higher summer temperatures brought by climate change than are Eastern Bluebirds. Photo by Michael Sciortino.
Volunteers Driving Monitoring Efforts
The 2025 season once again depended heavily on the dedication of Loudoun’s 240 volunteer trail leaders and monitors, who recorded activity across the county’s vast network of nest boxes. Their observations contribute to one of the most robust, community-driven bird-monitoring systems in Virginia.
The near-complete coverage enables comparative year-to-year analysis and helps reveal early signals of the impact of environmental change.
While countywide Bluebird numbers declined, some trails still saw strong success. The top three Bluebird trails and the trails’ leaders were:
• Morven Park, Cathy Lemmon – 95 fledglings
• Bolen Park, Thomas Letonja – 68 fledglings
• Zephaniah Vineyards, Diane Nastase – 54 fledglings
Tree Swallow productivity was even more striking, with four trails topping the list:
• Temple Hall Farm, Scott and Tamara McDonald – 103 fledglings
• Algonkian Golf Course (Front 9), Luke High – 83 fledglings
• Brambleton Northstar, Paul Thamasett – 69 fledglings
• Lovettsville Park, Keith Nusbaum – 69 fledglings
The high Tree Swallow counts at multiple trails further illustrate the species’ rising resilience and population strength in years with periods of hotter temperature.
Unwelcome Visitors: Snakes and House Sparrows
The 2025 report also tracks predation attempts from nest-box visitors, important metrics for trail managers. This year, Claude Moore Park, led by longtime trail leader Norman Gresley, reported the most snake predations, 13. Three other trails tied for second place, each with four snake encounters — the Academies of Loudoun and Loudoun Aquiary, both monitored by Conrad Varblow, and the 1757 Golf Course, led by Anne Cho.
On the invasive-species front, House Sparrows, a nonnative competitor that often displaces native birds, were deterred in high numbers thanks to diligent monitors on these trails:
• Dobbins Creek, Betty Hedges – 84 sparrows deterred
• Ida Lee Native Tree, Jesse Edwards – 76 sparrows deterred
• Ashby Ponds, Bill Davis – 31 sparrows deterred
House Sparrow control is a critical component of maintaining nest-box programs, and these leaders and their teams played an essential role in protecting native species during a challenging year.
Hanson Park: A Microcosm of 2025’s Climate Story
Hanson Park, where both temperature and rainfall data were paired with bird records, provided a valuable snapshot of how heat extremes shaped species differently. The park recorded fewer Bluebirds and more Tree Swallows, matching countywide patterns but also offering early evidence that Tree Swallows may be adjusting to warming conditions at a micro-habitat level.
That adaptability could make Tree Swallows a pivotal species in understanding how local bird communities respond to climate change.
Looking Ahead: Climate, Conservation, and Community Science
The 2025 results raise important questions for future management of nest-box trails across Loudoun County. As heat waves become more frequent and intense, Bluebirds — once the defining success story of nest-box conservation — may face increasing reproductive pressure.
Meanwhile, the surge in Tree Swallow activity hints that some species may adapt to changing conditions, reshaping local bird community dynamics.
What remains constant is the value of the county’s volunteer-based monitoring network. With 240 monitors tracking nearly 750 boxes, the program continues to generate one of the most detailed annual snapshots of cavity-nesting bird health in the region.
As Loudoun County heads into the 2026 season, conservationists, scientists, and community volunteers will be watching to see whether this year’s trends represent a temporary anomaly — or the early stages of a broader shift driven by rising temperatures.
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