Article image

Tree swallows have a surprising ability to beat the heat

Birds have it tough these days. With declining numbers, shrinking habitats, and increasing heat, the future seems grim for many species. Yet, in a surprising twist, the tree swallow is defying the odds and expanding its range into the hot Southeastern U.S.

How is this small but mighty bird beating the heat? The answer lies in a remarkable combination of behavioral and physiological adaptations, making the tree swallow a beacon of hope in our changing world.

The curious case of the tree swallow

Unlike most North American birds seeking cooler climates, the tree swallow caught the attention of scientists with its peculiar southward migration pattern.

This inquisitive observation sparked an investigation led by Indiana University researchers Dr. Kim Rosvall and her then-graduate student Mary Woodruff. The duo dedicated themselves to unraveling the mysteries of how these birds thrive in rising temperatures.

The science behind tree swallows heat resilience

Through years of meticulous research, Rosvall and Woodruff discovered that tree swallows have several tricks up their feathered sleeves:

Beat the heat with behavior

When temperatures rise, tree swallows employ several behavioral strategies to maintain their internal body temperature. Panting allows them to evaporate moisture and dissipate heat.

Additionally, spreading their wings increases the surface area exposed to the air, promoting heat loss through convection.

Cellular superheroes

In response to heat stress, tree swallows increase the production of specialized molecules known as heat shock proteins (HSPs). These proteins play a vital role in cellular protection.

They act as molecular chaperones, preventing the misfolding and clumping of proteins caused by heat. HSPs also assist in repairing damaged proteins, maintaining cellular function during periods of thermal stress.

Survival of the warmest 

Interestingly, the research suggests that exposure to moderate heat stress early in life may enhance the survival of tree swallows. This phenomenon indicates that heat stress could drive evolutionary adaptation.

“Stress can expose new variation upon which selection can act,” said Dr. Rosvall. Individuals with genetic traits better suited to tolerate heat are more likely to survive and reproduce, passing these beneficial traits to their offspring.

Tree swallows in the US

Woodruff’s research began with a meticulous analysis of tree swallow tissue samples collected from various locations throughout the United States.

Intriguingly, her analysis revealed a distinct pattern: birds nesting in warmer southern climates exhibited significantly higher levels of heat shock proteins (HSPs) compared to their northern counterparts. This compelling discovery fueled further exploration into the birds’ physiological responses to heat.

To delve deeper, the researchers designed controlled experiments in which tree swallows were exposed to elevated temperatures, carefully simulating heat stress conditions.

Increased antioxidants

The results were remarkable. In addition to behavioral adjustments aimed at cooling down, the birds unleashed a powerful physiological defense mechanism.

They boosted the production of antioxidants, which neutralize harmful molecules generated by heat stress, and initiated anti-inflammatory responses to minimize cellular damage.

“In a sense, behavior was not enough,” said Dr. Rosvall. “They needed that physiological change to cope with heat.” This observation highlights the intricate interplay between behavioral adjustments and complex internal processes that allow tree swallows to withstand thermal challenges.

Tree swallows heat tolerance

The success of the tree swallow offers a positive perspective in the face of climate change. It shows that some species have the capacity to adapt, at least for now. “I was surprised that the story was so hopeful, but I’m thrilled that it is,” said Woodruff.

The tree swallow provides a springboard for further research and raises a crucial question: “If a little bit of heat is good for an animal, is there something we can do to help animals thrive in a warming world?”

Experts are exploring the potential of “priming” animals by exposing them to brief bouts of heat, helping them trigger the helpful changes similar to those in tree swallows.

The work of researchers like Dr. Rosvall and Mary Woodruff is crucial. Their studies not only shed light on how birds like the resilient tree swallow handle heat stress but also pave the way for potential strategies to help other species survive the challenges of a warming planet.

More about tree swallows

Tree swallows are small, agile birds known for their brilliant iridescent blue-green upper bodies and clean, white underparts. These birds belong to the swallow family, which is famous for their aerial acrobatics and speed in flight. Here are some key points about tree swallows:

Habitat and distribution

Tree swallows are native to North America and are widely distributed across the continent. They breed in the northern regions, including Canada and the northern United States, and migrate south to spend the winter in warmer climates, ranging from the southern United States to Central America.

They are versatile in their habitat preferences but are commonly found near water bodies like lakes, rivers, and wetlands, which provide ample insect food for their diet.

Nesting and breeding

True to their name, tree swallows often nest in tree cavities, but they readily adapt to nest boxes if available. This adaptability has made them a favorite among bird watchers and conservationists, who often set up nest boxes to attract them.

During the breeding season, they display remarkable aerial displays and can become quite territorial over nesting sites.


Tree swallows primarily feed on insects, catching them mid-air with their mouths during flight. Their diet includes flies, beetles, mosquitoes, and other flying insects.

During the breeding season, when their energy needs increase, they can eat hundreds of insects per day. In some locations and seasons, they may also supplement their diet with berries and other small fruits, especially during their migration when insects are less abundant.


Tree swallows are migratory birds with a remarkable ability to cover long distances during their migration between breeding and wintering grounds. They typically travel in large flocks, which can include thousands of birds, creating impressive displays in the sky.

Migration allows them to exploit the summer breeding conditions in the north and the warmer wintering habitats in the south.

Conservation Status

Currently, tree swallows are considered to be of least concern by the International Union for Conservation of Nature (IUCN), indicating a stable population globally.

However, like many bird species, they face threats from habitat loss, climate change, and pollution. Conservation efforts, including habitat preservation and the installation of nest boxes, are important in supporting their populations.

Other interesting facts

  • Tree swallows are one of the few species of birds that can actually digest wax. They will sometimes eat the wax from bayberries and other fruit waxes during the winter months.
  • The iridescent colors of tree swallows come from the microscopic structure of their feathers, not pigments, reflecting light in a way that creates the brilliant blue-green sheen.

Tree swallows play a significant role in their ecosystems, controlling insect populations and acting as indicators of environmental health.

Their presence and behavior can provide valuable insights into the state of their habitats and the broader implications of environmental changes.

The study is published in Science of the Total Environment.


Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates. 

Check us out on EarthSnap, a free app brought to you by Eric Ralls and


News coming your way
The biggest news about our planet delivered to you each day